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Digital Comprehensive Summaries of Uppsala Dissertations
from the Faculty of Science and Technology 1279
The role of sediments in the
carbon cycle of boreal lakes
HANNAH ELISA CHMIEL
ACTA
UNIVERSITATIS
UPSALIENSIS
UPPSALA
2015
ISSN 1651-6214
ISBN 978-91-554-9318-9
urn:nbn:se:uu:diva-261157
Dissertation presented at Uppsala University to be publicly examined in Ekmans salen,
Norbyvägen, Uppsala, Friday, 16 October 2015 at 10:00 for the degree of Doctor of
Philosophy. The examination will be conducted in English. Faculty examiner: James Cotner.
Abstract
Chmiel, H. E. 2015. The role of sediments in the carbon cycle of boreal lakes. Digital
Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and
Technology 1279. 42 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-554-9318-9.
Inland waters are active sites of carbon (C) processing and emitters of carbon dioxide (CO2)
and methane (CH4) to the atmosphere. In the boreal zone, where surface waters receive large
quantities of organic carbon (OC) from surrounding forests and wetlands, lakes and streams act
as strong sources of these greenhouse gases. Lake sediments provide the only long-term sink
of C in boreal inland waters, through burial of OC. However, mineralization of OC counteracts
the efficiency of lake sediments in removing C from the short-term C cycle. In this context, this
thesis provides a better insight into the dual role of boreal lake sediments as C source and C sink.
The presented work is based on empirical assessments of OC burial and OC mineralization
rates in boreal lakes. The temporal variability of OC burial and the stability of the buried OC was
assessed on both centennial and millennial timescales. The quantitative importance of sediment
OC burial and mineralization in comparison both to other C fluxes within the lake, and to C
fluxes within the tributary stream network, was quantified. By simulating the effect of climate
change on water temperature, we also gauged the potential future efficiency of lake sediments
in storing C.
The results demonstrate that OC mineralization in sediments dominates three-fold over OC
burial when observed at a whole-basin and annual scale. The contribution of sediment OC
mineralization to annual C emission from the assessed study lake was, however, found to be
small (16%), when compared to OC mineralization in the water column (37%) and catchment
import of C (47%). Furthermore, C emission from headwater streams was found to dominate
greatly over the lake C emission, mainly triggered by the higher gas transfer velocity of streams
compared to lakes.
On a long-term (Holocene) scale, the continuous OC burial flux results in a large amount of C
stored in sediments. The temporal variability of this OC accumulation was found to vary across
lakes, with, however, time-dependent patterns: On a millennial scale, smaller lakes exhibited
a higher variability than larger lakes of the study area. For the last century, similar variability
and a trend to increased OC accumulation was found for most study lakes, irrespective of their
size. Analysis of lignin phenols in the accumulated OC did not indicated post-depositional
degradation, independent of the age of the sediment OC, implying that sediments are a very
stable sink for land-derived OC in boreal lakes.
Simulation of warming water temperatures in boreal lakes resulted in declines of the OC
burial efficiency BE (OCBE; OC burial/OCdeposition) up to 16%, depending, however, on basin
morphometry. Predicted declines in OCBE were higher for the more shallow lake compared to
the deeper lake.
In conclusion, this thesis illustrates that sediments play, despite a small quantitative impact
on aquatic C cycling, an important role as a very stable C sink in boreal lakes. However, the
efficiency of this C sink is likely to be reduced in the future.
Hannah Elisa Chmiel, Department of Ecology and Genetics, Limnology, Norbyv 18 D,
Uppsala University, SE-75236 Uppsala, Sweden.
© Hannah Elisa Chmiel 2015
ISSN 1651-6214
ISBN 978-91-554-9318-9
urn:nbn:se:uu:diva-261157 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-261157)
To my family
List of Papers
This thesis is based on the following papers, which are referred to in the text
by their Roman numerals.
I
Chmiel, H. E., Niggemann, J., Kokic, J., Ferland, M.-E., Dittmar, T.,
& Sobek, S. (2015) Organic matter burial and quality are uncoupled
in boreal lake sediments over the Holocene. J. Geophys. Res. Biogeosciences, in press.
II
Chmiel, H. E., Kokic, J., Denfeld, B. A., Einarsdóttir, K., Wallin, M.
