1. sports
2. fishing
3. freshwater fishing

UNIVERSITY OF COPENHAGEN
FACULTY OF SCIENCE
FRESHWATER BIOLOGICAL LABORATORY
Ecology in small aquatic ecosystems
Mikkel René Andersen
Cover photos
Top left: The study site.
Bottom right: The study site.
Background: Dense Chara aspera bed at the study site.
1
Ecology in small aquatic ecosystems
Ph.D. thesis
Author
Mikkel René Andersen
Freshwater Biological Laboratory
Universitetsparken 4, 3rd floor.
2100 Copenhagen Ø
Denmark
Principal
Kaj Sand-Jensen
supervisor
Freshwater Biological Laboratory
Universitetsparken 4, 3rd floor.
2100 Copenhagen Ø
Denmark
Co-supervisor
Peter A. Staehr
Department of Bioscience - Marine Diversity and Experimental
Ecology
Frederiksborgvej 399. B1.19
4000 Roskilde
Denmark
Committee
Photos
Dr. Ole Petersen (Chair)
University of Copenhagen
Dr. Eleanor Jennings
Dundalk Institute of Technology
Dr. Torben Linding Lauridsen
Aarhus University
All photos by Mikkel René Andersen, unless otherwise specified.
2
Table of contents
ABSTRACT ......................................................................................................................... 4
DANSK RESUMÉ ................................................................................................................ 4
INTRODUCTION ................................................................................................................. 8
AIM .................................................................................................................................... 17
PAPER SYNOPSIS ........................................................................................................... 17
CONCLUSIONS AND IMPLICATIONS ............................................................................. 25
REFERENCES .................................................................................................................. 28
PAPER 1 - PROFOUND DAILY VERTICAL STRATIFICATION AND MIXING IN A
SHALLOW, WIND-EXPOSED POND WITH SUBMERGED MACROPHYTES. ................ 30
PAPER 2 - RECURRING STRATIFICATION AND MIXING GENERATE EXTREME
DIURNAL OXYGEN AND CARBON CYCLES IN SHALLOW VEGETATED LAKES ........ 60
PAPER 3 - DISTINCT DIURNAL PATTERNS OF ECOSYSTEM METABOLISM IN A
SMALL CHAROPHYTE-LAKE ........................................................................................... 78
PAPER 4 - WHOLE-STREAM METABOLISM IN NUTRIENT-POOR CALCAREOUS
STREAMS ON ÖLAND, SWEDEN .................................................................................. 110
PAPER 5 - CAUGHT BETWEEN DROUGHT AND FLOODING ON ÖLANDS GREAT
ALVAR (IN SWEDISH, ENGLISH ABSTRACT) ............................................................... 139
ACKNOWLEDGEMENTS ............................................................................................... 148
3
Abstract
Small ecosystems are many-fold more abundant than their larger counterparts. Both
on regional and global scale small lakes outnumber medium and large lakes and
account for a much larger surface area. Small streams are also far more common than
rivers. Despite their abundance small ecosystems are grossly understudied.
In this thesis I present new insights into the dynamic nature of small aquatic
ecosystems. I show that small lakes can stratify and that the resulting gradients are
much steeper than in larger lakes. In a 30-40 cm shallow water-column the surface
waters can be more than 200 % supersaturated in oxygen while the bottom waters
becomes anoxic. Dense charophyte stands influenced the hydrodynamics and created
favorable conditions for the apical parts in the surface waters, while the basal parts
withstood anoxia for up to 12 hours in the bottom waters. Nocturnal convective
mixing oxygenated the bottom waters and replenished the DIC pool in the surface
waters every night. Nocturnal mixing and small distances resulted in similar
metabolic signals recorded by many oxygen sensors placed across the small lake.
Respiration and gross primary production (GPP) were tightly coupled (1:1 ratio) both
in the small lakes and in the small ephemeral streams on the Great Alvar.
Downstream respiration was decoupled from GPP as respiration rates were much
higher due to agricultural impact.
Dansk resumé
Små økosystemer meget mere almindelige end deres større modstykker. Små søer og
vandløb udgør det typiske ferskvandshabitat både i Danmark og globalt, men dette til
trods er de stærkt underrepræsenteret i videnskabelige undersøgelser hvor fokus
meget oftere har været på de større ikoniske søer og floder.
4
Man har ofte antaget at små søer var fuldt opblandede, mens større søer kan have
komplekse opblandings og lagdelings mønstre. Denne forsimpling af de små søer er
dog foretaget uden videnskabeligt belæg.
I kapitel 1 viser vi at helt små søer kan lagdele hvis indkommende solenergi afsættes i
toppen af vandsøjlen og den fysiske opblanding som følge af vindens friktion mod
vandoverfladen samtidigt hæmmes. Vi undersøger disse varmeflukse og den rolle tæt
bevoksning af kransnålalger har i en lille sø. Vi viser fysiske modeller som forudsiger
at søen i forhold til dens overflade areal hvorpå vinden afsættes skal være 4 gange
dybere end den er før den vil lagdele. Alligevel lagdeler den kun 30-40 cm dybe sø
næsten hver dag mellem slutningen af marts og slutningen af maj. Vi måler
temperaturforskelle på op til 15 °C mellem top og bund. Om natten opstår en meget
ustabil situation når overfladevandet køles ned til 1-5 grader under
bundvandstemperaturen, dette resulterer i kraftige konvektive strømme som
opblander vandsøjlen fuldstændigt.
Denne lagdeling muliggøres af kransnålebevoksningen, da op imod 90 % af den
indkommende solenergi afsættes i de øverste 5-20 cm af vandsøjlen som varmes op,
friktionen mellem vinden og søens overflade skaber strømhvirvler men disse svækkes
kraftigt af den tætte kransnålalgebevoksning og når ikke bundvandet der derfor
forbliver koldt. De natlige konvektive strømme påvirkes derimod kun lidt af
kransnålalgebevoksningen da de er retningsbestemte mod bunden som følge af
tyngdekraften. Dermed har kransnålalgerne en udtalt effekt på søen.
I kapitel 2 undersøger vi de gradienter som opstår som følge af daglige lagdeling af
søen. Om dagen overmættes overfladevandet med ilt til over 200 % af
atmosfæreligevægten, mens bundvandet bliver iltfrit. I overfladevandet hvor der er
lys, forbruges uorganisk kulstof i fotosyntesen og pH stiger, herved skabes de forhold
hvor kalk (CaCO3) kan fælde ud som krystaller, i denne proces frigives CO2 til
fotosyntesen uden at pH stiger yderligere.
5
CaCO3 krystallerne synker mod bunden, her i det kolde bundvand hvori
respirationsprocesser har frigivet CO2 og sænket pH opløses CaCO3 krystallerne,
herved ophobes uorganisk kulstof og CO2 ved bunden om dagen. I det iltfrie
bundvand reduceres desuden ferrijern og sulfat til ferrojern og sulfid som er giftig for
mange organismer.
Den natlige opblanding bringer ilt til bunden som oxiderer de reducerede stoffer og
samtidig føres det ophobede uorganiske kulstof tilbage til overfladevandet.
Disse processer betyder at denne næringsfattige sø kan have høj produktivitet og tæt
bevoksning.
I kapitel 3 viser vi at den høje produktivitet hænger sammen med en ligeledes høj
respiration, forskellen mellem disse er tæt på nul, hvilket vil sige at
nettoproduktiviteten i systemet er meget lav. Kun i nogle få timer først på dagen er
produktiviteten høj nok til at opveje respirationen, og allerede først på eftermiddagen
er respirationen større end produktionen. Da der er masser af lys tilstede hæmmes
produktionen af mangel på CO2, i stedet bruges bikarbonat og kalk.
Vi undersøger heterogeniteten i systemet ved dels at måle på metabolismen ned
gennem vandsøjlen og på tværs af søen med mange sensorer forskellige steder i
overfladevandet. Vi finder ensartede resultater ved målinger på tværs af søen, hvilket
formentligt skyldes den effektive natlige opblanding og de korte afstande i søen. Hvis
man ikke tager højde for lagdelingen i søen får man underestimeret respirationen som
dominerer i den nederste del af vandsøjlen.
Om natten aftager respirationsraten som følge af manglende substrat.
I kapitel 4 undersøger vi metabolismen i de små vandløb på Ölands Store Alvar, hvis
øvre dele ofte tørrer ud om sommeren. Her er flere ligheder med metabolismen i de
små søer. Produktiviteten og respirationen er tæt koblet og forhold i mellem dem er
tæt på 1:1 og nettoproduktiviteten er meget lav. Længere nedstrøms løber vandet
gennem landbrugspræget opland, her stiger respirationen og afkobles dermed fra
produktiviteten. Respirationsraten aftager også om natten i vandløbene.
6
Kapitel 5 (på svensk) opsummeres flere studier på Ölands Alvar, og vi viser
eksempler på de landskabsmæssige gradienter som findes her. Vandløb som tørrer ud,
rå kalkflader som oversvømmes skaber damme og udtørres igen, samt mere
permanente små søer hvor vandspejlet ændres dramatisk over året. Der gives
eksempler på biologien i disse habitater.
7
Introduction
Small aquatic ecosystems are many-fold more abundant than the larger and often
iconic lakes and rivers (Downing et al. 2006). Small ecosystems have highly dynamic
physico-chemical dynamics and house species that must be adapted to such dynamic
– often extreme – environments (Wesenberg-Lund 1915, Christensen et al. 2013).
Nonetheless, small lakes and streams have been greatly understudied despite that
many new aspects of ecosystem ecology and species adaptation can apparently be
discovered and understood in this abundant, widespread but forgotten environment
(Herb & Stefan 2005). I should add, that the small aquatic ecosystem are so common
that ecosystem processes such as storage of terrestrial fixed organic carbon and CO 2
emission to the atmosphere have a much greater contribution to these processes for
the entire landscape than that of medium-sized and large lakes and running waters
(Hanson et al. 2007, Sand-Jensen & Staehr 2012).
With this entrance I have very much argued for the focus and title of my
Ph.D. thesis: Ecology in small aquatic ecosystems. First of all, because small aquatic
ecosystems are greatly understudied my investigation was likely to offer new insight
and knowledge that would furthermore be relevant on the landscape level. Secondly,
being highly dynamic environments, small aquatic ecosystems are extremely
fascinating and appealing which is motivating during long field days and tiresome
data analysis. I was not let down by the results that appeared. Discovering recurring
daytime stratification and nocturnal mixing in a small charophyte-lake was a great
experience to me and my co-authors (paper 1), not to mention the documentation of
15 °C changes of water temperature and vertical gradients of the same magnitude
during a summer day. Finding anoxia in bottom waters during daytime alternating
with oxic conditions at night was a big surprise and an interesting discovery which is
obviously a huge challenge to the survival of plants, algae and immobile animals
(paper 2). Being able to present highly reproducible diurnal patterns of ecosystem
8
production and respiration in a small charophyte-lake and subsequently explain them
by relationships to characteristically diurnal courses of environmental variables was
highly rewarding. It comes at a time where recent reports of extreme spatial
variability of estimates of ecosystem metabolism in medium-sized lakes by multiple
oxygen sensors placed at different locations (Van de Bogert et al. 2012) threaten to
make the approach of deploying numerous temperature, O2, pH and other sensors in
the free-water less attractive because the ecosystem estimates attained can apparently
be so noisy that useful interpretation of results are difficult (paper 3). That we, in
contrast, could demonstrate distinct diurnal courses of ecosystem metabolism (i.e.
afternoon depression of photosynthesis and declining nocturnal respiration from
sunset to sunrise, paper 3) as well as approximately balanced production and
respiration rates under highly oligotrophic conditions in small charophyte-lakes as
well as in small ephemeral streams on the open, oligotrophic limestone grasslands of
Öland, SE Sweden was also a new finding and confirmation of a hypothesis that was
originally predicted by the father of aquatic ecosystem metabolism, Howard T. Odum
more than fifty years ago (Odum 1956) (papers 4 and 5).
Abundance of small ecosystems – With my Danish background I know that very
small lakes (< 1 ha) are counted in numbers exceeding 100,000 in the country, while
medium-sized and larger lakes (> 10 ha) are 100-fold less abundant (Sand-Jensen
2001). Also small narrow streams (< 2.5 m width) stretches for 48.000 km through
the landscape compared to 1.500 km for larger Danish streams (> 8 m width, SandJensen et al. 2006). If we turn to other lowland countries such as England (Biggs et
al. 2005, Davies et al. 2008) and the American mid-West (Hanson et al. 2007) the
dominance of small lakes and streams is repeated. The same pattern is even more
pronounced on the Arctic tundra (Anderson & Stedmon 2007) and in the deltas and
backwaters of major rivers (Emmerton et al. 2007).
9
On the large Swedish island of Öland, which became the region of my
investigation, there are no major streams and only one major lake (Lake Hornsjöen).
In contrast, there are many small, shallow streams, mostly ephemeral that dry out
during summer (Fig. 1) and there a many shallow lakes, often filled with charophytes
or submerged flowering plants because of the shallow, nutrient-poor and transparent
waters (paper 4 and 5).
Shallow lakes cover large surface areas of Ölands Great Alvar (Unesco
World Heritage) during winter, but are markedly reduced in size and numbers during
summer drought where many
waterbodies dry out completely. The
same changing water levels are
experienced by the many small lakes
in the abandoned limestone quarry
surrounded by the undisturbed
Räpplinge Alvar, where I made the
largest part of my Ph.D. study (Fig. 2).
Figure 1 Ephemeral stream on Öland (Åbybäcken) that has dried
This area has about twenty small lakes
out in the summer drought.
and my studies could be made
undisturbed by intruders and under kind observation of the owner such that we
experienced no loss of equipment during the three year study.
I should also emphasize that we selected small lakes and streams on
Öland as study objects because they are calcareous and oligotrophic and represents a
contrast to most ecosystems in other parts of the European lowlands that have been
greatly disturbed by excessive eutrophication from urban areas and intensely
cultivated farmland (paper 4 and 5). This destructive development has not occurred to
nearly the same extent on Öland as yet. Thus, we are studying ecosystems as they
10
were throughout Europe before World War II and as they still are in unspoiled
regions and as they may once again become following reduced nutrient loadings.
Small lakes: ephemeral or permanent
and terrestrial impact – Small shallow
lakes can dry out and undergo refilling
on a regular basis, though the nature
of the processes are stochastic such
that drying-out and refilling can occur
several times a year and in different
Figure 2 Arial photograph of an abandoned limestone quarry,
dominated by ponds and small lakes, seen as predominantly as
dark brown patches © Google.
seasons or not take place whatsoever
in wet years. The small charophytelake we studied lost 73% of its water
from winter to early summer in 2014 (paper 1). According to the landowner, it dries
out completely approximately once every 15 years. Charophytes and flowering plants
can sustain complete water loss, while fish populations are more susceptible, though
they were indeed present.
There are important implicit scaling functions concerning water retention
time, external loading of organic material and nutrients as well as incident irradiance
and wind exposure to surface area and water volume of lakes (Sand-Jensen & Staehr
2007, Staehr et al. 2012). Small lakes have a longer coastline relative to surface area
than large lakes; i.e. the contact to the surrounding terrestrial landscape is higher. In
fact for the same shape, the length of the coastline relative to surface area declines in
proportion to the linear dimensions of the lake. Therefore, small shallow lakes tend to
have shorter water retention time, greater external loading of organic material and
nutrients to surface area and volume than large deeper lakes, and are also likely to be
more shaded by riparian vegetation and less exposed to wind (Sand-Jensen & Staehr
11
2007, Staehr et al. 2012). Small forest lakes can be so extensively shaded that
autochthonous production is negligible and the metabolism is entirely based on input
of terrestrial material. In contrast, small lakes located in open landscapes host very
productive plant communities whose primary production by far outweighs the input
of allochthonous material that may come from the low plants in the surroundings.
Commonly, temperate lakes on open agricultural landscapes receive heavy external
nutrient loads resulting in blooms of microalgae, floating duckweeds or submerged
plants and anoxia following die-back and decomposition of the produced plant
material.
The small lakes on Öland, in contrast, are surrounded by nutrient-poor
open grassland with thin soils on the very slowly weathering Ordovician limestone.
Thus, external input of nutrients and organic material is low and the lake water is
crystal clear (Sand-Jensen et al. 2010, paper 5). Ecosystem processes should,
therefore, mostly be of autochthonous character and gross primary production and
community respiration pretty close to each other.
Warming, cooling and hydrodynamics – Small shallow lakes can respond much faster
to meteorological drivers than large, deep lakes because the smaller water column
requires less energy to heat up or cool down. It has mostly been over-looked and
forgotten, but already 40 years ago, Martin (1972) showed that very small lakes can
undergo daily thermal stratification. The fast response to meteorological conditions
and the fact that small shallow lakes can indeed stratify open up the possibility of
very dynamic and complex thermal regimes. Nonetheless, small shallow lakes are
usually assumed to be fully mixed with little justification (Branco & Torgersen
2009).
In contrast, it is realized that large lakes have complex thermodynamics
(Staehr et al. 2009), probably because scientific studies and knowledge of these lakes
are much more comprehensive. Since the early 1900, lakes have been classified based
12
on their stratification pattern (Hutchinson & Löffler 1956), and refined classifications
have been introduced more recently (Lewis (1983). A deep lake located at our
latitude stratifies during summer, mix in autumn, can have reverse temperature
stratification under winter-ice and become fully mixed following ice-out in spring.
We were very interested in analyzing how stratification and mixing
patterns are in small shallow lakes with dense submerged vegetation because
vegetation should greatly impede wind-induced mixing by offering great resistance to
water movements (Losee & Wetzel 1993, Sand-Jensen & Pedersen 1999). Also,
dense submerged vegetation generates extremely steep light attenuation and the
possibility of particularly strong surface warming. Both strong dissipation of
turbulent energy and strong warming of surface waters should facilitate formation of
vertical density gradients. Indeed it turns out that the temporal and spatial thermal
pattern of a deep temperate lake from spring, through summer to autumn is repeated
in the shallow charophyte-lake during every 24-hours day-night cycle in summer
(paper 1) with great consequences for water chemistry (paper 2).
Stratification and water chemistry – There are several reasons why temperature and
coupled stratification-mixing patterns are crucial for understanding physico-chemical
and biological processes in lakes. The thermocline acts like a boundary between
surface waters and bottom waters such that different processes result in diverging
chemistry. The density gradient between the two water layers marks the lower limit
to where turbulent mixing can effectively penetrate such that dissolved ions and gases
are predominantly transported by slow molecular diffusion, while dense particles
influenced by gravity can sink into the bottom waters (Boehrer & Schultze 2008).
In the surface mixed layer organic production by photosynthesis will
typically surpass respiration during daytime leading to accumulation of oxygen,
increase of pH and depletion of dissolved inorganic carbon (DIC). In the bottom
waters, being physically separated from surface waters by the density gradient, the
13
opposite processes (i.e. depletion of oxygen, decline of pH and DIC accumulation)
take place leading to strong vertical gradients.
The crucial question is now how these processes behave and influence
chemical gradients in small lakes that develop dense stands of macrophytes and
possibly form vertical density gradients despite shallow water. In dense stands of
charophytes and flowering plants with steep vertical light attenuation, the chemical
gradients are enhanced by the contrast between well-illuminated surface waters
warming up during daytime and shaded bottom waters. When vertical stratification
develops on a daily basis within macrophyte stands we anticipate the development of
much more profound temporal and vertical gradients of substrates and products
involved in photosynthesis and plant respiration as well as oxygenic and anoxic
bacterial processes than usually encountered in large lakes. pH changes coupled to
photosynthesis and respiration can also induce precipitation of calcium carbonate in
surface waters and dissolution in bottom waters. The rate of these processes and the
resulting vertical gradients should be particularly strong because all metabolic
activity is packed in a shallow water column. Perhaps the dynamics could become
intermediate between that experienced in the upper few millimeters of wellilluminated surface sediments where diffusion processes and extremely high
metabolic rates normalized to volume prevail (Jørgensen & Revsbech 1985) and the
conditions in meter-thick well-mixed water columns of lakes. If stratification lasts
long enough, surfaces waters may become depleted in plant nutrients and bottom
waters enriched in the same nutrients.
In the case of development of strong vertical oxygen gradients bacterial
processes and animals will be greatly influenced. Anoxia induces sulphate, nitrate,
manganese and iron reduction and animals may try to escape from the risk of anoxia
and accumulation of sulphide and reduced iron. The stratification-mixing regime sets
the scene for all the biological and chemical processes and their direct and indirect
ecological consequences. This scene should be quite different in small shallow lakes
14
with dense macrophyte stands than the well-known scene in the open water column
and even within the littoral vegetation of large lakes where vertical and horizontal
turbulence and water flow should be much stronger and weaken the gradients.
Ecosystem metabolism – Technological improvements of O2 sensors as well as other
sensors for free-water measurements of temperature, light, pH, conductivity, etc. have
made it possible to estimate ecosystem processes of gross primary production (GPP),
net ecosystem production (NEP) and respiration and the environmental conditions
regulating them without enclosing the organisms in bottles and chambers under
unnatural environmental conditions. After years of progress and optimism, we have
now been warned that perhaps we have been over-optimistic concerning the
potentials of free-water measurements to determine the processes and demonstrate
their regulation because of surprisingly high differences in metabolism estimates
derived from multiple sensors at different locations in medium-sized lakes (Van de
Bogert et al. 2007, Van de Bogert et al. 2012). Experience suggested that we ought to
maintain optimism regarding the ability of single and multiple sensors to yield
reproducible estimates of ecosystem metabolism in small lakes (Christensen et al.
2013) because shorter vertical and horizontal distances may reduce delays between
processes actually occurring and being registered and at the same time homogenizing
oxygen (and other chemical signals) signals from different sections of the small lakes.
This motivated us to perform the whole-lake studies in the small Charophyte-lake
(papers 2 and 3).
Studies of ecosystem studies of streams are the classical ones which were
initiated already in the 1920-1930ies. Because of the unidirectional flow, oxygen
balances can be established by the two-station method of H. T. Odum and the later
refinements. One challenge is to ensure accurate determination of air-water gas
exchange. We made direct measurements of gas exchange using flow chambers in the
small alvar streams on Öland and were able to obtain highly accurate and
15
reproducible results. We were particularly interested in these streams because they
are oligotrophic and collect virtually no sediment because most sediment is washed
out during winter stormflow following the low-flow spring and summer period and
what remains of organic particles is metabolized when the streams dry out in
summer. We hypothesized that autochthonous production and community respiration
should be very low compared to most other, more nutrient-rich and flow-stable
temperate streams and production and respiration should be in approximate balance.
The small shallow lakes, being filled with water for most of the year and indeed
collecting sediment, develop a dense cover of charophytes and should, thus, have
much higher rates of ecosystem production and respiration. This was an interesting
contrast that we wanted to evaluate (paper 5).
16
Aim
The aim of this thesis was to:
i.
Investigate surface heat fluxes and stratification-mixing in a small charophyte
dominated lake.
ii.
Investigate if macrophytes themselves can influence the hydrodynamics of the
lake sufficiently to cause it to stratify.
iii.
Investigate the physico-chemical gradients that may develop in a stratified
charophyte-lake.
iv.
Investigate ecosystem metabolism of small oligotrophic lakes and streams.
v.
Investigate if the horizontal and vertical heterogeneity of metabolic signals in
such systems differ from larger lakes and rivers.
Paper synopsis
Paper 1 - Profound daily vertical stratification and mixing in a small, shallow, windexposed lake with submerged macrophytes.
We studied a small (< 1000 m2), shallow (< 0.6 m) lake with dense submerged
macrophytes located in an open landscape on Öland, SE Sweden, between March and
May to investigate thermal regimes, surface heat fluxes and effects of macrophytes
Water temperature (C)
on stratification and mixing processes.
35
30
25
20
15
10
5
0
March
April
May
Figure 3 Surface water temperature (dashed line) and bottom water temperature (bold line) in the shallow lake
during the investigation.
17
The small lake heated up from March to May. Profound daytime temperature
differences developed between surface and bottom-waters ranging from 3 °C in
March to 15 °C in May (Fig 3).
Maximum relative thermal resistance to mixing (RTRM) exceeded 50 (a literaturederived value for the certain onset of stratification) on 11 days in April and 25 days in
May while the mixed depth dropped from 100 % of the water column to just 25 %
(calculations of mixed depth showed, however, that the small lake stratified
moderately even in late March). Nocturnal cooling of surface waters to 1-5 °C below
bottom water temperature led to full convective mixing of the water column every
night. Nocturnal surface cooling and convective mixing were enhanced by the
extraordinary daytime warming of surface waters above air temperatures. Convective
mixing was only weakly affected by the charophytes.
The daytime focal depth of the thermocline was 25 cm below the water surface in
early May and just 15 cm in late May following a parallel shallowing of the lake
bringing the charophyte canopy closer to the water surface. The strength of
stratification peaked in the early afternoon although diel wind speeds were highest at
this time (Fig. 4).
Mixed depth (%)
aa
RTRM
150
100
50
0
-50
00
03
06
09
12
15
18
21
24
Wind speed (m s -1)
200
100
1500
3
b ab
PAR (µmol m-2 s-1)
250
2
50
0
00
1
0
00
03
03
06
06
09
09
12
12
15
15
18
18
21
21
24
b
1000
500
0
00
24
03
06
25
100
Surf. water temp. (°C)
d averages of windspeed (m s -1)
Figure 4 Diel averages of RTRM (dimensionless) (panel a) and diel
Relative humidity (%)
09
12
15
18
Time of Day
of Day
TimeTime
of Day
Time of Day
90
( panel b). Blue line is March, green line is April and red line is May.
80
e
20
15
70
10
The coinciding peaks in wind speed and strength
of stratification was possible
60
5
because the dense macrophyte cover rapidly attenuated depth penetration of the
50
00
03
06
09
12
15
Time of Day
18
21
24
0
00
03
06
18
09
12
15
Time of Day
18
radiative fluxes, while also greatly attenuating the depth penetration of wind-induced
turbulent mixing. Thus, by facilitating build-up of temperature, chemical and density
gradients the macrophytes profoundly influenced their own environment.
Paper 2 - Recurring stratification and mixing generate extreme diurnal oxygen and
carbon cycles in shallow vegetated lakes.
Vertical stratification-mixing patterns are main determinants of biogeochemistry.
Here we show that a small, wind-exposed, shallow (ca 0.4 m) lake with submerged
macrophytes underwent recurring daytime stratification and nocturnal mixing during
summer accompanied by extreme variations in temperature, oxygen, pH and
dissolved inorganic carbon (DIC) with time and depth. During daytime stratification,
surface waters attained 230 % oxygen saturation and strong CO2 depletion (< 10 %
air saturation, Fig. 5).
19
Water depth (m)
0
35 C
a
30 C
0.1
25 C
0.2
20 C
15 C
0.3
10 C
0.4
500
5 C
b
O2 (M)
400
300
0.08 m
0.24 m
0.34 m
200
100
pH
14
c
12
9.0
10
ANC (meq L -1)
8.0
6
1.5
4
1.0
2
0.5
0.4
d
DIC
1.0
0.3
0.2
CO32-
0.5
0.1
CO2
0.0
0
12
0
12
0
12
0
12
0
12
0
12 August
13 August
14 August
15 August
16 August
CO32- / CO 2 (mM)
1.5
DIC (mM)
8
2.0
Calcite saturation index (green)
10.0
0.0
Figure 5 Time series of temperature, O2, pH, ANC, calcite saturation index, DIC, individual carbon species with depth in a
shallow charophyte-lake during six days.
a, Temperature isopleths calculated from measurements at 5-cm depth intervals.
b, Oxygen measured at 0.08 m (dark blue), 0.24 m (green) and 0.34 m (red) below the water surface.
c, pH (blue) and ANC (green) in surface waters (0.08 m).
d, DIC (red), CO32- (orange) and CO2 (blue) in surface waters (0.08 m).
Where b-d background color show day/night cycle (white = day, grey = night).
Deeper waters were colder and became anoxic. In the cold anoxic bottom waters,
reduced compounds such as ferrous iron and sulphide accumulated during the day
and CO2 built up to more than 1500 % super-saturation (Fig. 6).
20
Water depth (m)
0.0
0.1
0.2
0.3
0
10
20
30
Fe2+ (M)
40
0
0.1
0.2
0.3
0.4
Sulphide ( M)
1
2
3
DIC (mM)
Figure 6 Depth profiles of Fe2+, sulphide and DIC in a shallow charophyte lake during a diurnal cycle.
At 6:00 (red), 11:00 (blue), 16:00 (green) and 22:00 (orange) o’clock. The water column was vertically mixed at 6.00 o’clock
and stratified below 0.20 m at 11:00 - 16.00 and below 0.25 m at 22:00 o’clock.
High daytime pH in surface waters induced CaCO3 precipitation while releasing CO2
for ongoing photosynthesis without further pH rise. The majority of the precipitated
CaCO3 was re-dissolved in bottom waters leading to a buildup of DIC (Fig. 6).
Vertical gradients disappeared during nocturnal convective mixing which oxygenated
the bottom waters and regenerated the DIC pool in the surface waters.
These processes add new dimensions to our understanding of the regulation of
ecosystem photosynthesis and respiration and the adaptation of sessile plants and
mobile animals to the extreme variability of environmental stressors.
Paper 3 - Distinct diurnal patterns of ecosystem metabolism in a small charophytelake.
We wanted to characterize the temporal and spatial variability of metabolic
parameters: gross primary production (GPP), respiration (R) and net ecosystem
production (NEP) in a small, shallow lake with dense charophyte stands. To do so we
collected data from many O2 sensors placed along a vertical mid-lake profile and
across the lake surface in late May and early June. Similar diurnal patterns derived
both from individual surface sensors and multiple sensors. Maximum NEP-rates
21
occurred between 8:00 and 11:00 am and was followed by strong afternoon
depression with rates close to zero (Fig. 7).
While NEP rates dropped along with DIC and
CO2 concentrations, O2 concentration, pH and
temperature all rose profoundly in the surface
waters from morning to late afternoon.
Figure 7 Mean volume-weighted GPP (red) and NEP
(blue) for the entire lake, based on the seven oxygen
sensors
Inorganic carbon limitation of photosynthesis
and temperature enhancement of respiration
could account for the profound afternoon
depression of NEP. Nocturnal respiration declined systematically from sunset to
sunrise due to falling temperature and presumably depletion of respiratory substrates.
Mean temperature-corrected respiration rates at sunrise were 63% of that at sunset.
The dense charophyte canopy accounted for 90% of ecosystem respiration and the
entire primary production. Mean daily estimates of GPP and R varied only 2-fold and
small, negative NEP-rates varied less between surface sensors at different locations
across the lake (Table 1).
22
Table 1 . Mean daily rates of GPP, NEP and R derived from continuous oxygen
measurements at seven different positions (A to G) and the overall mean of all measurements
in a small lake during a week in early June. Daily minimum and maximum daily rates are in
parenthesis.
In the small oligotrophic lake rates of GPP and R were tight coupled and both about
20-fold higher than NEP (Table 1). Multiple oxygen sensors representing the main
depths and sections of the small lake could provide reliable and accurate
measurements of diurnal course and daily rates of metabolism, probably because a
relatively uniform oxygen signal was ensured by small distances and very efficient
nocturnal mixing.
Paper 4 - Whole-stream metabolism in nutrient-poor calcareous streams on Öland,
Sweden.
We studied whole-stream metabolism in three headwater non-forested stream reaches
on the island of Öland, Sweden in order to characterize the metabolism of this
unusual ecosystem and to compare it with other stream ecosystems in NW Europe.
Gross primary production (GPP) was low (< 4 g O2 m-2 d-1) with the lowest GPP
recorded in the most upstream, shallow reach draining the thin soils of the limestone
Alvar plains. Here, completely flooded terrestrial plants could account for the whole
primary production. Respiration (R) increased several-fold downstream with
increasing agricultural impact, resulting in heterotrophic stream conditions and higher
23
light requirements for GPP to outweigh the higher respiration.
Some similarities between the small oligotrophic lakes (paper 3) and the most
nutrient-poor reaches of the stream were observed. GPP and R were tightly coupled
and temperature-corrected respiration rates were highest in the beginning of the night
and decreased towards the end of the night (Fig. 8), indicating that nocturnal
respiration depleted photosynthetic products and became limited by organic
substrates in the streams too.
Broad-scale comparison of open NW
European streams showed a 1:1
relationship, indicating a tight link
between daily GPP and R during summer
(April-August) but not during winter.
Figure 8 Respiration rates declining during the night in a
We extended the range of GPP and R
small nutrient-poor Alvar stream.
measurements to include nutrient-poor
NW European streams, thereby increasing the knowledge on stream metabolism in
this region, otherwise, highly impacted by agriculture. We documented a strong
relationship between GPP and R in streams, ranging from extremely nutrient-poor to
moderately nutrient-rich conditions during spring and summer.
Paper 5 - Caught between drought and flooding on Ölands Great Alvar
The Great Alvar plain on the Swedish island of Öland is characterized by thin soils
covering the hard limestone pavements. This gives rise to widely fluctuating water
levels between winter flooding and summer drought and strong hydrological
gradients across small changes in elevation. In the semi-natural grassland, the
intermittent streams and the ponds are all strongly influenced by the fluctuating water
levels and the extremely low phosphorus availability. These factors have selected for
24
phototrophs with low metabolic rates and growth, and communities of low
photosynthesis and respiration.
Plant species were distinctively distributed according to their characteristic plant
traits along a moisture gradient from ponds to dry alvar. High root porosity to ensure
efficient oxygen transport was strongly selected for among species in wet soils, while
small, thick leaves were strongly selected for on thin, dry soils. Overall, six plant
traits could predict 66% of the variation in abundance of plant species in the
communities along the gradient.
The alvar streams had only modest biomasses during maximum development of
benthic algae in May, and community photosynthesis was 5–10 times lower than
corresponding levels in nutrient-rich streams in cultivated lowlands of Scandinavia.
During June–September streams dried out and the re-establishment of flow in winter
and spring led to an export of nutrients. Shallow ponds also dried out during summer
and had low metabolic rates just like the streams, while permanent ponds developed
dense stands of charophytes, despite undetectable levels of N and P in the water.
Photosynthesis and community respiration were in approximate balance in permanent
ponds. The maximum rates were comparable to those in eutrophic, phytoplanktonrich lakes.
Conclusions and implications
Several concrete results have emerged from this thesis. Small lakes can stratify and
exhibit temperature differences between the surface mixed layer and the hypolimnion
that rival that of much larger lakes. Because the stratification takes place in a very
shallow water column, the resulting gradients are extremely steep. It was a new
insight that a water column of just 0.3-0.4 meter could be more than 200 %
supersaturated in the surface waters and anoxic in the bottom waters.
25
The carbon pool in the surface waters in such a lake is regenerated in DIC and CO2
which is dissolved in the anoxic bottom waters during the day and distributed in the
water column by the efficient convective nocturnal mixing. On particularly cold days
when the water column does not stratify photosynthesis in the surface water becomes
carbon-limited much earlier during the day.
While the stratification presents plants and animals with challenges for survival, it
also provides the basis on which they depend and have adapted. Physical models of
mixing resulting from surface wind shear show that the lake would have to be
roughly 4 times as deep as it was in order to stratify, and the only explanation is that
the attenuation of both radiative heat fluxes and wind-induced turbulent mixing due
to the presence of macrophytes is the reason why the lake stratifies. The macrophytes
then create the physical and chemical environment on which they themselves depend.
Potential harmful effects of elevated pH levels in the surface waters were limited by
the fact that as the day progressed productivity depended more and more on CaCO3
precipitation which release CO2 and is pH neutral.
In this highly dynamic system we observed the same metabolic patterns with
individual oxygen sensors as we did with a volume-weighted average of many
sensors placed across the lake. Respiration was underestimated when calculations of
metabolic parameters did not account for stratification, but the results were still of the
same order of magnitude. We credit the small distances and the recurring nocturnal
mixing for integrating possible local differences in metabolic rates to a common
oxygen signal.
Investigating the metabolism in detail revealed some interesting patterns. Positive
NEP rates were restricted to mornings and early afternoons. Later in the day
respiration speeded up due to elevated temperatures and productivity dwindled as the
surface water became depleted on metabolic substrates. Nocturnal respiration slowed
down during the night, even when corrected for the drop of temperature. This finding
suggests that respiration too is limited by availability of substrate. Both productivity
26
and respiration rates were high on areal basis in this oligotrophic system, while NEP
was close to 0. This validates our metabolism model as the very shallow lake would
simply be filled up if respiration did not closely follow production, and in the ~30
years in which the small lake has existed, only a few (4-10) centimeters of sediment
have accumulated.
Some of the same patterns were observed in the small intermittent streams we
investigated on the Great Alvar. Productivity and respiration was tightly coupled with
a 1:1 ratio, and respiration rates dropped markedly as the night progressed. Both
productivity and respiration was several fold lower than in other low land streams,
owing to the extremely low nutrient- and DIC-concentrations in the water and the
winter wash-out of sediments. As we moved downstream into agricultural areas,
respiration was decoupled from production as it became much larger and the
differences between the Öland streams and other lowland streams was less marked,
although productivity remained reasonably low.
This thesis has implications for conservation and ecology on a broad scale. My coauthors and I show that stratification can occur in very small lakes. Two drivers are
necessary for a small lake to stratify:
 The incoming heat flux must be attenuated unevenly through the water column.
 The turbulent mixing must be insufficient to ensure full mixing of the water
column.
In this study the charophytes attenuated almost 90 % of the incoming short-wave
radiation in the top 5-20 cm of the water column. In other small lakes light could be
attenuated effectively in the surface water by dense microalgae or high densities of
humic substances.
The charophytes also effectively attenuated mixing, in other systems mixing could be
limited by the surroundings, for instance by forests or crevasses between fault lines in
mountains.
27
The implication is that many small lakes may have complex thermal regimes and may
behave vastly different than assumed in global estimates of temperature response,
carbon sinks etc. It also means that the palette of ecological niches may be larger on
local and global scales than previously assumed.
For system ecologists this thesis provides the basis for investigating physical,
metabolic and ecological patterns for a wider range of small lakes and streams.
It remains unanswered how sessile organisms survive the harsh conditions of the
bottom waters in the small charophyte-lakes.
It is my hope that this thesis may be an inspiration to physiological studies of
adaptations to extreme environmental conditions for algae, plants and animals in
small lakes.
References
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random sample of regional lake characteristics. Freshwater Biology 52:814-822.
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Hutchinson, G., and Löffler, H. 1956. The thermal classification of lakes. Proceedings of the National
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Staehr, P. A. 2010. Plant distribution patterns and adaptations in a limestone quarry on Oland. Svensk
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29
Paper 1 - Profound daily vertical stratification and
mixing in a shallow, wind-exposed pond with
submerged macrophytes.
30
Profound daily vertical stratification and mixing in a shallow, wind-exposed
pond with submerged macrophytes.
Mikkel René Andersen1, Kaj Sand-Jensen1, R. Iestyn Woolway2 and Ian D.
Jones3.
1
Biological Institute, Freshwater Biological Laboratory, University of
Copenhagen, Universitetsparken 4, 2100 Copenhagen, Denmark.
2
Department of Meteorology, Reading University, Reading, RG6 6BB, United
Kingdom.
3
Centre for Ecology & Hydrology, Lancaster Environment Centre, Library
Avenue,
Bailrigg, Lancaster, LA1 4AP, United Kingdom.
Submitted to Freshwater Biology
Keywords: temperature stratification, vertical mixing, pond, macrophytes,
charophytes
31
Summary
1) Ecology of small shallow lakes and ponds have been grossly understudied in
freshwater ecology although they are 100-fold more abundant than large, deep
lakes and cover a much larger area globally. Mixing patterns are essential
because they regulate distribution of gases, solutes and organisms. Here, we
studied a small (< 1000 m2), shallow (< 0.6 m) pond with dense submerged
macrophytes located in an open landscape on Öland SE Sweden between
March and May to investigate thermal regimes, surface heat fluxes and effects
of macrophytes on stratification and mixing processes.
2) The pond heated up from March to May as surface heat fluxes were positive.
Profound daytime temperature differences developed between surface and
bottom waters ranging from 3 °C in March to 15 °C in May. Maximum relative
thermal resistance to mixing (RTRM) exceeded a threshold of 50 on 11 days in
April and 25 days in May while the mixed depth dropped from 100 % of the
water column to just 25 %. Nocturnal cooling of surface waters to 1-5 °C below
bottom waters temperature led to full convective mixing of the water column
every night. Nocturnal surface cooling and convective mixing were enhanced
by the extraordinary daytime warming of surface waters above air
temperatures.
3) The daytime focal depth of the thermocline was 25 cm below the water surface
in early May and just 15 cm in late May following a parallel shallowing of the
pond bringing the charophyte canopy closer to the water surface. The strength
of stratification peaked in the early afternoon although diel wind speeds were
highest at this time. The dense macrophyte cover rapidly attenuated depth
penetration of radiative fluxes and wind-induced mixing.
4) Dense macrophyte stands can influence their own environment by facilitating
build-up of temperature, chemical and density gradients while lack of
macrophytes permits continuous mixing and uniform conditions.
32
Introduction
Ponds are far more abundant than lakes, and ponds have a combined surface area
which far exceed that of lakes (Downing et al. 2006). While it is well known that
lakes have complex thermodynamics (Boehrer & Schultze 2008), studies of surface
forcing, mixing and stratification in ponds are rare. Ponds are typically treated as
fully mixed systems with little or no justification (Branco & Torgersen 2009). This is
problematic as stratification and mixing dynamics in the water column are major
determinants of environmental conditions (Branco et al. 2005), distribution,
metabolism and survival of organisms (Vad et al. 2013). Ecosystem properties such
as carbon metabolism (Staehr et al. 2010), the flux of gases between water and air
(Boehrer & Schultze 2008, Coloso et al. 2011) and production rates of the
greenhouse gas, methane are also highly influenced by thermodynamics (Bastviken et
al. 2011).
Accurate characterization of heat exchange between lakes and atmosphere
is important for analysis of lake hydrodynamics (Lofgren & Zhu 2000). Positive net
heat input results in a positive buoyancy flux, stabilizing the warmer surface layer,
while a net surface heat loss cools the surface waters and promotes vertical mixing
(Imberger 1985). Wind-induced vertical mixing produced by wind shear on the
surface acts as a destabilizing force (Imberger & Hamblin 1982). This mechanical
vertical mixing deepens the mixed layer and reduces the likelihood of temperature
stratification in wind-exposed shallow lakes (Imberger 1985, Boehrer & Schultze
2008, Branco & Torgersen 2009). Thus, vertical temperature stratification in lentic
ecosystems takes place when the stabilizing influence of surface heating from solar
radiation and infrared radiation from the sky exceeds the destabilizing influence of
turbulent mixing generated by the wind and cooling of the surface waters (Gorham &
Boyce 1989, Imboden & Wüest 1995). In general, the shallower the lake, and the
33
more wind-exposed, the less likely is it to stratify (Imberger 1985, Gorham & Boyce
1989).
It was, therefore, a surprise that a very shallow (< 0.6 m) and small (<
1000 m2) pond exposed to the wind on the open calcareous Alvar plains on the Island
of Öland, Sweden, apparently underwent profound vertical stratification during most
days and full mixing every night according to vertical oxygen dynamics (Andersen et
al. 2015). Formation of anoxia in bottom waters during the day was probably made
possible by strong vertical density stratification, because surface waters were always
oxygenated and strongly supersaturated during the day. The pond, though, had a
dense macrophyte cover that can result in strong vertical light attenuation. High light
attenuation in humic waters can increase surface water temperatures due to absorbed
radiation (Persson & Jones 2008, Read & Rose 2013) but increase in surface water
temperature also causes a lake to lose more heat from sensible, latent and long-wave
fluxes than one with a cooler surface (Persson & Jones 2008) and, when this heat loss
exceeds incoming heat, will promote mixing. Thus, vegetation cover can potentially
affect the hydrodynamics in lakes if the macrophyte canopy is dense enough to
significantly enhance light attenuation. This effect will be stronger if the macrophytes
are located close to the surface as the macrophytes will absorb more energy relative
to the water column, and this energy will subsequently be dissipated in a smaller
volume of water. In transparent oligotrophic waters the strong light attenuation effect
of macrophytes will dominate as light attenuation directly in the water is small. Thus,
the effect of macrophyte light attenuation on hydrodynamics should be stronger in
clear than unclear waters. Similarly, if cover is sufficiently dense, macrophytes can
inhibit mechanical mixing by dissipating turbulent kinetic energy (Sand-Jensen &
Mebus 1996, Folkard et al. 2007).
We therefore initiated continuous measurements of temperature structure
at high spatial and temporal resolution in a calcareous pond with dense charophyte
beds, in order to investigate if the charophytes could influence the stratifying and
34
mixing dynamics sufficiently to cause diel stratification. To calculate heat fluxes a
meteorological measuring station was established next to the pond.
Our specific objectives in this study were to determine: (i) the heat fluxes
in a small pond with dense macrophyte cover, (ii) when and where stratification and
mixing take place during daily and monthly periods, and (iii) the impact of
macrophytes on stratification and mixing dynamics.
Materials and methods
Site description
The investigation was conducted in a small permanent pond in an abandoned
limestone quarry on Räpplinge Alvar on Öland, SE Sweden (56.81168°N, 16.6094°E;
Sand-Jensen & Jespersen 2012). The quarry supports about 20 temporary and
permanent small ponds (Sand-Jensen et al. 2010, Christensen et al. 2013). The
substratum in the quarry consists of exposed solid limestone pavements, which are
almost devoid of vegetation over large areas. The quarry which is surrounded by the
natural Alvar was abandoned about 30 years ago. The area is kept open by grazing
horses.