B., Isidorova, A., Koehler, B., Bastviken, D., Ferland, M.-E., & Sobek, S. (2015) The role of lake sediments in the carbon budget of a
small boreal lake. Submitted to Limnol. Oceanogr.
III Kokic, J., Wallin, M. B., Chmiel, H. E., Denfeld, B. A., & Sobek, S.
(2015) Carbon dioxide evasion from headwater systems strongly
contributes to the total export of carbon from a small boreal lake
catchment. J. Geophys. Res. Biogeosciences, 120:13–28
IV Chmiel, H. E., Natchimuthu, S., Bastviken, D., Ferland, M.-E. &
Sobek, S. Decreased efficiency of sediment carbon burial in boreal
lakes at warming lake water temperatures. Manuscript
Reprints were made with permission from Wiley.
Additional Papers
In addition to the thesis chapters, I have contributed to the following papers:
•
Denfeld, B. A., Wallin, M. B., Sahlée, E., Sobek, S., Kokic, J., Chmiel,
H. E., & Weyhenmeyer, G. A. (2015) Temporal and spatial carbon dioxide concentration patterns in a small boreal lake in relation to ice cover
dynamics. Boreal Env. Res., 20:1–14
•
Podgrajsek, E., Sahlée, E., Bastviken, D., Natchimuthu, S., Kljun, N.,
Chmiel, H. E., Klemedtsson, L., & Rutgersson, A. (2015) Methane fluxes from a small boreal lake measured with the eddy covariance method.
In revision at Limnol. Oceanogr.
•
Wallin, M. B., Weyhenmeyer, G. A., Bastviken, D., Chmiel, H. E., Peter, S., Sobek, S., & Klemedtsson, L. (2015) Temporal control on concentration, character, and export of dissolved organic carbon in two
hemiboreal headwater streams draining contrasting catchments. J. Geophys. Res. Biogeosciences, 120:832–846
Contents
Introduction ................................................................................................... 11 Boreal lakes and carbon cycling ............................................................... 11 The role of lake sediments for carbon cycling ......................................... 12 Knowns and Unknowns ............................................................................ 13 Aims of the Thesis ........................................................................................ 15 Methods ......................................................................................................... 16 Study sites ................................................................................................. 16 Analysis of carbon mass accumulation rates ............................................ 17 General approach ................................................................................. 17 Sampling, preparation, and analyses .................................................... 17 C contents, dry bulk density, and sedimentation rates ......................... 17 Lignin phenol analysis .............................................................................. 18 Sediment and water incubation experiments ............................................ 19 Sediment traps .......................................................................................... 19 Sub-bottom profiling ................................................................................ 19 Monitoring of lakes and streams .............................................................. 20 Water column monitoring and the diel DO technique ......................... 20 Stream monitoring of discharge and CO2 ............................................ 20 Lake warming simulations........................................................................ 20 Photochemical modeling .......................................................................... 21 Gas flux determination ............................................................................. 21 General approach ................................................................................. 21 Headspace equilibration method .......................................................... 21 Floating chamber method .................................................................... 22 Determination of k in streams by propane injections........................... 22 Results and Discussion .................................................................................. 23 Role of sediments for C cycling in a lake and catchment perspective ..... 23 Burial vs. mineralization of OC in lake sediments .............................. 23 Sediment C fluxes vs. emission C flux from lakes .............................. 23 Role of sediments in an annual lake C budget ..................................... 24 Comparison of lake- and catchment-scale C fluxes ............................. 26 Temporal variability of the lake sediment C sink..................................... 27 Variability in OC accumulation over the Holocene ............................. 27 Sources and stability of terrestrial OC in boreal lake sediments ......... 28 OC burial efficiency of lake sediments: now and in the future ........... 29 Conclusions and Perspective ......................................................................... 30 Sammanfattning på Svenska ......................................................................... 31 Acknowledgements ....................................................................................... 34 References ..................................................................................................... 38 Abbreviations
Ad/Al
CMAR
C/V
DIC
DO
DOC
GPP
IRGA
LPVI
NEP
OC
OCBE
pCO2
POC
Q
R
S/V
Acid-to-aldehyde ratio
Carbon mass accumulation rate
Cinnamyl phenol-to-vanillyl phenol ratio
Dissolved inorganic carbon
Dissolved oxygen
Dissolved organic carbon
Gross primary production
Infrared Gas Analyzer
Lignin Phenol Vegetation Index
Net ecosystem production
Organic carbon
Organic carbon burial efficiency
Partial pressure of carbon dioxide
Particulate organic carbon
Discharge
Respiration
Syringyl phenol-to-vanillyl phenol ratio
Introduction
Boreal lakes and carbon cycling
The boreal zone covers about 13% of the continental area on earth (Schultz,
2013) and has an important function in the climate system acting as a terrestrial carbon (C) sink (Pan et al. 2011). Boreal forests remove carbon dioxide
(CO2) from the atmosphere and transform it into biomass. The turnover time
of this biomass, i.e., the time until organic carbon (OC) is transformed back
to CO2 or methane (CH4), depends on various factors, such as respiration in
plants or soils, or the release during wildfires, but also on the interplay between the terrestrial and the aquatic C cycle (Cole et al. 2007; Tranvik et al.