The local climate is quite dry (mean annual precipitation 510 mm; 19601990), with moderately cold winters (January mean -1.2o C) and mild summers (July
mean 16.2o C) (SMHI 2013). The precipitation is evenly distributed throughout the
year (monthly mean range 32-54 mm), but temperature variations lead to large
seasonal differences in evapotranspiration and water availability (SMHI 2013).
Between April and August in 2010 local surface temperature on the exposed
limestone pavements exceeded 40o C on 37 days (Sand-Jensen & Jespersen 2012).
The drainage water from the limestone soils filling the shallow ponds has a high acidneutralizing capacity and a pH of 8.0 at air saturation (Sand-Jensen et al. 2010,
Christensen et al. 2013). The ponds have extremely low concentrations of soluble
35
inorganic nitrogen and phosphorus close to the limit of detection (Christensen et al.
2013).
A meteorological station was established next to the study pond 2.0 m
above the water surface. The station was equipped with sensors for incident
irradiance (HOBO PAR sensor (400-700 nm): S-LIA-M003, Onset Computers,
Bourne, MA, USA), wind speed and direction (HOBO anemometer and direction, SWSET-A, Onset Computers), air temperature and relative humidity (HOBO U23 Pro
v2, Onset Computers). Measurements were stored on a data logger (HOBO micro
station, H21-002, Onset Computers). A Swedish meteorological station is located at
Kalmar Airport 25 km away. This national station offered measurements of daily
precipitation, wind speed and direction when data collection failed for a short period
at our station. We used the regression equation established between the two sets of
measurements when both stations operated to convert Kalmar Airport measurements
to the pond setting. Wind speed was closely correlated (r2: 0.68, P < 0.0001) between
our station and the Kalmar station nearby.
Maximum water level, surface area and water volume in the pond are set
by overflow across its rim onto the adjacent Alvar plains. Water depths were
measured in a grid of 258 measurements across the pond surface. Water level in the
pond was measured at 10 minute intervals with an accuracy of 3 mm by recording
pressure differences between a submerged water level data logger (HOBO U-20-00104, Onset Computers) and a similar logger in air allowing continuous calculations of
water depth, surface area and water volume of the pond, while correcting for
atmospheric changes in barometric pressure.
From early March through May of 2014, maximum water depth dropped
from 0.59 to 0.30 m and surface area declined from 972 to 661 m2 (Fig. S1). The
maximum fetch across the pond in the main West-East wind direction ranged from 10
to 30 m. Water volume was reduced from early March through May from 343 to 99
m3 as a result of higher evaporation than precipitation (Fig. S1). Evaporation and
36
precipitation were the main components in the water balance during the period (data
not shown).
Sediments were dominated by fine mineral particles with a high content
of calcium carbonate (47% of dry mass) and a medium level of organic matter (10%)
(M. Andersen and K. Sand-Jensen, pers. comm. 2013). Sediments were deposited
directly on top of the hard limestone and varied in thickness from 4.0 to 9.5 cm
(mean 5.8 cm). The pond was covered by dense vegetation of charophytes. The
dominant species was Chara aspera and additional charophyte species included C.
vulgaris, C. virgata and C. globularis. Phanerogams comprised small populations of
Potamogeton crispus, Potamogeton natans and the emergent plants Phragmites
australis, Typha latifolia, Alisma plantago-aquatica and Alisma lanceolata.
Pond measurements
Vertical temperature and light profiles were measured in the middle of the pond at 810 positions and about 5-cm depth intervals from the water surface to the sediment
surface using small temperature-light sensors (HOBO UA-002-64, Onset Computers)
logging the signals every 10 minutes. Measurements were conducted from March to
late May 2014. Sensors were calibrated relative to each other before and after use by
setting them up in shallow water in the pond in natural daylight (10-cm depth and no
vegetation) for 24 hours and subsequently correcting the response of the individual
sensors relative to the mean value of all sensors. Temperature readings were in full
agreement with measurements by a high-precision thermometer. Because the HOBOloggers work in steps of 0.14 °C, we judge this as the absolute accuracy. Irradiance
data through the water column were recorded in May and used in relative mode to
calculate light attenuation between sensors positioned at different depths.
Temperature-light sensors were mounted on a vertical steel peg rising
from a heavy steel plate buried in the sediment. Individual sensors were fastened to
thin 5-cm long plastic brackets keeping the sensors horizontal and pointing in
37
different directions to avoid internal shading and minimize influence on the natural
temperature, light and flow regimes.
Temperature profiles were used to construct isopleths of temperature with
depth and time and to calculate the relative thermal resistance to mixing (RTRM,
Wetzel (2001)). RTRM is the non-dimensional ratio between the difference in density
of bottom and surface water normalized to the difference in density between waters at
temperatures of 4.0 and 5.0 °C:
RTRM=
(ρz -ρz )
2
1
(ρ4 -ρ5 )
,
where ρ is specific mass density of water (kg m-3), ρz1 is for the surface water, ρz2 is ρ
at the bottom water and ρ4-ρ5 is the difference in specific mass density of water at 4
°C and 5 °C respectively. Specific mass density of water was calculated from water
temperature according to Bigg (1967).
Temperature profiles through the water column at the time of maximum
Wetzel stability were used to determine the depth interval in which the maximum
change in water temperature occurred. The mid-point of this depth interval is an
analogue to the focal depth of the thermocline (zThCline) used for lakes.
Light data within the water column were not recorded for March and
April, but for May light data were collected and profiles of daily irradiance with
depth (z, m) below the water surface (Ez) were integrated over the day and used to
determine the mean daily light attenuation coefficients (η, m-1) with depth below the
water surface (z) by linear regression analysis according to:
Ln (Ez) = -η*z.
The attenuation coefficient was used to calculate the depth (z10%) at which the
subsurface irradiance was reduced to 10 % according to:
z10% = 2.3*η-1.
Biomass samples were collected in the charophyte bed on three occasions. Six to ten
randomly located cores (inner diameter 10 cm) were placed over the vegetation and
38
gently pushed into the sediment. All above-ground charophyte material within the
cores was removed by hand, carefully rinsed and dry weight (DW) determined after
48 h at 105 °C. Mean biomass density was calculated both per unit surface area and
per unit water volume.
Air temperature, relative humidity, PAR-light and wind speed were
measured every minute and stored every 10 minutes. The first HOBO temperaturelight sensor which was fully submerged was used for surface water temperature. The
MatLab© version of Lake Heat Flux Analyzer (Woolway et al. 2015) was used to
calculate surface heat fluxes, net incoming short-wave radiation (Qsin), the reflected
component of short-wave radiation (Qsr), sensible heat flux (Qh), latent heat flux (Qe),
incoming long-wave radiation (Qlin), outgoing long-wave radiation (Qlout), net longwave heat flux (Qlnet; Qlin - Qlout) and total surface heat flux (Qtot), as well as the
dimensionless drag coefficient, CD, and the transfer coefficient for latent heat, CE.
The transfer coefficient for sensible heat was assumed equal to that for latent heat
(Zeng et al. 1998). Lake Heat Flux Analyzer calculates fluxes and transfer
coefficients from standard, established equations in the air-water literature, including
calculating the transfer coefficients, CD10 and CE10, at the standard reference height of
10 m. Turbulent flux equations are based on Zeng et al. (1998), incoming long-wave
radiation is modelled after Crawford and Duchon (1999) and Fresnel’s equation is
used to calculate the reflected solar radiation (Woolway et al. 2015). The wind
energy flux, referenced to 10 m, P10, was calculated following (Wüest et al. 2000) as
P10 = ρa*CD10*U103, where ρa is the density of air calculated as Verburg and
Antenucci (2010) and U10 is the wind speed at 10 m, calculated from the measured
wind speed and atmospheric stability using Lake Heat Flux Analyzer.
Results
Meteorological variables
Surface irradiance reached daily peaks at noon above 1000 µmol m-2 s-1 on most of
the investigated days from March to May (Fig. 1). Days of lower irradiance were
39
scattered throughout the period. Wind speed was moderate (< 4 m s-1) during 94% of
the time and only on 12 % of the days was the maximum wind speed above 6 m s-1.
Wind speed peaked in the afternoon to 2-2.5 times the nocturnal wind
speed, and there was no difference between months (Fig. 2). PAR light had slightly
lower midday peaks in March than May. Surface water temperatures followed air
temperatures but were consistently higher; both peaked in the afternoon and both
increased from March to May. The relative humidity dropped in the afternoon at
increasing air temperature and declined from March to April, but did not differ
between April and May.
Light attenuation and charophyte density
Irradiance was rapidly attenuated with depth in the pond because of the high biomass
density of charophytes. The mean daily light attenuation coefficient from just below
the water surface to 8 cm above the sediment surface was 10-25 m-1, resulting in
absorption of 90 % of subsurface irradiance in the upper 9-23 cm of the water column
(Fig. 3). The depth at which subsurface irradiance was reduced to 10 % (z10%)
shallowed significantly during May along with falling maximum water depth (z10% =
0.75 * zmax - 0.12; r2: 0.39, P < 0.001) because falling water depth led to the canopy of
the charophyte vegetation being closer to the pond surface. The pond water itself is
highly transparent with light attenuation coefficients of only about 0.5 m-1. The steep
reduction of irradiance with water depth is due to charophytes having high areal
biomasses of 642-773 g DW m-2 in March-May. Biomass density per unit volume
was 2275-2569 g DW m-3 in March-May corresponding to biomass specific light
attenuation coefficients of 6-10 m-1 (kg DW m-3)-1 after correcting for background
light attenuation in the water.
Surface fluxes
The pond heated up throughout the period with consistently positive daily
accumulated Qtot values (Fig. 4). Net incoming solar radiation followed the expected
diurnal cycle. Reflected short-wave radiation was greatest during mornings and
40
evenings, even though the incoming radiation was low at this time, as the low angle
of the incident light resulted in a large fraction being reflected (Fig. 4). The net longwave radiation (with positive values indicating heating i.e. Qlin - Qlout), being
dependent on both outgoing and incoming long-wave radiation and therefore
dependent on air temperature, water temperature, relative humidity and cloud cover
showed a relatively complex diel cycle, but one that did not change greatly between
day and night. Both latent and sensible heat fluxes (where positive values indicate
cooling), however, had dramatic diel cycles with much greater cooling during the
day, driven by the large increases in wind speed and water temperature and the
marked day-time reduction in relative humidity (Figs. 4 and S2). As the wind energy
flux is proportional to the cube of the wind speed it was far greater during the day
than the night (Fig. 4).
Transfer coefficients
The transfer coefficients for latent heat, referenced to 10 m, CE10, ranged between
0.7x10-3 and 4.4x10-3, with an average of 2.13xI0-3, while the drag coefficients,
referenced to 10 m, CD10, generally had lower maximum and average values but the
same minimum, ranging between 0.7x10-3 and 3.2x10-3, with an average of 1.76x10-3.
Throughout the period CD10 was 17.6 % lower than CE10 (Fig. S3). Both transfer
functions dropped at midday to 45-66 % of the maximum nocturnal values and both
were slightly higher in May than in April. A few exceptional measuring points lead to
March standing out as very different from the other two months because a hail and
snow storm passed the site in two of the measuring days greatly affecting the
atmospheric stability (Fig. 5).
Stratification
The RTRM did not exceed the threshold value for stratification of 50 in March, and
RTRM was only above 50 once in the first half of April (Fig. 6). For the last half of
April and all of May the RTRM exceeded the threshold every day, except for six
particularly cold and windy days. The RTRM dropped to negative values almost
41
every night. In May the RTRM exceeded 50 around 9 am, peaked early in the
afternoon at 200 and stayed above 50 until 8 pm. While the RTRM exceeded 50 in
May, the mixed depth was reduced to 25 % of the water column during the day but
the pond was fully mixed every night (Fig. 7). Although the RTRM did not exceed 50
in March, day-time surface temperatures were regularly a few degrees higher than
bottom temperatures (Figs 8 & 9) and calculations showed that the mixed depth did
not remain at 100 % of the water column throughout the day, but dropped to 87 % of
the water column around midday. The temperature differences between surface and
bottom waters exceeded 15 °C on some warm days in late May (Fig. 8). Surface
waters were consistently colder than bottom waters at night. Isopleth plots of
temperature clearly showed daily stratification and nocturnal cooling (Fig. 9), weak
or no stratification in late March, stratification on most days in late April and strong
stratification on most days in late May. Regardless of the strength of stratification the
water column was fully mixed every night.
Discussion
Daily stratification
Daytime stratification and nocturnal mixing were recurring phenomena in this small
and densely vegetated pond on the open alvar. The mixed depth during daytime was
below 100 % of the water column for all three months, but at 100 % every night and
there was a distinct temperature decline from surface to bottom waters during the day
with a reversal during the night. From mid-April to the end of May the cycle was
profound; relative thermal resistance to mixing (RTRM) was above the threshold
value of 50 during most of these days and the daytime temperature gradients rose to
over 15 °C in less than half a meter of water. The strength of the diel stratification
typically peaked around midday despite the wind speed also peaking then, clearly
demonstrating that wind-induced mixing was not sufficient to fully mix the pond.
This consistent pattern developed despite the shallow water and the wind-exposed
location. Calculations of reduced mixed depth during the day throughout the period,
42
even on days when RTRM was below 50, suggest that the suggested threshold value
for stratification at RTRM > 50 is too large, at least in shallow systems.
Gorham and Boyce (1989) derived an empirical equation for determining which lakes
will typically stratify based on their length (L), approximated as the square root of the
surface area, and their depth (H): H = 0.34L0.5; a lake would not be expected to
stratify unless its actual depth was greater than H predicted by this equation.
Although both surface area and depth varied in the pond in this study, by this
estimate the pond would always be significantly shallower than the expected depth, at
least 1.7 m, necessary for stratification. Indeed, the implication of this empirical fit is
that any pond less than a meter deep would have to have a smaller surface area than
75 m2 in order to be likely to stratify. While this equation is only an approximation,
nevertheless it is indicative that, a priori, ponds of the size studied here might not be
expected to stratify, particularly if situated in an exposed location. This is reiterated
by Branco and Torgersen (2009) who also found that it was unlikely for a small
wind-exposed pond to stratify. Gorham and Boyce (1989) and Branco and Torgersen
(2009) worked with lakes with no macrophyte cover, and indeed in the pelagic of
larger lakes macrophytes are of little or no influence. However, in the littoral zone or
in small ponds macrophytes can enhance light attenuation, increase surface warming
and reduce vertical mixing to a large extent. It has been well documented that an
increase in light attenuation, by affecting the depths at which incoming solar radiation
is absorbed, can substantially reduce mixed depths and increase stratification (e.g.
Kling 1988, Persson & Jones 2008, Gaiser et al. 2009). Lake studies have
documented that submerged macrophyte beds can increase temperature stratification
by strong light absorption (Dale & Gillespie 1977). In the present study high density
of charophytes through most of the water column was, no doubt, a key to the
development of strong density gradients and restriction of the mixed layer to the
uppermost part of the water column during the day.
43
Incident irradiance only penetrated a few centimeters into the charophyte
canopy. The depth at which 10 % of surface irradiance is left then makes a proper
estimation of the depth of the upper canopy. During May, when continuous irradiance
data were available, the thermocline depth was just 0-9 cm (average 5 cm) into the
charophyte canopy (Fig. 3). Dense charophyte vegetation resulted in a very uneven
distribution of incoming short-wave energy throughout the water column as almost
all the energy was absorbed in the upper 3-cm of the vegetation. These findings
accord with model predictions and empirical comparisons of temperature dynamics
between macrophyte beds and open water locations by Herb and Stefan (2005a, b).
Falling water depth throughout May brought the upper part of the charophyte canopy
and the main attenuation of short-wave energy closer to the surface and, thereby,
further inhibited wind-induced mixing and reduced the mixed layer depth. As the
pond stratified all of the incoming heat would be confined to the mixed layer and
additionally stabilize the water column.
Strong reduction of local flow velocities within dense macrophyte stands
and restriction of strong turbulence to the upper few cm of the canopy have been
documented in shallow streams (Sand-Jensen & Mebus 1996, Sand‐ Jensen &
Pedersen 1999). Effective dampening of local flow and turbulence by macrophyte
beds can also account for their ability to reduce mixing and thus stimulate particle
sedimentation and reduce resuspension (Barko & James 1998, Sand‐ Jensen 1998,
Vermaat et al. 2000). It is therefore likely that the macrophytes influenced the mixing
processes both through light attenuation and by inhibition of mechanical mixing.
Nocturnal mixing
The regular pattern of daytime stratification and nighttime mixing was driven by the
pronounced diel cycle in the heat fluxes and wind mixing. Solar and atmospheric
long-wave radiation heated the pond during the day, but the shallowness of the mixed
layer enabled surface temperatures to climb rapidly, promoting significant cooling
through outward long-wave and turbulent heat loss. Although winds calmed
44
markedly during the night, the temperature differential between water and air was
still sufficiently high to allow significant cooling to take place each night. The heat
loss cooled surface waters down to 1-5 °C below bottom waters temperatures and
produced an unstable water density profile resulting in penetrative convective mixing
(Imberger 1985) to the sediment surface. Whilst the wind energy flux would also
have some impact, it was an order of magnitude lower at night than during the day
(Fig. 4g), emphasizing the importance of the nocturnal cooling. The dominance of the
nocturnal cooling was in line with Read et al. (2012) who showed that for small lakes
penetrative cooling frequently generates more surface turbulence than wind shear.
High densities of submerged macrophytes have relatively little influence on mixing
processes by natural convection caused by surface cooling (Herb & Stefan 2005b).
Natural convection carries potential energy down from the surface via plunging
thermals and although submerged macrophyte surfaces may reduce the kinetic energy
generated by the plunging downward flow, the mixing depth is not greatly influenced
(Herb & Stefan 2005b). The sinking plumes imply the existence of coherent rising
plumes because of the continuity of mass, and the surface layer is subject to intense
stirring (Imboden & Wüest 1995). According to Deardorff et al. (1969) and Wüest
(1987), sinking water parcels still have part of their kinetic energy left (e.g. 30%)
when they reach the bottom of the mixed layer and can, therefore, penetrate the
density gradient and push heavier water from below into the mixed layer leading to
its deepening. This situation is very different from the influence of macrophytes on
turbulence induced by wind shear on the water surface which must penetrate the
canopy from above via undirected isotropic eddies which are effectively dissipated by
contact with macrophyte surfaces (Sand‐ Jensen & Pedersen 1999, Herb & Stefan
2005b). Because the energy of sinking plumes is more directed than that of windinduced mixing, plumes generated by surface cooling generally have greater mixing
efficiency (Imboden & Wüest 1995).
45
A further impact of the temperature increases associated with heating being
concentrated in a shallow surface layer was for there to be a strongly unstable
atmosphere above the pond. This resulted in transfer coefficients being higher than
often reported (see Verburg & Antenucci 2010), which, in turn, contributed to
increased wind mixing and turbulent heat loss. The transfer coefficients, CD10 and
CE10, were also around 25 % higher during the night than the day (Fig.5), owing to
the low nocturnal wind speeds, unstable atmospheric conditions and smooth flow
conditions (see Verburg & Antenucci, 2010). This nighttime increase in transfer
coefficients was a further factor driving the nocturnal mixing processes.
Implications of stratification-mixing patterns
The dynamics of vertical mixing is crucial because of its overriding impact on
physical, chemical and biological conditions (Branco et al. 2005). The recurring
daytime stratification and nighttime mixing is not unique for the shallow open ponds
with dense charophyte vegetation studied here. It can also develop in the littoral zone
with submerged vegetation of large lakes (Herb & Stefan 2005a, Herb & Stefan
2005b, Coates & Folkard 2009) as well as in small, wind-protected lakes devoid of
submerged macrophytes where dense growth of phytoplankton or humic water lead to
strong vertical light attenuation and surface warming (Gu et al. 1996, Ford et al.
2002, Song et al. 2013).
Temperature itself is a key variable for the distribution and metabolic
activity of all organisms. In our study pond, sessile organisms and macrophytes were
exposed to diel temperature amplitudes of 1.9°C-18.9°C in the surface waters and
0.5°C-6.1°C in the bottom waters between late March and the end of May. This
highly variable environment is both a challenge to survival and a trigger of highly
variable metabolic rates with time. Assuming that only the temperature range of 17.0
°C influences metabolism, then for typical Q10-values of 1.5-3 metabolic rates can be
expected to change between 2 and 8-fold.
46
The diurnal stratification and mixing of the shallow lakes and ponds are
accompanied by profound vertical dynamics of pH, oxygen, nutrients and redox
potentials (Gu et al. 1996, Ford et al. 2002, Branco et al. 2005, Song et al. 2013). In
our study pond, for example, daytime stratification is accompanied by anoxia and
accumulation of CO2, sulphide and reduced ferrous-Fe in the lower 10 cm of the
water column, while oxygen reappears and sulphide and ferrous-Fe disappear in the
bottom waters during nocturnal mixing (Andersen et al. 2015). Macrophytes and the
sessile fauna in the sediment and in the lower part of the water column do not only
have to withstand profound diel temperature excursions but also alternating oxicanoxic and oxidized-reduced conditions.
Small water bodies are far more common world-wide than the iconic, charismatic
lakes often studied (Downing et al. 2006). Their temperature, mixing, and
stratification dynamics, which drive the lake ecology, are therefore of great interest; a
huge number of them will have complex thermal patterns resulting in very dynamic
stratification and mixing mechanisms as documented here.
A priori, it is not obvious that a pond, such as the one studied here, should stratify at
all. Almost certainly this regular diel stratification is promoted by the macrophyte
presence both influencing the vertical absorption of solar radiation and the
mechanical mixing within the pond. The macrophytes are thereby profoundly
influencing their own environment.
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Figure legends.
Figure 1.
Surface irradiance (a), air temperature (b), relative humidity (c) and wind speed (d)
measured next to the pond at 2.0 m above ground level during the investigation.
49
Figure 2.
Diel averages of wind speed (a), surface irradiance (b), air temperature (c), relative
humidity (d) and surface water temperature (e) for March (blue line), April (green
line) and May (red line).
Figure 3.
Daily mean light attenuation coefficients in the pond from immediately below the
surface to 8 cm above the sediment surface (dotted line), daily mean depth to which
10 % of subsurface light remains (white surface) and daily mean focal depth of the
thermocline (full line). The sediment surface is shown in black.
Figure 4.
Diel averages of incoming short-wave radiation (a), reflected short-wave radiation
(b), net long-wave radiation (c), latent heat flux (d), sensible heat flux (e),total heat
flux (f) and wind energy flux (g) for March (blue line), April (green line) and May
(red line).
Figure 5.