2009).
Boreal lakes and streams receive, in addition to their internal (autochthonous) OC production, large quantities of terrestrial (allochthonous) OC from
the surrounding catchment (Birge and Juday 1927). The largest share (9095%) of this allochthonous OC is present in form of dissolved organic carbon (DOC), while particulate organic carbon (POC) forms the smaller fraction (Wetzel, 2001). Allochthonous DOC is mostly derived through leaching
of OC from organic-rich soil horizons of forest and wetlands, and is responsible for the brownish colour of boreal surface waters (Rasmussen et al.
1989).
When allochthonous OC enters boreal lakes, it fuels the metabolism of
heterotrophic microorganisms in the water column (i.e., bacterioplankton),
which results in the production of CO2 (Tranvik 1988). Aquatic primary
producers, such as phytoplankton, partially transform the CO2 into biomass
again, however, the dark colour of allochthonous DOC strongly inhibits photosynthesis in boreal lakes (Karlsson et al. 2009). Therefore, the majority of
boreal lakes are net heterotrophic ecosystems, in which respiration is higher
than primary production (Del Giorgio and Peters 1994; Jansson et al. 2000).
Besides microbial degradation, OC can be mineralized photochemically by
sunlight, either directly to CO2 or by cleavage of structurally complex organic macromolecules to readily bioavailable molecules that stimulate bacterial
production (Granéli et al. 1996; Bertilsson and Tranvik 1998). Microbial and
photochemical degradation of OC is one of the reasons why most lakes of
the boreal zone are supersaturated with CO2 and emit CO2 to the atmosphere
(Sobek et al. 2003, 2005). Another reason is dissolved CO2 entering lakes
via surface water or ground water inflow (Stets et al. 2009). Also for boreal
11
streams, high CO2 emissions were reported and largely attributed to the injection of CO2 derived from soil respiration via groundwater inflow to the
stream (Öquist et al. 2009; Wallin et al. 2013). Hence, the release of CO2
from the entire aquatic continuum, i.e., both lakes and streams, is an important component of the boreal aquatic C cycle (Crawford et al. 2014) and
counteracts the terrestrial C sink.
At the same time as CO2 is emitted from lakes and streams to the atmosphere, OC is stored in lake sediments over long time scales.
The role of lake sediments for carbon cycling
In lakes, organic and inorganic particles of different origin settle down to the
lake bottom, where they form sediments. These sediments play a dual role in
the aquatic C cycle since they act both as a source and sink of C. The deposition of organic matter at the lake bottom results, on the one hand, in burial of
OC, which removes C from the active short-term C cycle. Hence, sediments
represent a long-term C sink. On the other hand, ongoing microbial degradation in the sediments prior to burial results in mineralization of OC to CO2
and CH4, which are recycled again in the lake or emitted to the atmosphere
(Figure 1).
In the boreal zone, it has been shown that lake sediments store more C
than the surrounding forest soils and biomass (Kortelainen et al. 2004). The
transport of POC to boreal lakes certainly contributes to sediment OC burial,
and it seems that in-lake flocculation of DOC to POC is an additional important source of sediment OC in boreal lakes (von Wachenfeldt and Tranvik
2008).
CO2 and CH4 !
emission!
stream
export!
stream !
import!
OC photomineralization!
internal !
C cycling !
OC sedimentation!
CO2!
CH4!
groundwater !
inflow!
OC mineralization!
OC burial!
Figure 1. Carbon fluxes and processes in the aquatic C cycle of lakes.