Diel averages of transfer coefficients (dimensionless) for March (blue line), April
(green line) and May (red line). CD10 (a) and CE10 (b).
Figure 6.
Relative thermal resistance to mixing (RTRM) calculated during the investigation.
Threshold value for onset of stratification (RTRM = 50) and no relative thermal
resistance to mixing (RTRM = 0) shown as dotted lines.
Figure 7.
Diel averages of relative thermal resistance to mixing (RTRM) (a) and mixed depth
(percentage of the water column) (b) for March (blue line), April (green line) and
May (red line).
Figure 8
Surface water temperature (dashed line) and bottom water temperature (bold line) in
the pond during the investigation.
50
Figure 9
Daily time course of water temperature with depth in the pond during 5 days in late
March, April and May based on measurements at 5-cm depth intervals every 10
minutes. The white area (25th May) marks a period when sensors were retrieved and
redeployed.
Supplementary figure legends
Figure S1.
Temporal changes in surface area (dashed line) and water volume of the pond (full
line; upper panel) and maximum water depth (dotted line) and precipitation (columns;
lower panel).
Figure S2.
Heat fluxes calculated during the investigation. Incoming short-wave radiation (a),
reflected short-wave radiation (b), net long-wave radiation (c), latent heat flux (d),
sensible heat flux (e) and total heat flux (f).
Figure S3.
Transfer coefficients (dimensionless) calculated during the investigation. CD10 (top
panel) and CE10 (bottom panel).
51
Wind speed (m s -1)
Relative humidity (%)
Air temperature ( C)
PAR (µmol m-2 s-1)
Figures
Figure 1
3000
25
80
a
2000
1000
0
b
20
15
10
5
-5
0
60
c
40
20
0
d
6
4
2
0
March
April
May
52
Figure 2
2
1
0
00
03
06
09
12
15
18
21
1000
500
0
00
24
20
b
Air temp. (C)
1500
a
PAR (µmol m-2 s-1)
Wind speed (m s -1)
3
03
06
25
d
Surf. water temp. (°C)
Relative humidity (%)
100
90
80
70
60
50
00
03
06
09
12
15
Time of Day
09
12
15
18
21
24
Time of Day
Time of Day
18
21
24
c
15
10
5
0
00
03
06
09
12
15
18
21
24
Time of Day
e
20
15
10
5
0
00
03
06
09
12
15
18
21
24
Time of Day
Figure 3
53
Figure 4
600
400
0
b
Qlnet (W m -2)
60
a
Qsr (W m -2)
Qsin (W m -2)
800
40
20
200
0
00
03
06
09
12
15
18
21
0
00
24
03
06
Time of Day
15
18
21
-150
00
24
100
06
09
12
15
06
18
21
24
12
15
18
21
24
18
21
24
f
30
20
0
00
09
600
10
03
03
Time of Day
Qtot (W m -2)
150
0
00
-100
e
Qh (W m -2)
Qe (W m -2)
d
50
03
06
09
12
15
18
21
400
200
0
-200
00
24
03
Time of Day
Time of Day
Wind mixing (W m -2)
12
-50
Time of Day
40
200
0.25
09
c
06
09
12
15
Time of Day
g
0.20
0.15
0.10
0.05
0.00
00
03
06
09
12
15
18
21
24
Time of Day
Figure 5
0.004
0.004
a
0.003
CE10
CD10
0.003
0.002
0.001
0.000
00
b
0.002
0.001
03
06
09
12
15
Time of Day
18
21
24
0.000
00
03
06
09
12
15
18
21
24
Time of Day
54
Figure 6
500
RTRM
400
300
200
100
50
0
-100
March
April
May
Figure 7
250
100
a
Mixed depth (%)
200
RTRM
150
100
50
0
-50
00
03
06
09
12
15
Time of Day
18
21
24
b
50
0
00
03
06
09
12
15
18
21
24
Time of Day
Water temperature (C)
Figure 8
35
30
25
20
15
10
5
0
March
April
May
55
Figure 9
26-30 March
22-26 April
23-27 May
56
Supplementary figures
500
1100
400
1000
900
300
800
200
700
100
600
0
500
Max depth (m)
16
0.5
0.4
12
0.3
8
0.2
4
0.1
0.0
March
April
May
Precipitation (mm day -1)
0.6
Surface area (m2)
Volume (m3)
Figure S1
0
57
Figure S2
1500
Qsin (W m -2)
a
1000
500
0
Qsr (W m -2)
100
b
50
0
Qlnet (W m -2)
0
c
-50
-100
-150
-200
Qe (W m -2)
400
d
300
200
100
0
Qh (W m -2)
80
e
60
40
20
0
-10
Qtot (W m -2)
1200
f
800
400
0
-400
March
April
May
58
Figure S3
0.004
CD10
0.003
0.002
0.001
0.000
CE10
0.004
0.003
0.002
0.001
0.000
March
April
May
59
Paper 2 - Recurring stratification and mixing generate
extreme diurnal oxygen and carbon cycles in shallow
vegetated lakes
60
Recurring stratification and mixing generate extreme diurnal oxygen and
carbon cycles in shallow vegetated lakes
Mikkel Rene Andersen, Theis Kragh and Kaj Sand-Jensen
Freshwater Biological Laboratory, Biological Institute, University of Copenhagen,
Universitetsparken 4, DK-2100 Copenhagen Denmark
Target Journal: Nature Communications
Environmental conditions in small lakes (< 1 ha) have been grossly understudied
although they globally exist in millions and are several-fold more abundant than
larger lakes1,2. Vertical stratification-mixing patterns are main determinants of
biogeochemistry and metabolism of organisms3, but small, shallow lakes have
usually been assumed to be homogeneously mixed, though with little
justification4. Here we show that a small, wind-exposed, shallow (ca 0.4 m) lake
with submerged macrophytes underwent recurring daytime stratification and
nocturnal mixing during summer accompanied by extreme variations in
temperature, oxygen, pH and inorganic carbon with time and depth. During
daytime stratification, surface waters attained 230 % oxygen saturation and strong
CO2 depletion (<10 % air saturation), while 6-14 oC colder bottom waters
developed anoxia and accumulated reduced iron, sulphide and >1500 % CO2
saturation. High daytime pH in surface waters induced carbonate precipitation
releasing CO2 for ongoing photosynthesis without further pH rise, while most
precipitated CaCO3 was re-dissolved in bottom waters. Vertical gradients
disappeared during nocturnal mixing injecting oxygen into bottom waters for
aerobic respiration and regenerated inorganic carbon into surface waters for
61
photosynthesis. These processes add new dimensions to our understanding of the
regulation of ecosystem photosynthesis and respiration and the adaptation of
sessile plants and mobile animals to the extreme variability of environmental
stressors.
Turbulent mixing of the water column in shallow lakes is expected to prevent
formation of vertical gradients of temperature, oxygen and solutes. Hydraulic models
predict that small, wind-exposed temperate lakes (1000 m2) should be deeper than 3
m to become vertically stratified5. While no vertical gradients are expected to form,
intense daytime photosynthesis and nocturnal respiration may generate profound
diurnal cycles of temperature and solutes6,7. It was, therefore, unexpected that a 0.4
m-deep lake underwent strong temperature stratification during daytime and
nocturnal mixing. Here we show that recurring stratification and mixing, because of
the presence of submerged macrophytes, generate extreme vertical and diurnal cycles
of pH, inorganic carbon and oxygen - including anoxia and reducing conditions in
bottom waters during daytime and oxic conditions at night. This overlooked
dynamics should be common in small vegetated lakes and within macrophyte beds in
large lakes and adds new dimensions to our understanding of biogeochemical cycles,
macrophyte adaptation and animal behavior.
We recently discovered an unexpected behavior of the great pond snail
(Lymnaea stagnalis) in shallow lakes dominated by characean macroalgae during
summer in open south-Swedish habitats8. The snails concentrated in surface waters
before noon to ventilate the vascularized lung with atmospheric air as if they rapidly
needed to recover from a threatening oxygen debt. A similar behavior has been
described for dragonfly larvae actively ventilating the gills in the rectum with
62
atmospheric air9. Oxygen concentrations were high in the surface waters before noon8
and it was unclear where the snails had developed the suggested oxygen debt that led
to their awkward behavior and risky exposure to predators. The environmental
conditions should be highly variable considering the high macrophyte density in the
shallow water. To document the conditions, we measured light, temperature, oxygen,
pH and conductivity at high vertical resolution in one of the shallow lakes during
weeks in May and August along with meteorological parameters. During a diurnal
cycle, we also determined the vertical distribution of dissolved inorganic nutrients,
sulphide, ferrous iron, calcium, total (DIC) and individual carbon species (i.e., DIC =
CO2 + HCO3- + CO32-) closely linked to photosynthesis and respiration. Acid
neutralizing capacity (ANC = HCO3- + 2 CO32- + OH- - H+) was measured directly
during the diurnal cycle and was calculated from conductivity during longer periods
because HCO3- is the main determinant of ANC and the dominant anion is closely
linearly related to conductivity (Suppl. Fig. S1).
The shallow lake developed strong daytime vertical stratification of
temperature and specific density on four days during the week in May (Fig. 1) and all
examined days in August (Fig. 2). No vertical stratification formed on two cold
windy days in May (Fig. 1). At night, the water column was always fully mixed.
Maximum daytime differences between temperatures in surface and bottom waters on
days with vertical stratification were 11.2-14.4oC in May and 6.2-8.9oC in August
(Suppl. table S1). The steepest vertical temperature gradient (0.3-1.9oC cm-1) formed
at 14.5-18.5-cm depth in May and 15.9-22.5-cm in August. This exceptionally strong
daytime vertical stratification can be explained by the dense charophyte vegetation
(5685-11340 g fresh weight m-3) reaching up to from the bottom to a few cm below
the water surface and effectively attenuating both the radiative heat flux and the
turbulent wind-driven mixing with depth. Thus, 90% of the daytime radiative heat
flux was absorbed within the upper 11-cm of water column in May (Suppl. table. S2).
The lake water was transparent and the vegetation was responsible for most (98%) of
63
the vertical light attenuation. Water turbulence generated by wind shear on the lake
surface must penetrate the macrophyte canopy from above via undirected isotropic
eddies which are effectively dissipated by contact with macrophyte surfaces10,11,
thereby, preventing full mixing. On two cold and windy days in May turbulence was
sufficiently strong to disrupt the formation of vertical temperature stratification. At
night, cooling of surface waters to 0.1-4.1 oC below bottom temperatures produced
inverse unstable water density profiles resulting in penetrative convective mixing of
the entire shallow water column. Submerged macrophytes, even of high density, have
relative little influence on convective mixing caused by surface cooling12. Natural
convection carries potential energy down from the surface via directed plunging
thermals and although submerged macrophyte surfaces may reduce the kinetic energy
generated by the plunging downward flow, the mixing depth is not greatly
influenced10,11. The sinking plumes induce coherent rising plumes because of the
continuity of mass resulting in intense mixing13. Because the daytime heat flux is
absorbed within a very thin surface layer it becomes markedly warmer than the air
(Suppl. Fig. S2) and is cooled that more strongly during the evening and night
inducing effective convective mixing.
Alternating daytime stratification and nocturnal mixing and steep vertical
attenuation of photosynthetic irradiance can account for the astonishing vertical and
temporal dynamics of oxygen, pH, ANC and inorganic carbon species (Figs 1 and 2).
Oxygen and pH in surface waters (2, 9 and 16 cm) followed the same diurnal course
on six days in May according to the balance between photosynthesis and respiration.
Maximum values (205-235 % O2 saturation, pH 9.1-9.6) were recorded in the early
afternoon and minimum values (12-25 % O2 saturation, pH 7.1-8.0) shortly after
sunrise. The lowest oxygen concentrations at sunrise were observed after warm
nights because nocturnal respiration increased with temperature14. During vertical
mixing, oxygen and pH at 23 cm below the water surface followed the same pattern
as at the surface. During stratification, in contrast, oxygen dropped to zero and pH to
64
7.0-7.5 at 23 cm water depth because of respiratory oxygen consumption and CO2
release. Intermittent vertical stratification (at noon, May 28) led to immediate decline
of oxygen and pH showing that respiration was much higher than photosynthesis at
this depth.
Continued photosynthesis and rising pH in the surface waters from
morning to afternoon were accompanied by falling DIC, ANC and free CO2 and
increasing CO32- as a result of coupled photosynthesis and calcification (i.e., Ca2+ + 2
HCO3-→ CaCO3 (precipitated) + CO2 (assimilated)15. The decline of ANC in surface
waters could either be due to CaCO3 precipitating directly on charophyte surfaces
coupled to photosynthesis and/or to formation and sinking of minute calcite crystals
in the water because of high pH and CO32- (i.e. HCO3- + OH-→ CO32-)16. The calcite
saturation index, representing the ionic molar product of Ca2+ and CO32- relative to
the solubility product at ambient temperature3,16, was high in the afternoon on all days
(i.e. >10) and should promote calcite formation (Fig. 2). Daytime loss of ANC in
surface waters was apparently due to precipitation and sinking of calcite crystals
which were re-dissolved in bottom waters because of high CO2 concentrations
resulting in accumulation of DIC and ANC until vertical mixing in the evening and at
night. This interpretation was supported by measurements of diurnal cycles and
vertical chemical profiles (Fig. 3).
Diurnal cycles resembled each other very closely during all examined
days in August (Fig. 2). Following minimum oxygen concentrations at all depths
shortly after sunrise, oxygen concentration rapidly increased in surface waters as
temperature stratification was established, while oxygen simultaneously declined in
bottom waters turning fully anoxic around midday and staying anoxic for the next 12
hours until midnight before effective convective mixing generated homogeneous
physico-chemical conditions again. Surface water pH was also lowest (about 8)
shortly after sunrise and highest (9.4-9.6) in the afternoon after many hours of
photosynthesis and was accompanied by DIC and ANC decline, drop of CO2 to
65
almost zero and increase of CO32-. Daytime loss of ANC in surface waters caused by
CaCO3 precipitation averaged 0.57 meq L-1 on five consecutive days, but 0.51 meq L(90%) had returned to surface waters on the next day following vertical mixing
because most CaCO3 precipitated from surface waters during daytime photosynthesis
was solubilized in bottom waters with low pH and excess CO2.
This interpretation of carbon dynamics was confirmed by direct analyses
of the vertical distribution of ANC and Ca2+ during a full diurnal cycle (Fig. 3).
Daytime decline of ANC and Ca2+ in surface waters by calcification was
accompanied by increasing concentrations in bottom waters by carbonate dissolution.
The latter process continued throughout the night giving rise to increasing
concentrations of ANC, Ca2+ and DIC in the water column until photosynthesis
picked up after sunrise. These tightly coupled vertical and diurnal cycles of
precipitation and dissolution of carbonates have not been disclosed before. The
implications are very important because daily calcification delivers protons and
ensures conversion of HCO3- to free CO2 for continued photosynthesis preventing pH
to rise to strongly inhibiting levels (i.e., Ca2+ + 2 HCO3-→ CaCO3 + CO2)8,15, while
direct HCO3- use without calcification leads to release of OH- and continued pH rise
(HCO3- → CO2 + OH-)17. The daytime net decline of DIC in surface waters averaged
0.77 mM, and 0.48 mM of this amount represented direct uptake and 0.29 mM
represented CO2 use coupled with CaCO3 precipitation of the same magnitude. Most
of the daytime decline of the DIC pool (91%) was restored by respiration and
carbonate dissolution before the next morning. Sediment incubations confirmed this
pattern showing the release of two moles of DIC for every mole of O2 consumed in
the process, because of concomitant respiration and CaCO3 dissolution (org. C + O2 +
CaCO3 → Ca2+ + H2O + 2HCO3-, Sand-Jensen, Petersen, Kragh & Andersen pers.
comm.).
The contrast to deeper lakes is stunning. In deep lakes temperature
stratification is permanent during several summer months with surface waters
66
gradually being depleted and bottom waters gradually enriched in DIC, ANC and
Ca2+ 16,18. The injection of these soluble substances are delayed until autumn overturn
after light has become limiting to photosynthesis thereby preventing the stimulation
of photosynthesis that takes place daily by nocturnal mixing in the shallow lake.
Anoxic conditions in bottom waters after midday on days of vertical
temperature stratification had further consequences (Fig. 3). During anoxia, sulphide
and ferrous iron accumulated because of reduction of sulphate and ferric iron served
as electron acceptors during anaerobic respiration19. Sulphide accumulation in
bottoms waters was small, while accumulation of ferrous iron was larger and
continued throughout the stratification period showing that ferric iron was a more
important electron acceptor than sulphate during anoxia. Sulphide and ferrous iron
were re-oxidized following vertical mixing when oxygen was reintroduced to the
bottom waters. Ammonium accumulated to a small extent in the bottom waters
during temperature stratification, while concentrations of nitrate and phosphate
remained close to the limits of detection (< 0.3 µM, data not shown) stressing the
oligotrophic nature of the calcareous lake.
This study adds new dimensions to the understanding of environmental
conditions and biogeochemistry in shallow vegetated lakes, in that recurring vertical
stratification during daytime and nocturnal mixing generated unexpected and
profound diurnal cycles of oxygen, pH and inorganic carbon species. Shallow lakes
in open habitats have hitherto been considered to be permanently mixed and if
profound oxygen depletion developed it was confined to heavily organically polluted
lakes and to nocturnal periods during degradation of algal blooms in hypereutrophic
lakes2,3. Our study showed that temperature stratification and mixing were recurring
diurnal phenomena during summer in a shallow vegetated lake because dense
charophyte stands strongly attenuated depth penetration of light and wind-driven
turbulence during the day, while surface cooling at night induced penetrative
convective mixing. Daytime decoupling between photosynthesis at high irradiance in
67
the upper canopy and respiration in darkness in the lower canopy generated oxygen
accumulation and DIC depletion in surface waters and anoxia and DIC accumulation
in bottom waters, while nocturnal mixing injected regenerated DIC for
photosynthesis in surface waters and oxygen for oxygenic respiration in bottom
waters. If regenerated DIC had been trapped in stagnant bottom water, charophyte
photosynthesis would become severely constrained. Despite the oligotrophic
environment and low growth rates typical of charophytes20, profound diurnal cycles
of oxygen, pH and DIC, nonetheless, developed in the lake because of gradual
development of a substantial charophyte-biomass attaining high metabolic rates of
the community8.
Concerning adaptations to the extreme environmental variability, basal
parts of charophyte tissues exposed to bottom waters would have to withstand up to
12 hours of anoxia and accumulation of potentially toxic sulphide and ferrous iron
every day, while apical tissues experienced wider diurnal amplitudes in temperature
and oxygen, though no anoxia. Charophytes, in contrast to flowering plants, lack air
lacunae for longitudinal oxygen transport from apical to basal tissues located in
anoxic water and rhizoids in anoxic sediments. How they cope with this metabolic
challenge is not known, though anoxic fermentation is most likely. Preliminary
experiments show that basal parts are indeed alive and have retained their
photosynthetic and respiratory activity (data not shown). Mobile animals can move
over short distances in the shallow lake to escape from the worst environmental
stress21. For example, vertical upward movement from 23 to 16 cm below the surface
in the lake during daytime stratification in May would be sufficient to escape anoxia.
In order to unravel whether the large facultative air-breathing Lymnaea snails had
suffered from earlier exposure to anoxia and toxic reduced ions in the bottom waters
or low oxygen concentrations in surface waters at sunrise, we would need to track
their position over time. This requires new technology not yet available.
68
More generally, our findings of extreme vertical and diurnal variability of
temperature, oxygen, pH and solutes should be widespread in shallow vegetated lakes
that are found in millions throughout the world. Small lentic water bodies (< 1 ha)
have been grossly understudied, although they are 100-fold more abundant than
larger (> 10 ha) intensely studied lakes1,2. The preference for studying large lakes
have given us a wrong perception of environmental conditions, species adaptations
and ecosystem processes for the natural range of lentic water bodies22,23. Our findings
suggest that the evolutionary processes and the geographical dispersal of organisms
in freshwaters could be particularly important in small lakes because they are highly
abundant and variable24 and the environmental conditions can be extremely
challenging and drive adaptation to high and globally rising temperatures and oxygen
stress25.
Methods summary
The study was conducted in one of several small, shallow lakes located in the
sparsely vegetated calcareous grassland at Greby on Öland, SE Sweden (56.81168°N,
16.6094°E)8. The examined lake varied in surface area from 630-745 m2 in late-May
to 825-859 m2 in mid-August and maximum depth from 0.30 to 0.46 m. According to
methods described previously26, meteorological parameters (incident light, wind
speed and temperature) were measured next to the lake at 2.0 m above the ground and
vertical profiles of light and temperature were measured at the deepest site at 5 cm
depth intervals at 10 minutes intervals. Depth profiles of dissolved oxygen and pH
were measured at 7cm depth intervals at 1 minute time intervals logging the mean
signal every 10 minutes. Conductivity was also measured at 10-minutes intervals and
corrected to 20 °C.
During a diurnal cycle, water samples were collected at 7 cm depth
intervals and analyzed for DIC27, ANC, calcium, ferrous iron, sulphide, orthophosphate, nitrate and ammonium by standard methods28. Weekly surface samples for
69
measurements of ANC (meq. L-1), pH and specific conductivity (Cond, µ Siemens
cm-1) were used to construct closely linear relationships of conductivity to total ANC
(= HCO3- + 2 CO32- + OH- + H+): total ANC = 0.009399 * Cond - 0.1410 (r2 = 0.72).
This relationship enabled continuous estimates of DIC and proportions of individual
carbon species from measurements of temperature, pH and specific conductivity29.
70
Water depth (m)
0
30 C
25 C
0.1
20 C
15 C
0.2
10 C
600
500
O2 (M)
35 C
a
5 C
b
400
300
200
100
10
0.02 m
0.09 m
0.16 m
0.23 m
c
pH
9
8
7
2.5
d
0.02 m
DIC (mM)
0.6
1.5
0.5
DIC
0.4
1.0
0.3
CO2
0.5
2.5
0.2
CO32-
e
0.1
0.23 m
DIC
0.8
0.7
0.6
0.5
1.5
0.4
1.0
CO2
0.3
0.2
0.5
0.1
CO320.0
0
12
25 May
0
12
26 May
0
12
27 May
0
CO32- / CO 2 (mM)
2.0
DIC (mM)
0.7
CO32- / CO 2 (mM)
2.0
0.8
12
28 May
0
12
29 May
0
0.0
28
71
Fig. 1.Time series of temperature, O2, pH, ANC, DIC and individual carbon
species with depth in a shallow charophyte lake during six days in May.
a, Temperature isopleths calculated from measurements at 5-cm depth intervals.
b, c, Oxygen and pH measured at 0.02 m (dark blue),0.09 m (light blue), 0.16 m
(green) and 0.23 m (red) below the water surface.
d, DIC (red), CO32- (orange) and CO2 (blue) in surface waters (0.02 m).
e, DIC (red), CO32- (orange) and CO2 (blue) in deeper waters (0.23 m).
Where b-e background color show day/night cycle (white = day, grey = night).
72
Water depth (m)
0
35 C
a
30 C
0.1
25 C
0.2
20 C
15 C
0.3
10 C
0.4
500
5 C
b
O2 (M)
400
300
0.08 m
0.24 m
0.34 m
200
100
pH
14
c
12
9.0
10
ANC (meq L -1)
8.0
2.0
6
1.5
4
1.0
2
0.5
0.4
d
DIC
1.0
0.3
0.2
CO32-
0.5
0.1
CO2
0.0
0
12
0
12
0
12
0
12
0
12
0
12 August
13 August
14 August
15 August
16 August
CO32- / CO 2 (mM)
1.5
DIC (mM)
8
Calcite saturation index (green)
10.0
0.0
Fig. 2. Time series of temperature, O2, pH, ANC, calcite saturation index, DIC,
individual carbon species with depth in a shallow charophyte-lake during six
days in August.
73
a, Temperature isopleths calculated from measurements at 5-cm depth intervals.
b, Oxygen measured at 0.08 m (dark blue), 0.24 m (green) and 0.34 m (red) below
the water surface.
c, pH (blue) and ANC (green) in surface waters (0.08 m).
d, DIC (red), CO32- (orange) and CO2 (blue) in surface waters (0.08 m).
Where b-d background color show day/night cycle (white = day, grey = night).
Water depth (m)
0.0
06:00
11:00
16:00
22:00
0.1
0.2
0.3
1
2
3
0
0.5
DIC (mM)
1
1.5
2
0
1
Ca2+ (mM)
2
3
4
ANC (meq L -1)
Water depth (m)
0.0
0.1
0.2
0.3
0
10
20
30
Fe2+ (M)
40
0
0.1
0.2
0.3
Sulphide ( M)
0.4
0
5
10
15
20
25
30
NH4+ (M)
Fig. 3. Depth profiles of DIC, ANC, Ca2+, Fe2+, ∑H2S and NH4+ in a shallow
charophyte lake during a diurnal cycle. Measurements on May 26 at 6.00 (red),
11.00 (blue), 16.00 (green) and 22.00 o’clock (orange). The water column was
vertically mixed at 6.00 o’clock and stratified below 0.20 m at 16.00 and below 0.25
m at 22.00 o’clock.
74
Supplementary
ANC (meq L-1)
4
3
2
1
0
0
100
200
300
400
Sp. conductivity (S cm-1)
Fig S1. ANC in lake water as a function of specific conductivity. ANC =
0.009399*Sp.cond.-0.1410. R2=0.72. The line is forced through coordinates (15,0).
The specific conductivity of rainwater in the region is 15 μS cm-1.
Temperature (C)
35
30
25
20
15
10
5
0 12 0 12 0 12 0 12 0 12 0 0 12 0 12 0 12 0 12 0 12 0
May
25
26
27
August
28
29
12
13
14
15
16
Fig S2. Surface water temperature (full line) and air temperature (dashed line)
for 5 days in May and 5 days in August.
75
Table S1. Diel minimum and maximum differences between surface and bottom
water temperature in May and August. Negative values denote surface water
colder than bottom water.
Date
Minimum Maximum
25-05-2014
-0.10
12.86
26-05-2014
-1.43
14.36
27-05-2014
-3.98
0.57
28-05-2014
-4.11
3.93
29-05-2014
-3.63
11.18
12-08-2013
-1.62
6.22
13-08-2013
-1.52
8.63
14-08-2013
-1.24
6.20
15-08-2013
-1.43
8.91
16-08-2013
-0.96
7.94
Table S2. Mean daily vertical attenuation coefficients and depths at which 10 %
of surface light remains.
1
2
3
Light attenuation
Depth at which 10 % of
Date
coefficient (m-1)
surface light remains (m)
25-05-2014
22.7
0.10
26-05-2014
17.5
0.13
27-05-2014
21.6
0.11
28-05-2014
25.3
0.09
29-05-2014
22.7
0.10
Downing, J. et al. The global abundance and size distribution of lakes, ponds, and impoundments.
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Sand-Jensen, K. Lakes - A Protected Nature Type (Danish). (Gad Publishing, 2001).
Kalff, J. Limnology: inland water ecosystems. (Prentice Hall New Jersey, 2002).
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5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
Branco, B. F. & Torgersen, T. Predicting the onset of thermal stratification in shallow inland
waterbodies. Aquatic Sciences-Research Across Boundaries 71, 65-79 (2009).
Gorham, E. & Boyce, F. M. Influence of lake surface area and depth upon thermal stratification and
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Paper 3 - Distinct diurnal patterns of ecosystem
metabolism in a small charophyte-lake
78
Distinct diurnal patterns of ecosystem metabolism in a small charophyte-lake
Theis Kragh, Mikkel René Andersen and Kaj Sand-Jensen, Freshwater Biological
Section, Biological Institute, University of Copenhagen, Universitetsparken 4, 2100
Copenhagen
Target journal: Limnology and Oceanography
79
Abstract
To characterize the temporal and spatial variability of metabolism (gross primary
production (GPP), respiration (R) and net ecosystem production (NEP)) in a small,
shallow Swedish lake with dense charophyte stands, we collected data from many O2
sensors placed along a vertical mid-lake profile and across the lake surface in late
May and early June. Similar diurnal patterns derived from single surface sensors and
multiple sensors showed maximum NEP-rates between 8 and 11 am and strong
afternoon depression with rates close to zero accompanying profound rise of O2, pH
and temperature and depletion of inorganic carbon and CO2 from morning to late
afternoon. Inorganic carbon limitation of photosynthesis and temperature
enhancement of respiration could account for profound afternoon depression of NEP.
Nocturnal respiration declined from sunset to sunrise due to falling temperature and
presumably depletion of respiratory substrates. Mean temperature-corrected
respiration rates at sunrise were 63% of that at sunset. The dense charophyte canopy
accounted for 90% of ecosystem respiration and the entire primary production. Mean
daily estimates of GPP and R varied only 2-fold and small, negative NEP-rates
varied less between surface sensors at different locations across the lake. In
conclusion, multiple oxygen sensors representing the main depths and sections of the
lake can provide reliable and accurate measurements of diurnal course and daily
rates of metabolism in small lakes probably because a relatively uniform oxygen
signal is ensured by small distances and nocturnal mixing. During colder periods of
continuous mixing a single mid-station sensor should provide reliable metabolism
estimates.
Introduction
Ecosystem metabolism is essential to the understanding of carbon cycling and
functional properties of gross primary production (GPP), respiration (R) and net
ecosystem production (NEP; Odum 1957, Kelly et al. 1983, Staehr et al. 2012c).
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During recent years there has been renewed interest in ecosystem metabolism of
freshwaters because of technical improvements and the recognition that freshwaters
are hot spots for storage of terrestrially fixed carbon and CO2 emission to the
atmosphere (Cole et al. 2007, Sand-Jensen & Staehr 2012).
Most determinations of ecosystem characteristics and metabolism are from large to
medium-sized lakes (> 10 ha), while small lakes and ponds (< 1 ha) which are
particularly abundant in the landscape are grossly underrepresented in the studies
(Downing et al. 2006, Hanson et al. 2007, Staehr et al. 2012a). In Denmark, for
example, there are about 1000 lakes larger than 10 ha but more than 100,000 lakes
smaller than 1 ha, whose combined contribution to CO2 emission by far exceeds that
of the larger lakes (Sand-Jensen & Staehr 2007, Staehr et al. 2012a). Ecosystem
metabolism changes markedly from small to large lakes because of the reduced input
of water, organic carbon and nutrients relative to surface area and volume in
gradually larger and deeper lakes and the increase of incident irradiance, wind
exposure and turbulence (Sand-Jensen & Staehr 2007, Staehr et al. 2012a). Thus,
most small lakes exhibit strongly negative rates of NEP and high atmospheric CO2
evasion relative to surface area (Staehr et al. 2012a), though nutrient-poor lakes in
terrestrial landscapes with thin soils of low organic carbon export may show positive
NEP and release O2 to the atmosphere during summer (Christensen et al. 2013). This
situation calls for stronger future emphasis on rates and regulations of ecosystem
metabolism in small lakes.
Technological improvements of O2 sensors have made it easier to use free-water
measurements to obtain continuous estimates of ecosystem metabolism in many
freshwater localities and reach broad-scale overviews of their role in carbon balances
of the landscape. Estimates of whole-lake metabolism have usually been based on a
single O2 sensor located in surface waters at the deepest point of the lake with the
implicit assumption that it is representative of the pelagic waters, if not the
metabolism of the entire lake. However, spatial heterogeneity of estimates of
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ecosystem metabolism may exists both within and between pelagic and littoral
habitats depending on underlying differences in ecosystem structure and extent of
physical isolation of water masses (Vadeboncoeur et al. 2002, Vadeboncoeur et al.
2006, Van de Bogert et al. 2007, Langman et al. 2010, Staehr et al. 2012b, Van de
Bogert et al. 2012). Within-lake heterogeneity may be due to measurements of strong
O2 signals from “hot spots” of intensive daytime photosynthesis and nocturnal
respiration in dense periphyton or macrophyte communities alternating with weak O2
signals from “cold spots” of low metabolism on naked sediments and clear pelagic
waters (Van de Bogert et al. 2012). Detected O2 signals are, in addition to the
underlying metabolic activity, influenced by physical processes of transport, mixing
and atmospheric gas exchange causing sensors to measure on water parcels of
different metabolic and physical time history (Mackay et al. 2011, Van de Bogert et
al. 2012). Under conditions of low wind and water mixing, distinct differences in
recorded metabolism may exist between sensors located in littoral and pelagic
habitats (Van de Bogert et al. 2007, Sadro et al. 2011a). In contrast, homogenization
of water masses due to strong wind forcing and mixing and/or short horizontal and
vertical distances may generate a uniform integrated O2 signal in the water across the
lake despite variation of metabolism among sites and over time.
Here we focused on ecosystem metabolism of a small lake (ca. 1000 m2) and tested
the reproducibility of estimates of multiple sensors covering the horizontal and
vertical gradients in the lake. We measured directly gas exchange velocity as a
function of wind velocity and fetch allowing us to construct a model to account for
atmospheric gas exchange for every 10-minute estimate of metabolism among sites.
Because the small, shallow lake is covered by charophytes across the entire bottom
and undergoes recurring mixing during summer nights, we expect that the variability
of daily estimates of metabolism derived from O2 sensors located in surface waters
across the lake should be relatively small compared with the surprisingly high
variability published for larger lakes (Van de Bogert et al. 2012).
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Regulation of metabolic rates takes place at temporal scales of seconds, minutes and
hours, while ecosystem metabolism based on free-water measurements is usually
expressed on a daily basis due to lack of sensitivity at shorter time scales. Many
important metabolic regulations (i.e. afternoon depression of productivity by lower
CO2 and higher O2, and falling respiration during the night by declining temperature
and lower availability of respiratory substrates; Markager & Sand-Jensen 1989,
Alnoee et al. 2015) are missed at the integrated daily scale, but may turn up on
shorter time scales and may also reveal greater short time variability among sensors
with different spatial location. The temporal variability of temperature, O2, CO2 and
pH can be very profound in small lakes like the one we studied (e.g. Christensen et
al. 2013).
Our overall goal was to determine rates of GPP and NEP at 10-minute intervals to
test whether the mean metabolic signal for all sensors and the signal for individual
sensors resembled each other and showed the same diurnal course. The two specific
goals was to test the possibility of afternoon depression of GPP and NEP and the
decline of R as the night progresses. We tested directly the possibility of afternoon
depression of NEP because of inorganic carbon limitation by comparing
photosynthetic rates of apical charophyte shoots during in situ incubations in the
afternoon in naturally DIC- and CO2-depleted surface water versus enriched bottom
waters. We also tested whether respiration rates declined as the night progresses and
evaluated to what extent falling temperature and depletion of respiratory substrates,
following temperature correction, could account for the predicted decline. Finally,
direct measurements of sediment respiration allowed us to evaluate its contribution to
ecosystem respiration relative to that of the dense charophyte canopy as respiration in
the clear water was negligible.
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Materials and methods
Study site and lake characteristics - The study was conducted in a small lake located
in a limestone quarry abandoned 30 years ago on Räpplinge Alvar on Öland, SE
Sweden (56.81168°N, 16.6094°E). The area is very sparsely vegetated grassland
(alvar) with thin soils covering the hard Ordovician limestone. The lake is fed by
rainwater and runoff from the almost naked limestone surfaces and overflow from
nearby lakes during periods of heavy rain (Christensen et al. 2013). The lake was 847
m2 large and had a maximum depth of 46 cm and a mean depth of 21 cm during the
first intensive study period in June 8th -15th , 2013, while it was 700 m2 large and had
maximum and minimum depths of 34 and 29 cm during the second study period in
May 24th – 30th, 2014. A georeferenced bathymetric chart was created from 258
measurements of depth and position in a grid allowing extrapolation limits between
measurements of less than 1 m across the lake surface. Changes in water level were
followed continuously with an accuracy of 3 mm by recording pressure differences
between a submerged water level data logger (HOBO U 20 – 001-04, Onset
Computers, Bourne, USA) and a similar logger in air. The lake had transparent water
and very low summer concentrations of ammonium (about 2 µM), nitrate
(undetectable), ortho-phosphate (about 0.06 µM) and phytoplankton chlorophyll
(about 1 µg L-1; Sand-Jensen et al. 2010). The lake sediment was 4-10 cm thick
calcareous gyttja of low organic content (10% of dry mass) deposited on top of the
solid limestone surface (Sand-Jensen et al. 2010, Andersen and Sand-Jensen 2013,
pers. comm). The lake bottom was covered by dense charophyte vegetation (mainly
Chara aspera and some C. contraria, C. virgata and C. vulgaris) with less than 10%
representation of submerged angiosperms (Myriophyllum spicatum, Potamogeton
crispus and Zannichellia palustris; Sand-Jensen et al. 2010). The lake was free of
shading bank vegetation and mostly of emergent vegetation except for scattered
Typha angustifolia and Phragmites australis at location F (see map in Fig. 3).
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A meteorological station was established next to the lake at 2.0 m above the ground.
The station was equipped with sensors for incident irradiance (HOBO PAR sensor
(400-700 nm) S-LIA-M003, Bourne Ma, USA), wind speed and direction (Hobo
anemometer and direction, S-WSET-A), air temperature and relative humidity
(HOBO U23 Pro v2) that took measurements every 1 minute and averaged readings
for every 10 minutes on a HOBO micro station data logger (H21-002).
Temperature, oxygen and air-water gas exchange - The horizontal variability of
water temperature and dissolved O2 was determined across the lake in June 2013 by
deploying seven MiniDOT sensors (PME, Vista, Ca, USA) at different positions
immediately above the charophyte canopy and 8-12 cm below the lake surface
(positions in Fig. 3). In May 2014, the vertical variation of temperature and dissolved
O2 was recorded at five depths through the water column and the charophyte canopy
at a 31-cm deep site by deploying one MiniDOT sensor at 2 cm and four Firesting
Pyroscience fiber optic sensors (Aachen, Germany) at 9, 16, 23 and 30 cm depth
below the surface. Before each deployment MiniDOT sensors and Firesting optodes
were calibrated in air-saturated and anoxic water. After each deployment, all sensors
were tested in air-saturated water for several hours to make sure that no sensor drift
had occurred. In no instance did we need to compensate for drift during
measurements. During O2 measurements in May 2014 we also recorded pH (pHTemp2000 Madgetech, Warner, NH, USA)and conductivity (HOBO U24-001)
continuously in surface waters. All pH electrodes were calibrated before deployment
and controlled for drift after deployment. A close linear relationship between parallel
direct measurements of carbonate alkalinity (= HCO3- + 2 CO32-) and conductivity
normalized to 20 oC has previously been established for the lake (Andersen et al.
2015a,b). This enabled calculations of total dissolved inorganic carbon (DIC) and
CO2 from carbonate alkalinity, pH, temperature and conductivity according to
Mackereth et al. (1978).
85
Previous measurements on a similar small lake dominated by charophytes
(Christensen et al. 2013) have shown that wind-based models of air-water oxygen
exchange rate such as those in Jähne et al. (1987) and Cole and Caraco (1998)
overestimated exchange rates and produced erratic model predictions. We, therefore,
made a series of direct gas exchange measurements to account for the characteristics
of this particular lake (e.g., size, depth and charophyte cover) and the sitespecific
effects of wind velocity and fetch across the lake. Air-water gas exchange was
measured as CO2 exchange in free-floating flux chambers resembling those applied
by Raymond et al. (2012). CO2 was measured continuously in the flux chamber by a
portable IRGA (LiCor 840, Lincoln, Ne, USA) and CO2 in the water was determined
by continuous measurements of pH and temperature and frequent measurements of
DIC (dissolved inorganic carbon). Calculations of CO2 in the water and gas exchange
rates followed the procedures used by Sand-Jensen and Staehr (2012) and Alnoee et
al. (2015) and the precision and reproducibility were as high as in their
measurements. To convert measured gas exchange rates for CO2 to O2, we corrected
for chemical enhancement according to the measured temperature and pH (Bade &
Cole 2006) and the basic difference in exchange rate between the two gases
according to molecular weight (Wanninkhof 1992, Sand-Jensen & Staehr 2012). For
every location of the flux chamber we made three consecutive measurements and
used the average in further evaluations. Overall, we determined air-water exchange
rate at 21 locations, wind velocities and fetches. The air-water exchange rate (k600)
was positively related to mean wind speed, mean wind gust speed and to fetch at the
particular location of the flux chamber according to wind direction. The best
relationship to k600 (y, cm h-1) found by multiple regression using bidirectional
stepwise selection was for mean wind gust speed (x, m s-1) averaged for 2 hours prior
to measurements and mean wind speed grouped below and above 2 m s-1 during the
same period. At mean wind speed below 2 m s-1 the relationship was: y = 0.14 x +
0.04 (R² = 0.60, n=6). Above 2 m s-1 the relationship was: y = 0.87 x – 2.10 (R² =
86
0.76, n=15). The oxygen flux between the atmosphere and the water (F) was
calculated in time steps of 10 minutes as: F = k600 (O2surface - O2sat), where O2surface is
the measured concentration in surface waters and O2sat is the concentration in water at
equilibrium with the atmosphere at ambient water temperature and atmospheric
pressure. The flux was calculated for each time step using one of the two equations
for k600 depending on average wind speed and wind gust speed.
Oxygen metabolism
The vertical series - In measurements in May 2014 the O2 pool through the water
column was weighted according to the depth and water volumes represented by each
of the five sensors. For calculation of the air-water flux only temperature and O2
concentration in the surface water were used.
The horizontal series - Measurements in June 2013, using O2 concentrations and
temperature from the horizontally dispersed sensors, were averaged and NEP rates
normalized to surface area determined. Ecosystem rates for the entire lake was
determined after weighting according to the surface area represented by each sensor.
The sensor position was plotted on the map created and half of the distances to
neighbouring sensors were used as boundaries of the area represented by the sensor.
Boundaries were extrapolated onto the shore. The area covered by each sensor could
be calculated using the georeferenced map and weighted against the total lake area.
Ecosystem Respiration and GPP - Respiration rates in the lake during the night were
measured directly as the decline of O2 concentration in the water corrected for gas
exchange with the atmosphere. Respiration rates were integrated for the entire night
defined by incident photon irradiance below 1.3 µmol m-2 s-1 (PAR). Nocturnal
respiration rates were also determined for the initial 30 minutes (RIni) and the final 90
minutes (REnd) of the night by calculating the mean rate of O2 decline for 3 and 9
pairs of O2 measurements, respectively. These data allowed us to test whether
respiration rates declined from early to late during the night for temperature87
uncorrected rates and rates normalized to 20 oC applying a general Q10 value of 2.0.
Thus, the decline of temperature-corrected respiration rates during the night can be
represented by the quotient REnd (corr.)/RIni (corr) with values below 1.0 suggesting a
depletion of respiratory organic substrates.
Daytime respiration rates were calculated from the initial rate of O2 decline during
the first 30 minutes of the following night (RIni). This initial dark respiration rate was
assumed to be equal to the daytime respiration rate as organic carbon limitation of
respiration can be expected to be small and of the same magnitude. Daytime
respiration was corrected to ambient temperature applying the Q10 value of 2. GPP
was calculated for each ten minute step as the increase of O2 concentration corrected
for atmospheric exchange and with respiration added.
Averages of NEP and GPP were calculated for the entire measuring period, but only
including measurements for entire days. Standardized diurnal courses (Fig. 2 and 3)
were calculated as the mean of measurements in each time interval on the different
days. Values of NEP-day and NEP-night (= nocturnal respiration) were calculated for
the daytime and the night period as described above, and mean hourly respiration
rates were calculated by dividing cumulated NEP-night by the duration of the night
for comparison with sediment respiration rates.
Sediment respiration - Five sediment cores representing the different regions of the
small lake were retrieved in late May for measurements of aerobic sediment
respiration at ambient temperature (15 oC). Cores were made of Acrylic plastic of low
gas permeability, were 40 cm long, had an inner diameter of 5.2 cm and were closed
with rubber stoppers. Cores were incubated in a temperature-controlled water bath
filled with anoxic water preventing significant O2 influx to the sediment cores while
O2 was gradually depleted by sediment consumption over 4 days. Water within the
cores was kept homogeneous by a magnetic stirrer bars fastened to the upper rubber
stopper and driven by a large slowly-rotating magnet placed in the center and
88
surrounded by the five sediment cores. Oxygen concentrations were continuously
recorded in each core by a Firesting Pyroscience fiber optic sensor. Oxygen readings
showed constant sediment respiration rates during the first three days of incubation,
then used for calculation, and a decline on the fourth day.
Charophyte production - Charophyte production rates and the influence of gradual
self-limitation by depletion of CO2 and enrichment of O2 in the water during
photosynthesis were measured in situ in early June by incubating small apical shoots
of Chara aspera in 50 ml glass bottles placed immediately above the charophyte
canopy. Four replicate bottles were filled with surface water and four additional
replicate bottles were filled with bottom water at 2 pm on a sunny day with profound
density stratification typical of summer (Andersen et al. 2015a). Three blanks without
charophytes were incubated with either surface or bootom water and showed
negligible O2 changes. Before incubation, surface water had been enriched in O2 (520
µM, 206% saturation) and depleted in CO2 and DIC (1.37 mM) by photosynthesis
before noon, while bottom water located in the shade below the canopy had been
depleted in O2 (95 µM, 35% saturation) and enriched in CO2 and DIC (2.37 mM)
forming a distinct contrast and test for afternoon depression of photosynthesis in
surface water because of O2 accumulation and DIC depletion by ongoing
photosynthesis. Net photosynthesis was calculated as the release of O2 from 2 to 4 pm
relative to plant dry mass.