12
The production of CO2 and CH4 in sediments depends on different factors
that act on OC mineralization: The source of the organic material (i.e., autochthonous or allochthonous origin) influences mineralization rates (Sobek
et al. 2009) as autochthonous OC is more easily degraded by microorganisms than allochthonous OC (Burdige 2007). Furthermore, temperature
strongly controls OC mineralization rates in sediments, with warmer temperatures stimulating higher mineralization rates (Bergström et al. 2010; Gudasz
et al. 2010). Additionally, mineralization rates and the production of CH4
depend on the presence and absence of oxygen: Under anoxic conditions,
degradation rates typically slow down, lowering the production rates of CO2
(Zehnder and Svensson 1986) while methanogenesis occurs. In lakes, CH4
typically has a small contribution in C units, however, CH4 is a 28-fold more
powerful greenhouse gas than CO2 on a 100 year scale (Intergovernmental
Panel on Climate Change, IPCC, 2013) and therefore important to consider
with respect to climate.
To quantify this dual function of lake sediments as C source and sink, the
OC burial efficiency (OCBE, %) is calculated. It is defined as the ratio of
OC burial:OC deposition onto the sediment surface, and hence represents the
fraction of OC that remains permanently in the sediments after deposition.
Since the OCBE of sediments is intimately linked to OC mineralization (OC
that is not mineralized is buried), it depends on the same factors, i.e., temperature, organic matter sources and the exposure time to oxygen, but also
on the sediment accumulation rate (Burdige 2007; Sobek et al. 2009).
Knowns and Unknowns
The role of lake sediments for aquatic C cycling has been investigated in a
multitude of studies (e.g., Mulholland and Elwood 1982; Molot and Dillon
1996; Kortelainen et al. 2004, 2013; Algesten et al. 2005; Sobek et al. 2009;
Bergström et al. 2010; Gudasz et al. 2010; Ferland et al. 2012; Fenner and
Freeman 2013). Some of these studies have focused on regulatory factors of
OC mineralization and preservation in boreal lakes. For instance, the temperature dependence of OC mineralization rates in lake sediments was investigated in a literature survey and exemplified for boreal lakes of different
trophic state (Gudasz et al. 2010). Also, the exposure time of oxygen and
anoxic conditions in lakes have been highlighted as key factors for OC
preservation in boreal lake sediments (Sobek et al. 2009; Fenner and
Freeman 2013).
Other studies have explored quantitative aspects of C fluxes at the lake
bottom, in order to gauge the importance of sediments for aquatic C cycling.
For example, a large-scale study on Finnish lakes concluded that OC mineralization in sediments was an important driver for CO2 emission (Kortelainen et al. 2006). However, an investigation of sediment OC mineralization
13
in boreal lakes during summer, demonstrates that this is not always the case
(Algesten et al. 2005). Also, studies on the OCBE have come to divergent
results. In a survey of sediments, which were investigated at the deepest
point in lakes, it was found that boreal lake sediments had a comparatively
high OCBE (range, 45-67%; Sobek et al. 2009), while a study on the OCBE
in boreal lake sediments in Québec revealed a much wider range (4-62%), by
accounting for the spatial variability in sediment deposition across lake basins (Ferland et al. 2014). Hence, dissimilar findings of studies that investigate sediment C fluxes in lakes may reflect the difference in spatial and temporal scale on which these studies were carried out (Hobbs et al. 2013). For
better comparison between studies, and a comprehensive assessment of the
role of lake sediments in the aquatic C cycle, it is therefore necessary to integrate spatial and temporal patterns in sediment-, lake-, and catchment-scale
C fluxes.
Furthermore, it is important to look at different time scales, when assessing OC accumulation in lake sediments. Studies that address the temporal variability in OC accumulation mostly focus on trends in the recent
past, i.e., the last century (Anderson et al. 2013; Dietz et al. 2015). The variability in OC accumulation on long-term scales, i.e., over millennia of the
Holocene is, however, often neglected. Instead, when addressing Holocenescale OC accumulation it is mostly referred to it as mean values, and less is
known about the variability (Anderson et al. 2009; Kastowski et al. 2011;
Kortelainen et al. 2013). Long-term variations in OC burial, however, might
provide important insights into the functioning and evolution of lake sediments as a C sink, in response to past environmental change.