Results
Environmental conditions - The sky was clear and the incident irradiance followed a
regular sinusoid course on day 1, 2 and 5 in late May (Fig. 1). The water column
underwent strong daytime surface heating and vertical stratification at about 20 cm
depth on day 1 and 2 (Andersen et al. 2015a). Day 5 was colder and surface heating
and stratification were much weaker than on day 1 and 2 (Fig. 1 and Table 1). The
89
weather was partly overcast, colder and windy on day 3 and 4 and no vertical
stratification developed. The water column underwent convective mixing by surface
cooling every night (Andersen et al. 2015a). Dissolved O2 and pH in surface waters
exhibited profound and regular sinusoid diurnal courses on day 1 and 2, while diurnal
amplitudes were smaller and less regular on the other days. Photosynthesis during the
day depleted CO2 in surface waters below air saturation to only 0.42-2.92 µM late in
the afternoon on the five days, while O2 rose above air saturation to 430-509 µM. The
molar quotient of CO2 to O2 dropped to only 0.0009-0.0063.
Vertical daytime stratification was accompanied by strong DIC depletion by
photosynthesis and CaCO3 precipitation from surface waters at high pH, while DIC
was replenished by respiration and dissolution of CaCO3 at high CO2 concentration at
low pH in bottom waters during the day and continued throughout the night restoring
the DIC pool for photosynthesis on the following day (Andersen et al. 2015b). DIC
replenishment in bottom waters was weaker on day 3-5, because daytime vertical
mixing prevented the build-up of high CO2 concentrations in bottom waters
conducive to CaCO3 dissolution, leading to a marked decline of the mean DIC
concentration before noon from 1.30 mM on day 2 to only 0.84 mM on day 5 (Table
1).
Surface and vertically integrated metabolism - Average diurnal patterns of GPP and
NEP for the week in late May based on the surface sensor at 2 cm depth and all five
sensors dispersed through the water column at a mid-site were both highly regular
(Fig. 2) compared to the more irregular course of NEP estimated from the surface
sensor during individual days (Fig. 1). The average course of GPP reached a
maximum between 8 and 12 o’clock, while rates were lower in the afternoon. GPP
was slightly higher when based on measurements in surface waters alone than for the
entire water column (Fig. 2). Rates of NEP were positive from sunrise to the early
afternoon, close to zero in the late afternoon and negative during the evening and the
night. Nocturnal respiration declined from sunset to sunrise. Overall, diurnal patterns
90
of NEP were the same when based on measurements in surface waters or the entire
water column except that dark respiration was about 50 % higher when calculated
with data for the entire water column rather than surface waters alone.
Horizontal variability and integrated lake metabolism - Mean diurnal patterns of GPP
and NEP for the week in early June 2013 resembled each other between the seven
different positions of the O2 sensors and the volume-weighted average for the entire
lake (Fig. 3). Diurnal patterns also resembled those already described for the mid-site
in late May (Fig. 2). As expected, GPP was close to zero during the night, reached a
daytime maximum before noon, was somewhat lower in the afternoon and then
dropped steeply in the late afternoon and early evening. The lower GPP in the
afternoon than before noon took place at higher temperatures, O2 concentrations and
pH and lower availability of CO2 and DIC than before noon as described before (Fig.
1). Afternoon depression was more profound for NEP than GPP reflecting the much
higher respiration rate at high temperature and O2 concentration in the afternoon than
at lower temperature and O2 before noon. Positive values of NEP were typically
recorded for 12 hours between 5 and 17 o’clock and maximum rates between 8 and
11 o’clock. At night, respiration rates dropped from sunset to sunrise as temperature
and O2 declined similar to the nocturnal course in late May (Figs. 1 and 2).
Mean daily NEP (mmol O2 m-2 d-1) for the week was almost the same (-5 to -9) for
the different horizontal positions and the integrated average of the lake (Table 2). The
minimum (-7 to – 14) and maximum daily NEP rates (6 to 13) also resembled each
other among locations. Rates of GPP and R were both about 20-fold higher than the
small difference (NEP) between them. Mean rates of GPP and R varied about 2-fold
between different horizontal positions (e.g. GPP: 125-266 mmol m-2 d-1) and tended
to be smaller in very shallow water with a short charophyte canopy (e.g., location E)
than in deeper water with a taller canopy (locations B and D).
Ecosystem and sediment respiration – During all 11 nights in late May and early
June, ecosystem respiration rates declined significantly as the night progressed.
91
Respiration rates during the last 90 minutes of the night were 28-84% (avg. 58%) of
the rate during the first 30 minutes immediately after sunset (Table 2). After
correction for falling temperature during the night, respiration before sunrise still
remained 29-90% (avg. 63%) of the rate immediately after sunset.
The charophyte canopy was more active in terms of GPP and nocturnal respiration in
late May than in early June although temperature was higher in June (Tables 1 and 2).
Thus, the mean nocturnal respiration was 7.9 mmol m-2 h-1 in late May and 6.8 mmol
m-2 h-1 in early June (Table 1). The mean respiration rate after sunset corrected to a
common temperature of 20 oC was significantly higher in late May (15.7±1.8) than
in early June (8.2±1.4 mmol m-2 h-1, mean ± 95% C.L).
Sediment respiration in late May at 15 oC was 0.70±0.15 mmol m-2 h-1 (mean ± 95%
C.L.) and only about 10% of total ecosystem respiration. Because respiration in the
transparent, oligotrophic water was negligible, the dense charophyte canopy
accounted for about 90% of total ecosystem respiration.
On a diurnal basis sediment respiration averaged 16.8 mmol m-2 d-1 which is of the
same order of magnitude as the mean diurnal NEP in late May (5 mmol m-2 d-1) and
early June (- 7 mmol m-2 d-1). When sediment respiration was accounted for in the
ecosystem balance, mean net production of the charophyte canopy was slightly
positive in late May and early June (21.8 and 9.8 mmol m-2 d-1, respectively.
Regulation of daily and short-time variability of GPP and NEP – Rates of GPP and
NEP can be influenced by day-to-day and and short-time variability of irradiance,
temperature, DIC, CO2 and O2 concentrations through their impact on photosynthesis
and respiration. Daily GPP and NEP were not significantly correlated to day-to-day
variability of daily irradiance and temperature during the two measuring weeks
(Table 1). Though too few days were examined to test the relationship between daily
GPP and NEP and available DIC and CO2 in late May, data pointed at distinct
impacts. On day 5 with very low DIC and CO2 concentrations before noon, daytime
NEP was extremely low and GPP was 2 times lower than rates on day 1 and 2 when
92
DIC and CO2 were much higher (Table 1). The data suggest that availability of DIC
and CO2 represents a stronger constraint on daily GPP and NEP than irradiance and
temperature.
We evaluated further the influence of irradiance and pH on short-time variations og
GPP and NEP during 10-minutes intervals. pH was used as a proxy for inorganic
carbon availability relative to O2 because progressively higher pH during the day
accompanied depletion of DIC and CO2 and accumulation of O2 (Fig. 1, Andersen et
al. 2015b). We examined production rates as a function of irradiance and pH
separately before and after noon (Fig. 4). We only examined the patterns on sunny
days with clear skies where irradiance, temperature, O2 and pH follow regular
courses. It was apparent that GPP as a function of incident irradiance above 50 µmol
m-2 s-1 followed almost the same pattern before and after noon with approximate
saturation being attained above 500 µmol m-2 s-1. In contrast, rates of NEP were a
positive function of irradiance before noon, while rates were much lower and close to
zero and virtually independent of irradiance during the afternoon supporting that
other environmental variables constrained NEP. Rates of GPP were more similar
before and after noon than those of NEP because respiration rates are higher in the
afternoon at higher temperatures and GPP is the sum of NEP and respiration.
The relationship of GPP and NEP to pH between 8.0 (typical late morning pH) and
9.3 (typical late afternoon pH) was examined at irradiances above 500 µmol m-2 s-1 to
ensure light saturation of photosynthesis (Fig. 4) The relationship of GPP to
increasing pH was slightly negative, while that of NEP to pH was strongly negative
reaching zero at about pH 9.3. In the relationships of GPP and NEP to pH, afternoon
measurements took place at systematically higher temperature than before noon
conducive to higher afternoon respiration and, therefore, particularly low rates of
NEP compared with GPP.
Test of afternoon depression – We performed a direct test of afternoon depression of
charophyte photosynthesis as a result of DIC depletion and O2 accumulation in
93
surface waters by comparing rates of O2 release in surface waters (DIC/O2 molar
quotient of 2.7) with those obtained in DIC- enriched and O2-depleted bottom waters
(DIC/O2 quotient of 29). Net photosynthesis of apical shoots at high irradiance were
stimulated almost 4-fold by incubation in water collected near the bottom (151±52)
relative to shoots incubated in surface waters (40± 28 µmol g -1 DM h-1, mean±95
C.L.) confirming strong afternoon depression of photosynthesis by the build-up of O2
and the drawdown of DIC and CO2.
Discussion
Diurnal course of metabolism – Net ecosystem metabolism (NEP) exhibited two
profound features: afternoon depression and falling respiration during the night at all
locations and during all days of measurements. Positive NEP was mainly restricted to
the period from sunrise to shortly after noon , while NEP was close to zero during the
later afternoon and strongly negative during the evening until midnight. From the
morning to the afternoon the marked rise of temperature, O2 and pH and the decline
of CO2 and DIC can probably account for the distinct afternoon depression of NEP.
This explanation was supported by the steep decline of NEP with rising pH at high
irradiance and confirmed by in situ experiments showing almost 4-fold higher
photosynthesis of apical charophyte shoots incubated in DIC-enriched and O2depleted bottom waters compared with DIC-depleted and O2-enriched surface waters.
While NEP represents direct estimates from changes of concentrations and
atmospheric exchange of O2, GPP is the sum of NEP and R and is, therefore,
dependent on the selected temperature coefficient for respiration and the assumption
that daytime respiration rates is best represented by respiration rates immediately
after sunset, which is supported by most measurements (Markager & Sand-Jensen
1994). Thus, a lower estimate of the basic respiration rate (e.g. the average for the
night instead of the early night) and a lower temperature coefficient of respiration
than the selected Q10 of 2.0 would reduce respiration and, thus, depress GPP further in
94
the afternoon relative to rates before noon.Temperature rose 2-12 oC from sunrise to
late afternoon during our measurements. For a Q10 of 2.0, respiration would increase
0.4-2.4-fold. Because GPP and R are high relative to NEP (= GPP – R), the increase
of respiration contributed to the shift from maximum NEP values before noon to very
low NEP values during late afternoon. As a result afternoon depression is more
prominent for NEP than GPP.
The daytime depletion of DIC by photosynthesis and carbonate precipitation and the
even stronger decline of CO2 due to rising pH and temperature will increase inorganic
carbon limitation of photosynthesis (Madsen & Sand-Jensen 1991, Pedersen et al.
2013). Maximum inorganic carbon concentrations in surface waters in the morning
were 32-337 µM CO2 and 0.75-1.26 mM HCO3-, while minima late in the afternoon
were 0.39-2.7 µM CO2 and 0.62-0.82 mM HCO3-. Charophytes are capable of using
both HCO3- and CO2 for photosynthesis, but the affinity is higher for CO2 than HCO3(Lucas 1985). Earlier in situ incubation experiments at light saturation showed that
net photosynthesis of communities of the charophyte, Chara virgata in these small
lakes declined almost 2-fold when exposed to higher pH (9.5) and lower CO2 (1 µM)
characteristic of the afternoon relative to the rate under early morning conditions
(e.g., pH 7.5 and 150 µM CO2; Christensen et al. 2013). While the latter experiment
reflects the direct impact on photosynthesis of the charophyte community of higher
pH and lower CO2 at the same concentrations of DIC and O2, the stronger almost 4fold stimulation of net photosynthesis in the incubations of small charophyte shoots
in surface waters versus bottom waters reported here can be explained by the
additional constraints by lower DIC and higher O2 accompanying higher pH and
lower CO2 in surface waters during the afternoon. Lower DIC and higher O2
concentrations in itself and added lower molar quotients of DIC/O2 and CO2/O2
restrict photosynthesis and enhance photorespiration of submerged plants and algae
(Sand-Jensen & Frost-Christensen 1998) and high water temperatures enhance
mitochondrial respiration and photorespiration even further (Beardall et al. 2003).
95
These general phenomena will apply to charophytes, though their specific responses
have not been studied. The inhibition of net carbon fixation depends on the ability of
charophytes to increase CO2 in the cells by active carbon concentrating mechanisms
(Lucas 1985, McConnaughey 1991). The CO2/O2 quotient at the site of Rubisco
activity is a main regulator of the balance between photosynthetic CO2 assimilation
and CO2 loss by photorespiration because of competitive inhibition between
carboxylase and oxygenase activity of Rubisco (Sand-Jensen & Frost-Christensen
1998, Beardall et al. 2003). Incubation experiments in bottles in the lake with the
dominant Chara species confirmed that net photosynthesis dropped from the morning
to almost zero in the afternoon when O2 within incubation bottles approached 700800 µM (3-fold supersaturation) and CO2 declined below 1 µM (Andersen, Kragh and
Sand-Jensen pers, comm, 2015). Our field measurements also confirmed that NEP
reached zero in the afternoon when O2 approaced 400 µM and CO2 dropped to 0.5-5
µM in surface waters. All together these measurements stress the strong selflimitation of net photosynthesis of the dense charophyte stands in the shallow water.
As predicted, respiration rates declined progressively during the night. On average,
respiration rates in the lake before sunrise were 58% of the rate immediately after
sunset and 63% when respiration rates were corrected for falling nocturnal
temperatures from sunset to sunrise. Because charophytes are responsible for 90% of
ecosystem respiration, we suggest that diminishing pools of respiratory substrates due
to ongoing consumption during the evening and the night can account for the
declining rates following temperature correction. Declining respiratory rates during
the night have been observed in other field studies in oligotrophic lakes and streams
of low organic productivity (Sadro et al. 2011b, Solomon et al. 2013, Alnoee et al.
2015), but also in dense communities of microalgae (Gibson 1975, Markager &
Sand-Jensen 1989). The falling respiratory rates during the night can be correlated to
consumption of the main pools of soluble carbohydrates and neutral lipids (Gibson
1975, Lacour et al. 2012).We cannot exclude that synchronization of metabolic
96
activity involved in different synthetic pathways during diurnal cycles can also
influence the course of nocturnal respiration. In whole-system measurements
endogenous rhytms in synthetic pathways cannot be separated from the influence of
variable respiratory substrate pools.
Resemblance of GPP and R – Mean daily rates of GPP and R integrated across the
entire lake during the week in early June (198 and 191 mmol O 2 m-2 d-1) were close to
each other such that NEP shifted from slightly negative to slightly positive rates
between days (-15 to 10 mmol m-2 d-1), though with an overall negative average for
the week (-7 mmol m-2 d-1). Because of the complete cover of the lake bottom by
charophytes they were responsible for the entire gross production and 90% of
respiration in the lake. Sediment respiration was mainly fueled by degradation of
dead charophyte debris, the surrounding terrestrial landscape being very oligotrophic,
sparsely vegetated and having thin soils (Baastrup‐ Spohr et al. 2015). Low sediment
respiration rates can be explained by the low organic content in sediments (about
10% of DM) and regular exposure of shallow sediments to atmospheric air during
summer drought (Christensen et al. 2013, Sand-Jensen et al. 2015) resulting in
enhanced decomposition of organic material with access to the atmosphere and no O2
consumption from the lake water.
Net production of the charophyte community was slightly positive during the two
weeks in late May and early June (about 22 and 10 mmol m-2 d-1, respectively). These
low rates are expected when the charophyte community has obtained the observed
high density at which a net increase of the biomass and an associated positive daily
O2 balance for the charophyte community is prevented by profound self-shading,
depletion of inorganic carbon and accumulation of O2 during the day. Slightly
negative daily NEP rates have been observed before in similar lakes dominated by
dense charophyte communities during mid-summer, when GPP and NEP reached
about 200-300 mmol O2 m-2 d-1, while positive NEP rates (about 40 mmol m-2 d-1)
have been measured during active growth of the charophyte biomass in spring and in
97
late summer after refilling of the shallow waterbody following mid-summer drought
(Christensen et al. 2013).
Reproducible metabolic estimates across the lake – Trust in the reported metabolic
features is strengthened by the calculations of reproducible diurnal courses from
single surface sensors at different locations and integrated responses from many
sensors along the vertical profile and across the lake. The variability of estimated
rates between sensors was also relatively modest. For example, nocturnal respiration
exhibited a characteristic decline from sunset to sunrise during all 11 nights. Also, all
seven O2 sensors located across the lake showed almost the same average rates
(mmol m-2 d-1) of NEP during a week measurements (-5 to – 9), while GPP (125 to
266) and R (-118 to – 256) varied 2-fold. Some of the shallow sites had a shorter
charophyte canopy and a thinner sediment on the limestone (e.g. location E and G)
which may account for the lower rates of GPP and R than deeper sites with taller
charophytes.
The reproducible patterns are probably due to the continuous charophyte cover across
the small shallow lake. The short vertical and horizontal distances and the recurring
nocturnal mixing will tend to homogenize the water masses, the O2 signals and the
estimated metabolism despite vertical stratification of temperature and O2 at the
deepest sites during summer days. In addition, we made a special effort to directly
determine gas exchange with the atmosphere as a function of wind speed as part of
the metabolic estimates permitting us to perform running corrections for the air-water
O2 exchange. Outside the summer, continuous mixing of the entire water mass should
make the O2 signal from a single mid-station sensor in this small lake sufficiently
reliable for estimating whole-system metabolism.
In many other lakes, metabolic activity and physical dispersion of O2 signals are
much more complex and several sensors and more sophisticated models of air-water
gas exchange are needed to obtain reliable metabolic rates(Lauster et al. 2006, Van
de Bogert et al. 2007, Sadro et al. 2011a, Van de Bogert et al. 2012). This is the
98
situation in lakes with a high spatial physical and biological complexity because of a
shoreline with variable development of benthic phototrophs and heterotrophs on
different sediment types and partly separated surface and bottom waters of variable
volumes and diverging metabolic activity (Staehr et al. 2012b). However, even in
lakes with a regular shape and bathymetry, variability of metabolic estimates between
sensors can be astonishingly high; e.g. GPP from - 132 to 250 mmol O2 m-2 d-1 during
ten days in Sparkling Lake (Van de Bogert et al. 2012). Using four randomly placed
oxygen sensors increased the precision of daily metabolism estimates 4-fold over
single-location measures in Van de Bogert et al.’s study. The generality and possible
reasons for the high variability of metabolism estimates should be checked in future
studies because the results sincerely question the reliability of free-water approaches
to estimate metabolism in large and medium-sized lakes. This high within-lake
variability could explain why long-time series of metabolism estimates may show
extensive day-to-day variation that is only partially explainable by known driving
variables such as irradiance, temperature, vertical mixing and desiccation-refilling
(Sand-Jensen & Staehr 2007, Coloso et al. 2011, Christensen et al. 2013).
Alternatively, if there are time-delays between driving variables (e.g. mixing) and
metabolism, or important driving variable (e.g. nutrient availability) are not
accounted for in model analyses, metabolism estimates could still reflect the actual
conditions.
In the present multiple-sensor study and a previous single-sensor study of metabolism
in small charophyte-dominated lakes (Christensen et al. 2013), we found
reproducible diurnal patterns and long-term patterns closely related to environmental
characteristics known to regulate photosynthesis and respiration (e.g., irradiance,
temperature, DIC, pH, O2, and phototroph biomass). To confirm the effect of these
variables it is essential, that the free-water approach are combined with controlled
experiments in the field or in the laboratory on field samples retrieved for immediate
analysis. Such experiments allowed us to confirm afternoon depression of charophyte
99
photosynthesis by elevated O2 and pH and declining DIC and CO2. It is
recommended to perform controlled experiments with field material concurrent with
free-water measurements (Alnoee et al. 2015). Controlled chamber experiments
coupled to open-water measurements help identifying the main processes and
mechanisms that are driving ecosystem metabolism and its photosynthetic and
respiratory processes.
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Legends
Fig. 1. Diurnal course of incident irradiance and surface temperature (upper panel),
dissolved oxygen and pH in surface waters (middle panel), and GPP and NEP (lower
panel) based on measurements of a single oxygen sensor in surface waters in the
middle of the 0.31-m deep lake during six days in late May 2014.
Fig. 2. Mean diurnal patterns of GPP and NEP based on measurements during six
days in late May 2014 from two oxygen sensors near the water surface (2 and 9 cm,
upper panel) and a total of five sensors placed along the vertical profile (2 to 30 cm,
lower panel) at a 31-cm deep site in the middle of the lake.
Fig. 3. Mean diurnal patterns of GPP and NEP based on measurements during seven
days in early June from seven oxygen sensors (A-G) placed at 8-12 cm depth below
the surface at different locations in a small charophyte-dominated lake. The mean
volume-weighted GPP and NEP for the entire lake based on the seven oxygen sensors
was determined. Depth contours and location of sensors are shown on the map.
102
Fig. 4. Rates of GPP and NEP as a function of irradiance and pH before noon and in
the afternoon on three days in late May with a clear sky and regular sinusoid diel
changes in irradiance, temperature, oxygen and pH. Rates are based on measurements
of dissolved oxygen every 10 minutes from five oxygen sensors placed across the
vertical profile (2 to 30-cm depth) at a 31-cm deep site in the middle of the lake.
Measurements in the left panels were restricted to irradiances above 50 µmol m-2 s-1,
while measurements in the right panels were restricted to irradiances above 500 µmol
m-2 s-1.
103
1000
10
0
600
0
Temp (C)
PAR (µmol m-2 s-1)
2000
PAR
Temp 20
300
200
100
0
40
GPP and NEP
(mmol m-2 h-1)
9
400
pH
Oxygen (µM)
500
O2
8
pH
GPP
7
NEP
20
0
-20
Day 1 Day 2 Day 3 Day 4 Day 5 Day 6
104
Surface waters
30
GPP and NEP (mmol m-2 h-1 )
20
10
0
-10
30
Water column
GPP
NEP
20
10
0
-10
0
4
8
12
16
20
24
Time of day
105
20
20
A
10
10
0
0
F
-10
E
-10
G
20
20
B
F
A
10
10
B
0
0
C
-10
-10
20
E
C
10
0
-10
20
D
GPP and NEP (mmol m -2 h-1 )
GPP and NEP (mmol m -2 h-1)
D
20
10
0
-10
20
10
10
0
0
-10
-10
0
4
8
12
16
Time of day
20
24
G
0
M ean
4
8
12
16
20
24
Time of day
106
GPP (mmol m-2 h-1)
30
20
10
0
NEP (mmol m-2 h-1)
-10
30
Before noon
Afternoon
20
10
0
-10
0
500
1000
1500
Irradiance (µmol m-2 s-1)
8
9
10
pH
107
Table 1. Daily incident photon irradiance (PAR), surface temperature, surface inorganic carbon
concentrations (DIC and CO2) and integrated daytime NEP, nocturnal NEP (= nocturnal respiration)
and diurnal NEP during 5-6 days in late May and early June in a small lake. Photon irradiance and
temperature are mean values for the entire day, while DIC and CO2 are concentrations at noon.
Mean duration of the night was 6.7 hours in late May and 5.7 hours in early June resulting in a
mean nocturnal ecosystem respiration rate of 7.87 and 8.23 mmol O2 m-2 h-1 in late May and early
June, respectively.
108
Table 2. Nocturnal respiration rates in the small lake for 5-6 days in late May and early June during
the first 30 minutes (RIni) and the last 90 minutes (REnd) of the night. Respiration rates and the
quotient REnd/RIni are shown both uncorrected and corrected for temperature changes (Q10 = 2)
during the night.
Table 3. Mean daily rates of GPP, NEP and R derived from continuous oxygen measurements at
seven different positions (A to G) and the overall mean of all measurements in a small lake during a
week in early June. Daily minimum and maximum daily rates are in parenthesis.
109
Paper 4 - Whole-stream metabolism in nutrient-poor
calcareous streams on Öland, Sweden
110
Whole stream metabolism in nutrient-poor calcareous streams on Öland,
Sweden
Anette B. Alnoee 1), Tenna Riis1), Mikkel R. Andersen2), Annette Baattrup-Pedersen1) and Kaj
Sand-Jensen2)
1)
Department of Bioscience, Aarhus University, Ole Worms Alle 1, 8000 Aarhus C, Denmark
2)
Department of Biology, Freshwater Biology, University of Copenhagen, Universitetsparken 4,
2100 København Ø, Denmark
Keywords: seasonal metabolism, primary production, ecosystem respiration, surface irradiance,
headwater streams
Aquatic Sciences 2015, 77(2), 207-219
111
Abstract
We studied whole-stream metabolism in three headwater non-forested stream reaches on the island
of Öland, Sweden in order to characterize the metabolism of this unusual ecosystem and to compare
it with other stream ecosystems in NW Europe. Gross primary production (GPP) was generally low
(< 4 g O2 m-2 d-1) with the lowest GPP recorded in the most upstream, shallow reach draining the
thin soils of the limestone Alvar plains. Here, completely flooded terrestrial plants could account for
the whole primary production at baseflow. Ecosystem respiration (ER) increased several fold with
agricultural impact, resulting in heterotrophic stream conditions downstream and higher light
requirements for photosynthesis to outweigh respiration. A strong relationship between daily GPP
and ER was found at the two most nutrient-poor sites. Temperature corrected instantaneous ER rate
was highest in the beginning of the night, but decreased at the end of the night at the same reaches,
indicating that dark respiration depleted photosynthetic products and became limited by organic
substrates. The broad-scale comparison of open NW European streams showed a 1:1 relationship,
indicating a tight link between daily GPP and ER during summer (April-August) but not during
winter. This study has extended the range of GPP and ER measurements to include nutrient-poor
NW European streams, thereby increasing the knowledge on stream metabolism in this, otherwise,
highly agricultural impacted region. It also documented a strong relationship between GPP and ER
in streams, ranging from extremely nutrient poor to moderately nutrient rich conditions during
spring and summer.
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Introduction
Land use has proven to be an important factor determining whole stream metabolism.
Agricultural and urban streams receive additional nutrients and are more productive than natural
streams which are relatively unaffected by human activities (Bernot et al. 2010). Most open streams
in north-temperate lowland regions of Europe drain deep, fertile agricultural soils, they are rich in
dissolved nutrients and support high biomasses of benthic microalgae and submerged macrophytes
(e.g. Sand-Jensen et al. 1988; 1989; Kristensen and Hansen 1994; European Environment Agency
2005). Gross primary production (GPP) of benthic algae and macrophytes is high under wellilluminated conditions from spring to autumn, and ecosystem respiration (ER) is relatively high
year-round due to degradation of allochthonous and autochthonous material (Edwards and Owens
1962; Kelly et al. 1983). The open lowland streams are heterotrophic (ER > GPP) on an annual
basis, though GPP may exceed ER during periods in spring-summer with intense phototrophic
growth (Sand-Jensen 1997). Therefore, the magnitude and temporal pattern of GPP and ER reflect
both the intensity and timing of phototrophic production and organic decomposition which, in turn,
are influenced by light availability, nutrient richness and input of easily degradable organic matter
(Simonsen 1974; Mulholland et al. 2001).
Streams with very low nutrient availability due to thin, slowly mineralizing or wellleached soils in the catchment supposedly have low in-stream growth of benthic phototrophs and
heterotrophic communities. Daily GPP and ER should be low and in approximate balance provided
that the external input of easily degradable organic matter is low and less than the autochthonous
material, and that the temporal and spatial coupling between production and decomposition is close
(Odum 1971; Solomon et al. 2013). Low daytime photosynthesis may also lead to extensive organic
carbon limitation of respiration by phototrophs and bacteria during the subsequent night period and
gradually lower respiration rates as the night progresses and respiratory organic pools are
consumed, as previously shown for pelagic algal communities (Gibson 1978; Markager and SandJensen 1989). In open shallow clear-water streams, the incident solar UV-flux during the day could
also disrupt the organic aromatic compounds and facilitate coupled bacterial degradation during the
early part of the night. We propose that close coupling of daily GPP and ER and gradually
decreasing night-time respiration rates may prevail for small, shallow nutrient-poor streams
draining the thin soils on the open Ordovician limestone pavements of the great Alvar plain on the
island of Öland, SW Sweden (Ekstam and Forshed 2002). The streams on Öland are small and
shallow, have low slopes and low current velocities, facilitating the coupling between production
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and decomposition. The small ephemeral and permanent ponds on the Alvar have highly transparent
waters and concentrations of organic matter and dissolved inorganic nutrients are notably low
(Sand-Jensen et al. 2010; Christensen et al. 2013). We expect that water chemistry is similar in the
streams on the Alvar plain and that this may explain why there is little visible growth of algae on
the stream bottom.
Here we present a case study including three stream reaches located on Öland,
Sweden in order to include hitherto unexplored nutrient-poor streams in a comparison of the
metabolism of open, NW European temperate streams to evaluate the range of metabolic rates and
the coupling between GPP and ER. The three stream reaches included an upstream reach, reach I,
draining the natural sparsely vegetated grassland on the thin soils of the great Alvar plain, reach II,
impacted by agricultural land use over short distances, and reach III draining deeper soils and
impacted by mixed forest and agricultural fields over longer distances. From reach I to reach III, our
predictions were that (i) the biomass of benthic algae would increase from very low to higher
values, (ii) GPP, and in particular ER, would increase from very low to higher values, generating
stronger heterotrophy, and (iii) the coupling between GPP and ER would decrease.
Material and methods
Study site description
The study was conducted in two stream systems in separate catchments originating on
the great Alvar plain on the large Swedish island of Öland located in the southern Baltic Sea. These
Alvar streams experience approximately the same climatic conditions as the nutrient-rich Danish
and United Kingdom streams draining fertile agricultural soils (Edwards and Owens 1962;
Simonsen 1974). Annual mean precipitation is around 500 mm and mean temperature is -1°C in
January and 15°C in July (www.smhi.se). The great Alvar plain is a 260 km2, genuine nature
reserve (UNESCO World Heritage) having a species-rich, unproductive grassland vegetation with
very few deciduous trees occurring only in fissures in the horizontal, almost impermeable,
limestone plates and small areas with thicker soil layers covering the limestone surfaces. As the
streams leave the Alvar plain and pass through 0.5-4 km wide strips of agricultural land towards the
Baltic Sea located in the East, they become deeper and more influenced by bank shading and
agriculture.
The upstream reach I is supplied by rain water and aquifers close to the surface of the
Alvar plain and regularly dries out for 2-4 months between May and August, and occasionally also
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in September-October (Leberfinger and Hermann, 2011). Reach II and III receive some water from
deeper cultivated moraine soils and may retain small discharges during summer. Therefore, to allow
measurement and comparison, we conducted all measurements in May 2010 and May and October
2011 when all three reaches were transporting water.
Reach I (56° 32.473’N, 16° 34.037’E) was located in River Frösslundabäcken close to
the spring on the Alvar plain and 6.5 km from the Baltic Sea (Leberfinger and Hermann, 2011, their
Fig. 1). Reach II (56° 32.695’N, 16° 36.644’E) was located 4 km downstream of reach I in River
Frösslundabäcken after the stream had passed about 2 km of open Alvar and meadow and 2 km of
cultivated grain fields. Reach III (56° 35.680’N, 16° 39.165’E) was located in the lower part of
River Åbybäcken in an agricultural area east of Gårdby. This reach was 5 km from the spring on the
Alvar plain and 3.5 km from the sea. Reach I and II are in one catchment while reach III is in
another. The proportion of catchment occupied by Alvar plain ranged from 100% at reach I, to 40%
at reach II to 15% at reach III. In contrast, agricultural land use occupied 30% and 40% at reach II
and III, respectively, as determined by GIS analysis of maps showing land use.
The length of the study reaches varied from 130 to 246 m. To describe the physical
conditions and vegetation composition and abundance, transects were placed across the stream
every 10 m along the reaches. In each transect, stream width was measured and water depth,
substrate type and macrophyte species occurrence were determined at five points at equal distances.
In each reach, the proportions of run, riffle and pool flow types were estimated by visual assessment
at 10 m intervals. Average current velocity and travel time of water for the whole reach were
measured by injecting a pulse of NaCl and recording conductivity over time downstream of the
reach (White 1978). Discharge was calculated as the product of mean current velocity and mean
cross-sectional stream area for the whole reach. The slope of the reaches was determined during the
first visit using standardized leveling equipment to measure change in the surface water level along
the stream reach. Reach III had a much lower discharge in October 2011 than in May 2011, and
slope measurements were repeated.
In-stream plant cover on the reach scale was calculated by dividing the number of
points in transects with plant observations by the total number of points examined. The relative
frequency of species occurrence was calculated by dividing all observations per species by the total
number of plant observations.
Two water samples were collected at each reach and during each sampling period, and
analyzed in the laboratory for NO3- and NH4+ using a flow injection analyzer (Lachat, QuikChem
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FIA+, 8000series, Method 10-107-04-1-C and Method 10-107-06-3-D, respectively). Detection
limits are 0.01 mg N L-1 for NO3- and 0.005 mg N L-1 for NH4+. Soluble reactive phosphorus (SRP)
and total phosphorus (TP) were measured using the methods described by Brix and Schierup (2001)
based on spectrophometry, and total nitrogen (TN) and particulate organic carbon (POC) was
determined on a TOC-VCPH analyzer (Shimadzu, TNMU). Detection limits were 1 µg P L-1 for TP
and SRP, 0.1 mg N L-1 for TN, and 0.56 mg C L-1 for POC. Water samples for NO3-, NH4+ and SRP
were filtered before analyses. Alkalinity was determined by acidimetric end point titration (pH 3.7)
using 0.05 M HCl (Rebsdorf 1972). From alkalinity and pH, we determined free CO2 content using
Table 3 in Pedersen et al. (2013).
Stream reach metabolism
Stream reach metabolism was measured by the upstream-downstream two-station
oxygen change technique described by Odum (1956) following improvements by Marzolf et al.
(1994, 1998) and Young et al. (1998). We used YSI 6600 V2-2, Multiparameter Water Quality
Probes to measure dissolved O2 (DO, mg L-1), pH and temperature every 10 minutes. Surface
irradiance was measured with a LiCor (Li-1400) quantum sensor (Lincoln, Nebraska) placed on the
bank. The probes were calibrated at 100% saturation in calibration caps before they were deployed
in the streams. To correct for drift of O2, the probes were placed together in the stream before and
after measurements for at least half an hour. Drift in the O2 sensors was assumed to be linear over
time, and any difference between sensors over time was corrected accordingly. Average drift
between two sensors was 0.08 mg O2 L-1 day-1.
We measured reaeration rate based on measurements of gas exchange velocity (min-1)
for gas exchange over the air-water interface using cylindrical chambers of the same type as
described by Sand-Jensen and Staehr (2012). Bott et al (2006) recommends to measure reaeration
using propane gas, but a gas chromotograph (GC) could not be brought for the remote fieldwork, so
the chamber method was used. The chambers permit a natural, undisturbed water flow below a
water-air interface area of 0.39 m2 inside the chamber holding an air volume of 47.9 L. The
chamber was placed at the water surface and flushed with N2-gas from a pressure tank.
Subsequently, the influx rate of CO2 from the water to the enclosed air volume was recorded at 3
second intervals using an infrared gas analyzer (LI-COR Environmental Li-840) connected to a
laptop computer. The gas exchange was recorded for 10 minute intervals and repeated three times at
each location. CO2 concentrations in the chamber increased linearly over time (r2: 0.994-0.999), and
the mean coefficient of variation of triplicate calculation of evasion rates (CV: SD/xmean) was only
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0.08. The calculated values for reaeration was normalized to 20 ˚C, which corresponds to a Schmidt
number of 600 (k600), to allow comparisons between gas exchange velocities measured at ambient
temperatures. The reaeration speeds was normalized according to the equation by Raymond et al.
(2012):
(1)
k gas1
k gas2
 Sc gas1

 Sc gas
2





n
which is based on work done by Jähne et al. (1987) and Wanninkhof (1992), where kgas1 and kgas2 is
the reaeration rate at ambient temperature and 20˚C respectively, Sc is Schmidt number for the two
gasses and n is a constant expressing the mixing regime. In streams, n is set to 0.5 according to
Benson et al. (2014). The same equation was used to convert reaeration rates for CO2 to reaeration
rates for O2. Schmidt numbers for CO2 and O2 at different temperatures are available in
Wanninkhof (1992). The reaeration rate was measured for the different flow types (run, riffle and
pool). At each site, we measured reaeration three times in a row and the measurements were
averaged. Relative cover of flow types was multiplied by the respective reaeration rates and
summed to attain a whole-reach flow-weighted reaeration. In May 2010, reaeration rates were
measured at two runs at reach I, and at two runs and one riffle at reach II. In May 2011 at reach II,
we measured reaeration at two runs (one slow and one fast) and at two riffles. No pools were
present in the reach, but one of the runs had deep slow flowing water. At reach III, we measured
reaeration at two runs (one slow and one fast). In October 2011, we measured reaeration rates at
two runs and one riffle (the only one on the reach) at reach I. At reach II, we measured reaeration at
two riffles (one slow and one fast), at one run and one pool, and at reach III, we measured
reaeration rate at two runs (one slow and one fast).
Metabolism was measured on sunny days, and therefore, measured GPP (g O2 m-2 d-1)
is considered as maximum values for the reaches and periods. Net O2 changes corrected for
reaeration were calculated at 10-min intervals during the 24 hours. Metabolism calculations
followed Bott (2006). Daily net ecosystem production (NEP, g O2 m-2 d-1) was determined from one
hour after sunrise to one hour before sunset (according to timeanddate.com). ER (g O2 m-2 d-1) was
determined from changes in metabolism in the dark period between 0 and 3 a.m. and multiplied by
24 to attain daily ER. GPP was calculated as the sum of NEP during the photic period plus hourly
ER rates multiplied by the photic hours. NEP was converted to NEP g O2 m-2 min-1 from the 10
minute measurements and plotted against surface irradiance to compare the potential production
between the three reaches. Here we omitted measurements from the afternoon in reach I and
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measurements before noon in reach II and III due to shading from bank vegetation to be able to
compare measurements made in full sun between the reaches. During night-time, the progressive
changes in dark respiration were determined after correcting the metabolism (R15) for temperature
changes according to an Arrhenius equation (2) with a Q10 of 2.0 as being a general Q10 for
biological processes:
(2)
R15 = Ra * Exp[(15-T a)*0.0693]
where, R15 is the calculated metabolism at 15 °C, Ra (g O2 m-2 min-1) is the measured respiration
rate at ambient temperature (Ta) and 0.0693 (˚C-1) is a constant temperature coefficient
corresponding to Q10 of 2 (Sand-Jensen et al. 2005).
Chlorophyll a (g chl. a m-2) was measured on four sediment types (mud, sand, gravel and stone).
Five replicates were collected on mud, sand and gravel and 10 on stone (five stones from run and
five from riffles). Sand and mud were sampled using a small cylindrical core (area = 6.7 cm2) and
gravel with a large core (area = 22.2 cm2). Chlorophyll was extracted from a 1 cm (sand and mud)
and a 3 cm (gravel) surface sediment core. Chlorophyll on stones was measured by scrubbing the
stones thoroughly and filtering the particles onto glass fiber filters (GFC) for ethanol extraction.
Chl. a was measured in triplicate in May and October 2011 by filtering stream water onto glass
fiber filters. All chlorophyll extractions were made in 90% ethanol for 24 hours, and chlorophyll
was measured in a spectrophotometer (Shimadzu, UV-1800) and calculated according to
Lichtenthaler (1987).
Organic matter (ash-free dry mass, AFDM) in sediment samples was also measured at
all reaches and for all sediment types using the same number and type of samples as for chlorophyll
determination. Organic matter was measured as loss on ignition at 550 °C for 24 hours of 60 °C
dried samples.
For chl. a and organic matter, we calculated habitat-weighted chl. a (mg chl. a m-2)
and organic matter (g AFDM m-2) by multiplying chl. a biomass and organic matter content with
the proportional cover of the different sediments and adding all values together.
Photosynthesis experiment
For the dominant in-stream plant species (Alopecurus geniculatus L., Carex flacca
Schreb., Galium palustre L., Mentha aquatic L.) and algae species (Spirogyra sp. and Cladophora
sp.) at reach I and II, a photosynthetic experiment was performed in May 2010 to estimate the
possible contribution of plants to in-stream primary production and respiration. Before the
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experiment, the oxygen electrode (OX500; Unisense, Aarhus, Denmark) was calibrated in water
with 0% and 100% air saturation obtained by bubbling with N2 and atmospheric air, respectively.
Separate plant leaves or algal filaments were incubated in triplicate in ambient stream water in 50
mL glass-stoppered glass bottles, placed on a rotating wheel in a temperature-constant incubator (15
o
C) and illuminated with 450 µmol m-2 s-1 for 2 hours, without reaching supersaturation.
Subsequently, the bottles were transferred to the dark for 20 hours to measure respiration. Ambient
stream water without plants was used in four blank bottles. Oxygen concentration was measured in
samples and blanks after light and dark incubation. Plants and algae were then freeze-dried and
weighed to calculate the photosynthetic rate (Pambient, mg O2 g DW-1 d-1) and dark respiration (R, mg
O2 g DW-1 d-1) relative to dry weight (DW).
To estimate the contribution from the different plants and algae to the metabolism of the entire
reach, we multiplied net photosynthesis and respiration relative to DW with the biomass in the field
expressed as DW m-2 d-1, thereby getting GPP (g O2 m-2 d-1) and R24 (g O2 m-2 d-1). Hourly rates
were converted to daily rates by multiplying with day length. Photosynthetic rates will, therefore,
overestimate in situ rates because the calculation assumes that tissues are fully light saturated
throughout the day, even in dense stands, which is unlikely to be fulfilled. Furthermore, the
measurements did not include respiration from roots, thus, the photosynthetic calculations represent
maximum capacity under ambient conditions rather than realized rates. In the initial survey, we
determined the coverage (25, 50 or 75% cover) of the following plants: A. geniculatus, C. flacca, G.
palustre, and M.aquatica, and of two macroalgae: Spirogyra sp. and Cladophora sp.. We sampled
the different species in triplicate (bottom area = 22.2 cm2) within the 25, 50 and 75% cover, dried
the samples and measured DW, after which the data obtained were used to calculate in-stream plant
DW m-2 stream bed.
Comparison of metabolism among streams
To identify variability and regulating factors for the metabolic parameters (GPP, ER,
GPP/ER), we compared daily values for stream reaches on Öland with those reported for other open
reaches in North European lowland streams (Edwards and Owens 1962; Simonsen 1974; Kelly et
al. 1983; Riis et al. 2012; 2014; Alnoee unpubl. data). We examined the relationship between daily
GPP and ER for the spring-summer period (April-August) when irradiance is high and the autumnwinter period (September-March) when irradiance is low. Furthermore, we compared GPP/ER with
surface irradiance for open reaches from all over the world where these values have been measured
simultaneously (Edwards and Owens 1962; Simonsen 1974; Fellows et al. 2001; Mulholland et al.
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2001; Acuña et al. 2011; Rasmussen et al. 2011). This relationship was constructed to evaluate if
Öland streams separate from other streams due to strong nutrient limitation. If the streams are
nutrient limited, one should expect that the GPP/ER ratio would be lower than in streams in
nutrient-rich areas at the same irradiance.
Results
Physics and chemistry
All three stream reaches on Öland were small, had low downstream slopes and low
current velocities (Table 1). Reach I and II ran close to the riparian ground surface, whereas reach
III was incised and more shaded by emergent plants on the south bank. Travel time varied from 41
to 256 min between reaches.
Alkalinity was consistently high (3.01-4.88 meq L-1, Table 2), and the concentrations
of TP (4.89-14.88 µg P L-1, Table 2) and SRP were extremely low (<0.01-7.73 µg P L-1). Reach I
on the Alvar plain was only weakly supersaturated (about two-fold) with CO2 (30 µmol L-1), while
saturation was about 20-fold higher in reach III (250-380 µmol L-1). Water pH declined with
increasing concentrations of CO2. Oxygen concentrations were close to air saturation in reach I and
II and consistently undersaturated in reach III.
While NO3- was under the detection limit and NH4+ was very low (0.01-0.03 mg N L1
) in reach I, both NO3- (0.05-0.24 mg N L-1) and NH4+ (0.02-0.08 mg N L-1) increased in reach II
after passage through an agricultural section (Table 2). The influence of agriculture and more fertile
soils on NO3- concentrations (0.70-1.99 mg N L-1) was even stronger in reach III.
Sediment, vegetation and organic matter
Sediment cover varied between sampling periods and reaches (Table 3). The main
sediment types were thin deposits of sand and gravel on the limestone pavement in reach I, bare
limestone pavement and thin organic deposits on the limestone pavements in reach II, and thicker
sand deposits in reach III. The in-stream plant species were A. geniculatus, C. flacca, G. palustre,
M. aquatica and Potentilla acaulis L. covering 73-78 % of reach I, Menyanthes trifoliata L., Berula
erecta (Huds.) Coville, Sium latifolium L. and M. aquatica covering 27-29% of reach II, and B.
erecta, S. latifolium and Phragmites australis (Cav.) Trin. ex Steud covering 43-69% of reach III.
These species were amphibious plants able to live submerged as well as emerged. Obligate
submerged macrophytes were only represented by filamentous green algae (Cladophora sp. and
Spirogyra sp.) covering 5-17% of reach II.