Also, the source of the buried OC is important, particularly in terms of
how the lake sediment C sink is accounted. If the buried OC originates from
land (i.e., is allochthonous) it may simply be viewed as soil C that was transported to the lake bottom, and may therefore not be accounted as a new sink.
If the buried OC was produced within the lake, i.e., is autochthonous, it
would represent a new C sink. There are contrasting views on the source of
buried in OC in boreal lakes (Dean and Gorham 1998; Gudasz et al. 2012).
More specific knowledge on the different source types of terrestrial OC in
sediments and how their contribution changed over time is needed. In addition, there are indications that lake sediment OC can continue to degrade for
hundreds of years after deposition, albeit, at very low rates (Sobek et al.
2014). For boreal lakes, however, the long-term stability of the buried sediment OC is presently not well understood.
Apparently, important gaps remain in our understanding of lake sediments as sources and sinks of C.
14
Aims of the Thesis
This thesis aims at gaining more insights into the role of lake sediments in
the C cycle of boreal lakes. Thereby, the main focus is on assessing C fluxes
on integrated spatial and temporal scales, in order to put sediment C fluxes
in perspective to lake and catchment-scale C cycling. In addition, the thesis
addresses the role of lake sediments as past and future C sink.
More specifically, the different thesis chapters focus on:
1) the temporal variability of OC accumulation in boreal lake sediments
over the past 10,000 years, and the stability of the sediment C sink (Paper I),
2) the contemporary role of lake sediments as C source and sink in the annual C balance of boreal lakes (Papers II and IV),
3) the role of C loss and transport in the headwaters of a small boreal lake
in relation to lake-internal C cycling (Papers II and III),
4) the future role of lake sediments as C sink in a warmer climate (Paper
IV).
15
Methods
Study sites
The study lakes and streams (Table 1) are located in the boreal zone of central Sweden (Paper I-IV) and southwestern Sweden (Paper IV). They are
small sized systems (<2 km2) with medium to high DOC contents (range, 1028 mg L-1), brownish water colour, and a glacial origin some 8,000-10,000
years ago (Lundqvist 1986).
Table 1. Investigated lakes and streams in Papers I-IV.
Paper
I
I
I
I
I
I
II
16
Surface Area
km
2
Location
N
E
III
III
III
Lake
Dagarn
Övre Skärsjön
Oppsveten
Lilla Sångaren
Grästjärn
Gäddtjärn
Erssjön
Svarttjärn
Prästjärn
Kringeltjärn
Svintjärn
1.72
1.65
0.65
0.24
0.09
0.07
0.06
<0.01
<0.01
<0.01
<0.01
59°54’
59°51’
60°01’
59°54’
59°53’
59°51’
58°22’
59°53’
59°51’
59°51’
59°52’
15°42’
15°33’
15°28’
15°23’
15°21’
15°11’
12°09’
15°15’
15°12’
15°12’
15°12’
III
III
III
III
III
Stream
Gäddtjärn inlet 1
Gäddtjärn inlet 2
Gäddtjärn outlet
Wetland outlet
Svintjärn outlet
<0.01
<0.01
<0.01
<0.01
<0.01
59°51’
59°51’ 59°51’ 59°51’ 59°52’ 15°11’
15°11’ 15°11’ 15°12’ 15°12’ III
I
Study site
IV
IV
Analysis of carbon mass accumulation rates
General approach
Carbon mass accumulation rates (CMARs; g C m-2 yr-1) in sediments were
calculated from the dry bulk density, the carbon content, and from sedimentation rates in vertical sediment core profiles according to the equation:
!"#$ = !! ∗ !!"#$ ∗ !"
where !! is the C content in mass %, !!"#$ is the dry bulk density in g cm-3,
and !" is sedimentation rate in cm yr-1.
Sampling, preparation, and analyses
To investigate CMARs over the Holocene (Paper I) sediment long-cores
(1m) were sampled with a Livingstone corer from deep areas of the lakes
with maximum sediment thickness. The cores were taken in overlapping
sections and split and sliced into 1-5 cm thick subsamples. For determinations of CMARs rates over the past century (Papers I, II, and IV) sediment
cores were sampled with a gravity corer (UWITEK) from the deepest point
of the lakes. These cores (20 cm) were sliced into 0.5-1.0 cm increments. All
subsamples were freeze-dried and homogenized for further analyses.