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Habitat-weighted benthic chlorophyll a varied from 32 to 398 mg chl. a m-2 with
reach II having higher content than reach I and III (Table 3). The chl. a content was consistently
low in the water at the three reaches (0.6 to 1.1 μg chl. a L-1). Compared to benthic chl. a, the chl. a
content in pelagic microalgae relative to stream bed area was negligible (i.e. < 0.5 mg chl. a m-2).
Habitat-weighted organic matter (AFDM) varied from 338 g m-2 to 1221 g m-2 (Table 3). Values
varied the most between sampling periods at reach II and the least at reach III. There was no
tendency of reach III having higher organic matter content on the stream bed (Table 3) or suspended
in the water (Table 2).
Stream metabolism
Daily GPP was generally low but reach II attained moderately high values in May
2010 (Fig. 1, App. 1). Daily ER increased several fold from reach I to reach III in both May and
October. As a result, GPP/ER declined downstream mainly due to the increase of ER. In May,
GPP/ER was close to 1.0 in reach I and II, but lower in reach III. In October, all GPP/ER values
were below 0.32. Variation in GPP, ER and GPP/ER was small between days within sampling
periods (Fig. 1).
NEP during the day exhibited a hyperbolic saturation response to surface irradiance
for measurements conducted before noon in reach I and in the afternoon in reach II and III when
shading by bank vegetation was minimal (Fig. 2). NEP required gradually higher incident
irradiances to reach zero further downstream. Thus, 80% of the maximum NEP rate was obtained at
1370 µmol PAR m-2 s-1 in reach I, whereas reach II and III had not reached this level at 2500 µmol
PAR m-2 s-1. Temperature-corrected dark respiration rates were very low and decreased as the night
progressed in reach I and II, except for October in reach II (Fig. 3). In reach III, respiration rates
were much higher and changed less during the night compared to reach I and II (Fig. 3).
The contribution of submerged plants to the overall stream metabolism showed that
the photosynthetic rates varied seven-fold among species, with the filamentous macroalgae,
Cladophora sp. and Spirogyra sp., being the most productive (Table 4). The habitat-weighted
metabolism of the different species derived from the laboratory measurements showed that they
could contribute substantially to the in situ rates of GPP (1.2 to 14.1 g O2 m-2 d-1, corresponding to
52% to 210%; Table 4) and R24 (0.2-0.9 g O2 m-2 d-1, corresponding to 3% to 43%). Adding up the
estimates of Pambient to GPP from the different individual plant species, yielded high values for reach
I (6.1 g O2 m-2 d-1) and reach II (15.7 g O2 m-2 d-1), suggesting that in-stream plants, in theory, could
account for the measured whole-system rates in reach I (2.3 g O2 m-2 d-1, Table 4, Fig. 1, App. 1)
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and reach II (6.7 g O2 m-2 d-1, Table 4, Fig. 1, App. 1). The sum of habitat weighted dark respiration
rates of plants yielded rates (3.0 in reach I and 1.1 g O2 m-2 d-1 in reach II, Table 4) that were within
the range of the in situ measurements recorded in reach I (2.1 g O2 m-2 d-1, Fig. 1) and substantially
lower than the rates in reach II in May 2010 (6.4-7.8 g O2 m-2 d-1, Fig. 1).
Metabolic variability among Northern European streams
There was a strong positive relationship between GPP and ER in the April-August
measurements in NW European open lowland streams (r2 = 0.78, Fig. 4a). GPP values for the most
nutrient-poor reach, reach I, on the Alvar plain were 5-10-fold lower than most values from
nutrient-rich streams in the United Kingdom and Denmark. GPP and ER values for reach I and II on
Öland were close to the 1:1 line, while the deviation was higher for reach III (Fig. 4a). The
regression line including all points in figure 4a did not have a significantly different slope (p = 0.61)
or intercept (p = 0.47) from the 1:1 line. During autumn-winter (September-March, Fig. 4b), GPP
rates were low and typically lower than the ER rates. In accordance with these findings, the ratio of
GPP to ER in a range of stream reaches increased with mean surface irradiance, reaching values
above 1.0 at 43 mol PAR m-2 day-1 and a mean value of about 1.2 for even higher irradiances (Fig.
5). The shift from heterotrophy to autotrophy at 43 mol PAR m-2 d-1 can take place from April to
September (5 months) in Denmark and South-Sweden following the seasonal change in irradiance
at these latitudes.
Discussion
Downstream gradients in chemistry, biomass and metabolism
We found a generally low GPP and ER in the upstream extremely nutrient-poor reach I. GPP
peaked in reach II moderately affected by agriculture and ER peaked with very high values in reach
III. SRP remained low along the streams from the Alvar plain to the reaches located 4-6 km
downstream after passage of cultivated moraine soils, whereas NO3- gradually increased from
below detection limits in reach I to relatively high concentration in reach III. Therefore, growth of
benthic algae should be severely constrained by P availability at all reaches (Bothwell 1985;
Kjeldsen et al. 1996), although we expect that the P input to the stream increases downstream due to
more agricultural activity in the catchment. The lower P concentration at reach III compared to
reach II in May 2010 and from reach I to downstream stations in October 2011 could be due to
concomitant P uptake by P-limited benthic algae and bacteria.
As originally predicted, the chl. a content increased from reach I to II but decreased
from reach II to reach III along with more agricultural impact. This decline in chl. a could support
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the presence of P-limitation on benthic chl. a or be explained by less available light reaching the
benthic algae in the deeper and more shaded reach III, which, in contrast to the shallower and less
shaded reach I and II, is incised and shaded by emergent plants and tall banks. Thus, no obligate
submerged plants were present in reach III.
As a consequence of changes in nutrient availability, benthic chl. a and light
availability, GPP peaked in reach II and was lower in reach I and III. There was also a high cover of
in-stream obligate macrophytes dominated by filamentous macroalgae at reach II. It is most likely
that benthic microalgae had a strong effect on GPP, because GPP in May 2010 was almost threefold higher and chl. a was four-fold higher in reach II than in reach I. The same pattern was found
in October where reach I and III had a lower chl. a content and GPP than reach II. In October, the
highest GPP for all three reaches was only 1.67 g O2 m-2 d-1, probably due to lower daily irradiance
and temperature. GPP was generally low in our study, but comparable with the findings of other
European studies conducted at oligotrophic conditions (Kelly et al. 1983; Von Schiller et al. 2008).
In the extremely nutrient-poor Alvar stream (reach I), GPP was only 2-3 g O2 m-2 d-1 during sunny
summer days with high irradiance and favorable temperatures. GPP between 2-3 g O2 m-2 d-1 is low
compared to other more nutrient-rich unshaded streams with similar irradiance and temperature
levels (Kelly et al. 1983), and our study thus increases the range of measured GPP in European
streams. However, our measurements were higher than those reported by Bunn et al. (1999) for
some shaded Australian forest streams (0.33-0.53 g O2 m-2 d-1) and by Fellows et al. (2006) for
some American montane and forest streams (0.05-1.4 g O2 m-2 d-1). Our result is consistent with
results found by Bernot et al. (2010) which showed that natural streams have a lower GPP than
agriculture and urban streams.
We found a close coupling between GPP and ER in the most nutrient-poor reach I and
II in May. This is consistent with the highly oligotrophic conditions and algae being the primary
source for heterotrophic metabolism, and it suggests low external input of degradable organic
matter at these two reaches and low influence of the hyporheric zone. A much lower GPP than ER
in October in reach I and II suggested supplementary decomposition of terrestrial material supplied
by senescing plants in autumn (Roberts et al. 2007). The high respiration but low primary
production in reach III indicates high influence of the hyporheic zone and high input of
allochthonous degradable material as also reported by Graeber et al. (2012), although we could not
measure higher content of organic matter in surface sediment or seston. The sediments in reach I
were solid limestone pavements with no or thin sand deposits, the sediment in reach II was bare
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limestone pavements covered by thin layers of organic matter, while the sediment in reach III was
mainly deeper sand deposits such that a substantial respiration may take place in the hyporheic zone
deeper down below the surface sediment. It is known that the hyporheric zone may account for 4093% of ER (Fellows et al. 2001). Reach III was always strongly heterotrophic and had a metabolism
comparable to that measured in Sonoran desert streams in Arizona (Uehlinger et al. 2002), which
showed GPP/ER values below 0.17. This pattern is accompanied by gradually higher irradiances
needed for photosynthesis to balance ecosystem respiration and for photosynthesis to become light
saturated. While the rates of ER in reach I were 5-10-fold below the summer values recorded in
nutrient-rich Danish streams with the same climate (7.3-22.9 g O2 m-2 d-1, Simonsen, 1974), the
rates in reach III approached similarly high levels.
Rates of photosynthesis and ecosystem respiration were particularly low in reach I and
II where dark respiration appeared to deplete photosynthetic products during the night to the extent
that respiration rates declined and approached zero as the night progressed. In contrast, in reach III
respiration rates and supposedly the supply rates of organic substrates were higher and respiration
remained approximately constant during the night. Such a decline in night-time respiration has to
our knowledge only been demonstrated a few times in streams (Tobias et al. 2007; Hotchkiss and
Hall 2014), but the mechanism is also known from phytoplankton communities (Gibson 1978;
Markager and Sand-Jensen 1989). Future studies should test whether the gradual respiration decline
during night is a general phenomenon for oligotrophic streams fuelled by daytime photosynthesis.
The importance of the Alvar vegetation
In-stream plant coverage was high in all three stream reaches and during all three sampling periods,
reach I having the highest cover (about 75%). By calculating the potential contribution of plants to
whole-stream metabolism using habitat-weighted estimates in reach I, we found that respiration was
less than 1 g O2 m-2 d-1 for all plants and that GPP varied from 1.2 to 14.1 g O2 m-2 d-1. The
estimated whole stream rates of GPP and R24 in reach I were 6.1 g O2 m-2 d-1 and 3.0 g O2 m-2 d-1,
respectively, and thus the plants dominated by terrestrial grass species, could account for the whole
stream metabolism. In reach II, respiration of submerged macrophytes dominated by filamentous
macroalgae could only account for one fifth of the total respiration, whereas they could account for
the entire production. Acuña et al. (2011) also found that macroalgae habitats in streams were able
to account for 30-90% of GPP at the Pampean investigated reach but only for 2-20% of ER. Our
results suggest that in-stream terrestrial plants may have a strong influence on whole-stream
metabolism in intermittent reaches where this plant type may cover large parts of the stream bed.
124
The terrestrial in-stream plants in May 2010 in reach I remained photosynthetically
active despite the restricted availability of 30 µmol L-1 of CO2 in the water. All former experiments
have shown that these plant types are unable to exploit the abundant source of HCO3- in the water
(Sand-Jensen et al. 1992; Maberly and Madsen 2002). However, some species (e.g. Carex flacca
and Alopecurus geniculatus) retain a thin gas film on the leaves under water, allowing stomata to
operate, gases to bypass the resistant cuticle and higher rates of photosynthesis and respiration to
take place than is the case for leaves devoid of a gas film (Colmer and Pedersen 2007). Inorganic
nutrients were mostly undetectable in the stream water in reach I, which suggests strong constraints
of the chl. a development and productivity of benthic algae. In contrast, vascular plants can enhance
the dissolution of phosphorus from the calcareous soils by root release of organic acids (Tyler and
Ström 1995; Ström et al. 2005). After decomposition of plant tissue and terrestrial detritus, nutrients
are transferred to the streams during high-flow periods and become available to benthic algae. The
intermittent flow in the Alvar streams increases the likelihood that dissolved nutrients are exported
during the early phase of resumed flow before benthic algae have had the time to develop an
appreciable biomass for nutrient uptake. Intermittent flow supposedly restricts benthic biomass and,
in particular, prevents development of perennial species, which contrasts the situation in permanent
ponds on the Alvar plain where a gradual build-up of a high charophyte biomass occurs via
exploitation of sediment rather than water nutrient resources (Christensen et al. 2013). Such
charophyte beds in Alvar ponds attain a high GPP and ER (about 10 g O2 m-2 d-1) at the biomass
peak in July (Christensen et al. 2013). Thus, provided a long time period and availability of
alternative sediment resource, slow-growing species can establish a high biomass and substantial
rates of GPP, although they do not reach the peak values recorded in nutrient-rich lowland streams
(e.g. 25 g O2 m-2 d-1, Fig. 4).
In conclusion, we found generally low GPP in the Öland streams and some six-fold
lower rates than the maximum rates found in shallow streams in agricultural landscape of NW
Europe. A strong positive relationship between daily GPP and ER was found at the two most
nutrient-poor sites with very thin sediments on limestone pavements and low influence from the
hyporheric zone. Temperature corrected instantaneous ER rate was highest in the beginning of the
night, but decreased at the end of the night at all three reaches, indicating that dark respiration
depleted photosynthetic products and became limited by organic substrates. This study has extended
the range of GPP and ER measurements in NW Europe by including a very nutrient poor stream,
increased the knowledge on stream metabolism in this, otherwise, highly agricultural impacted
125
region, and documented a strong relationship between GPP and ER in streams ranging from
extremely nutrient poor to moderately nutrient rich conditions during spring and summer.
Acknowledgements
The authors acknowledge The Danish Council for Independent Research, Carlsberg Foundation
and Villum Kann Rasmussen Foundation to "Centre of Excellence for Lake Restoration" (grants for
T. Riis and K. Sand-Jensen). We also thank Peter Anton Staehr for comments to the paper, Anne
Mette Poulsen for valuable editorial comments and four anonymous reviewers.
126
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130
Table 1 Mean physical conditions in reach I, II and III during the three sampling periods.
Further details on study sites are found in Methods
Slope (m km-1)
Velocity (cm s-1)
Discharge (L s-1)
Width (m)
Depth (m)
Travel time (min)
Reaeration rate (min-1)
Reach I
May 2010
Oct 2011
0.23
1.4
0.3
2.21
2.39
0.11
0.10
255.5
0.021
0.011
May 2010
0.23
3.7
22
3.85
0.15
130.3
0.015
Reach II
May 2011
2.8
11
3.24
0.13
148.9
0.008
Oct 2011
4.9
24
3.34
0.15
84.2
0.021
Reach III
May 2011
Oct 2011
0.07
0.04
6.0
1.3
130
23
4.56
4.01
0.47
0.33
40.58
183.9
0.005
0.009
Table 2 Water chemistry, oxygen concentration (O2, mg L-1) and temperature in reach I, II and III
for the three sampling periods
pH
Alkalinity (meq L-1)
CO2 (μmol L-1)
POC (mg C L-1)
TP (μg L-1)
SRP (μg L-1)
Org-N (mg L-1)
NH4+ (mg L-1)
NO3- (mg L-1)
O2 max (mg O2 L-1)
O2 min (mg O2 L-1)
Temperature (˚C)
Reach I
May 2010 Oct 2011
8.43
8.38
3.01
3.07
30
30
4.53
5.55
4.89
10.74
<0.01
3.43
0.29
0.31
0.01
0.03
<0.01
<0.01
12.1
13.7
8.9
11.2
14.39
5.63
May 2010
8.15
3.74
60
5.22
14.88
5.82
0.41
0.02
0.15
13.1
8.0
12.48
Reach II
May 2011
8.01
3.74
100
2.44
13.23
7.73
0.07
0.08
0.24
13.1
8.3
11.97
Oct 2011
8.02
4.12
90
3.68
11.66
<0.01
0.17
0.04
0.05
12.1
10.1
6.51
Reach III
May 2011 Oct 2011
7.48
7.64
4.24
4.88
380
250
3.69
4.68
11.34
10.93
2.72
<0.01
0
0.1
0.03
0.05
1.99
0.7
9.1
7.0
6.2
5.5
6.47
6.73
Table 3 Sediment cover (%), macrophyte cover (%), habitat-weighted benthic chl. a (mg chl. a m-2)
and habitat-weighted organic matter (g AFDM m-2) in reach I, II and III for the three sampling
periods
Sand (%)
Gravel (%)
Stone and limestone plates (%)
Obligate water plants (%)
Amphibious plant (%)
Benthic chl. a (mg chl. a m-2)
Organic matter (g AFDM m-2)
Reach I
May 2010 Oct 2011
48.1
50.6
41.2
32.6
10.7
16.8
0.9
1.7
77.7
73.0
99.6
31.9
939.4
840.6
May 2010
26.2
9.6
64.2
16.6
27.6
398.1
1220.7
Reach II
May 2011
19.0
23.5
57.5
5.0
28.6
194.4
690.6
Oct 2011
24.0
15.6
60.4
6.4
25.5
215.3
337.8
Reach III
May 2011 Oct 2011
63.6
68.1
20.3
18.9
16.1
13.0
0
0
42.5
69.2
108.8
169.9
557.9
525.5
131
Table 4 In-stream plant cover (%) in May 2010 in Reach I and II, dry weight (DW, g m-2), leaf
photosynthesis (Pambient, mg O2 g DW-1 d-1), leaf respiration (R, mg O2 g DW-1 d-1), and habitatweighted and reach scale GPP (mg O2 m-2 d-1) and R24 (mg O2 m-2 d-1) for the five dominant
submerged plants, obtained by ex situ experiments
n
Plant cover (%)
Reach I
Reach II
DW (g m-2)
Pambient (mg O2 g DW-1 d-1)
R (mg O2 g DW-1 d-1)
Habitat-weighted metabolism
GPP (g O2 m-2 d-1)
R24 (g O2 m-2 d-1)
Reach I
Reach II
Reach I
Reach II
Alopecurus
geniculatus
4
38.3
72.0
30.5 ±6.9
30.0 ±5.2
Carex
flacca
4
23.5
116.3
20.4 ±9.9
17.4 ±3.6
Galium
palustre
1
17.4
72.0
70.9
66.2
Mentha
aquatica
3
17.4
17.5
116.3
58.3 ±48.3
45.9 ±39.5
Spirogyra and
Cladophora
1
34.0
317.9
136.1
2.4
Total
1.6
0.8
-
1.2
0.5
-
1.7
0.8
-
1.6
1.6
0.9
0.9
14.1
0.2
6.1
15.7
3.0
1.1
132
Reach
scale
2.3
6.7
2.1
7.1
Fig. 1 Mean daily GPP (g O2 m-2 d-1), ER (g O2 m-2 d-1) and GPP/ER in the three stream reaches in
May 2010 and 2011 and October 2011. Dashed line shows GPP/ER = 1. In May, n = 1 in reach I, in
reach II n = 2 in 2010 and n = 5 in 2011, n = 2 in reach III in 2011. In October, n = 2 in reach I and
II and n = 3 in reach III. Exact numbers are shown in appendix 1
Fig. 2 Relationships between surface irradiance (μmol m-2 s-1) and NEP (g O2 m-2 min-1) in the three
reaches. To reduce the influence from bank shading, data in reach I were from before noon in May
2010, and data in reach II and III were from the afternoon in May 2011. Overall significant
hyperbolic relationships are shown
Fig. 3 Night-time respiration rate (g O2 m-2 min-1) corrected for temperature variations in the three
stream reaches for three sampling periods. One night per sampling period is shown. Please note that
the scale for the y-axis of reach III differs from that of reach I and II
Fig 4 Relationships between daily GPP (g O2 m-2 d-1) and ER (g O2 m-2 d-1) in different streams in
Northern Europe during two seasons; April-August (Spring-summer) and September-March
(Autumn). Overall significant linear regressions are shown, and the dashed line shows GPP/ER = 1.
Data were summarized from Edwards Owens (1962), Simonsen (1974), Kelly et al. (1983), Riis et
al. (2012;2014), Alnoee et al. (unpubl. data), and this study
Fig. 5 Relationships between GPP/ER and daily surface irradiance (mol m-2 d-1) in temperate
streams. An overall significant hyperbolic relationship is applied. Data were summarized from
Edwards and Owens (1962), Simonsen (1974), Fellows et al. (2001), Mulholland et al. (2001),
Rasmussen et al. (2011), Acuña et al. (2011), and this study
133
Figure 1
134
Figure 2
135
Figure 3
136
Figure 4
137
Figure 5
Appendix 1 Mean daily GPP (g O2 m-2 d-1), ER (g O2 m-2 d-1) and GPP/ER in the three stream reaches in May 2010 and 2011 and
October 2011. Values are shown in figure 1
n
GPP (g O2 m-2 d-1)
ER (g O2 m-2 d-1)
P/R
Reach I
May 2010 Oct 2011
1
2
2.26
0.38
2.05
1.20
1.10
0.32
May 2010
2
6.69
7.13
0.95
Reach II
May 2011
5
3.09 ±0.20
2.89 ±0.45
1.08 ±0.11
Oct 2011
2
1.67
6.35
0.26
Reach III
May 2011
Oct 2011
2
3
3.30
0.38 ±0.01
7.98
27.24 ±0.18
0.42
0.01 ±0.00
138
Paper 5 - Caught Between Drought and Flooding on
Ölands Great Alvar (in Swedish, English abstract)
© Ole Petersen
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Acknowledgements
First of all I want to thank my principal supervisor Kaj Sand-Jensen for taking me on this scientific
journey which has taught me so much! For your invaluable guidance and support, for taking time to
discuss matters even on the busiest of days and in weekends, for many fantastic days in the field,
“hunting” rare flowers and birds, for interesting talks and for almost coming to peace with the fact
that I don’t find grasses and half grasses interesting. I want to thank my co-supervisor Peter Stæhr
for always having an open door, for introducing me to the possibilities in international
collaborations, for great times in the field on all kind of projects and for technical guidance.
I am grateful to the VIPs at the department. Ole, Kirsten and Dean I have had the great pleasure to
teach courses with. Jens, dit skrummel, you are the heart, soul and entertainment of the department!
I want to thank Theis for technical guidance and many great hours on Öland.
I want to thank my old officemate Jesper for all the help and time you have given even in your free
time and for showing me how to relax in stressed situations. Lars I want to thank for many hours in
the field and for making the long days a little bit more fun.
I am grateful to have met so many fantastic people at the Freshwater Biological Laboratory: Trine,
Winkel, Anja, Ane, Claus Møller, Frandsen, Matteo, Laci, Stine, Petur, Søren, Søren, Jesper,
Magnus, Lasse, Dennis, Max, Jos, Iversen, Emilie, Anne, Emil and Kenneth. A special thanks to
kuttersøstrerne (our Kathrines) and Mikkel MØ who have all helped me in my field work, you make
our office a nicer place to work in.
I want to mention Eleanor, Bas, Biel and Rafa from NetLake, Anette and Tenna from Århus. In
England I met Ian, Iestyn, Alex and Stephen. Going there was a great experience.
The staff at Freshwater Biological Laboratory Anne, Finn, Allan, and their predecessors Birgit, Nils
and Flemming have all been incredibly helpful. Special thank goes to Ayoe, you are amazing, and
the department would never be as productive without you to kick the asses that need kicking or
without your incredibly helpful attitude when mountains of samples show up and need processing
“yesterday”.
Thanks and much love to my friends and family who have supported me all along.
Mange tak til jer alle!!
148