C contents, dry bulk density, and sedimentation rates
Carbon and nitrogen contents were measured on subsamples of both longand short-cores, using an elemental analyzer (ECS 4010 Elemental Combustion System, CHNS-O).
To derive the dry bulk density of sediment in long-core samples (Paper I) we
applied a core scanning technique before core splitting. This technique operates by measurements of gamma ray attenuation, which can be translated
into the wet bulk density of the material, from which the dry bulk density
was calculated. For measurements a GEOTEK Multi Sensor Core Logger
(MSCL) at the Department of Geological Sciences at Stockholm University
was used. The dry bulk density of sediment in short cores was determined
manually on 1 cm3 subsamples.
Sedimentation rates in long-core samples were derived through radiocarbon (14C) dating of macrofossils or bulk sediment samples taken at different
core depths. The 14C ages were measured on chemically pre-treated samples
by accelerated mass spectrometry (AMS) at the Ångstöm Laboratory at
Uppsala University. Calibrations of 14C ages were performed using the
IntCal09 calibration curve (Reimer et al. 2009). Sedimentation rates in short17
core samples were determined by lead (210Pb) dating. The unsupported 210Pb
activity was measured on chemically pre-treated subsamples using gamma
spectrometry. Sedimentation rates were calculated by assuming a constant
rate of supply of unsupported 210Pb to the sediment (Appleby and Oldfield
1978).
Lignin phenol analysis
The molecular composition of lignin phenols was analyzed in long-core sediment samples (Paper I). Lignin is the major component in the cell wall of
vascular plants, and its phenolic composition indicates the source of OC, i.e.,
the plant type, and the degree to which the organic material is degraded. A
set of lignin phenol parameters used to identify OC sources and degradation
state is presented in Table 2.
Table 2. Definitions of Lignin phenol parameters used to assess OC sources and
degradation state in sediment samples .
Lignin parameter
Definition
Source parameters
Xlignin
Λ8
PON/P
S/V
C/V
LPVI
sum of all vanillyl, syringyl, and cinnamyl phenols
(mmol C/mol OC in sample)
sum of all vanillyl, syringyl, and cinnamyl phenols
(mg phenol/100 mg OC in sample)
p-hydroxyacetophenone/p-hydroxyl phenols
(molar ratio)
syringyl phenols /vanillyl phenols
(molar ratio)
cinnamyl phenols /vanillyl phenols
(molar ratio)
= [S(S+1)/(V+1)+1] x [C(C+1)/(V+1)+1]
whereby S,C, and V are in % of Λ8
Degradation state parameters
P/(V+S)
Ad/Al)p
(Ad/Al)v
(Ad/Al)s
18
p-hydroxyl phenols/sum of vanillyl and syringyl phenols
(molar ratio)
p-hydrobenzoic acid/p-hydrobenzaldehyde
(molar ratio)
vanillic acid/vanillin
(molar ratio)
syringic acid/syringaldehyde
(molar ratio)
Lignin phenols were chemically extracted from sediment samples following
the cupric oxide (CuO) oxidation method (Hedges and Ertel 1982). The
sample extracts were measured on an ultra performance liquid chromatography (UPLC) system (Waters Acquity UPLC), using a modified method
after Lobbes et al. (1999).
Sediment and water incubation experiments
OC mineralization rates in water and sediment samples (Papers II and IV)
were determined by laboratory incubation experiments following the experimental setup by Gudasz et al. (2010). Oxic mineralization rates were quantified as the change in DIC concentration in water and in water overlying sediment samples over time. DIC concentrations were measured on a Total Carbon (TC) Analyzer (Sievers 900). For the determination of anoxic mineralization rates the change in CH4 concentration was additionally quantified,
using a gas chromatograph (7890A GC system, Agilent Technologies) for
quantifications. Both water and sediment samples were kept in temperaturecontrolled water chambers during the incubation periods to assure stable
conditions and to assess OC mineralization rates at different temperatures.
Sediment traps
To quantify the sinking flux of OC in the water column (Paper II) sediment
traps were deployed at the deep center of one study lake. The traps consisted
of cylindrical, polycarbonate tubes, which were open at the top and closed at
the bottom. The tubes were placed in tube holders that were deployed at 1 m
below the water surface and at 1 m above the lake bottom. The tube holders
were attached to a rope that was anchored in the sediment and held upright
with a buoy. The traps material was collected about monthly during the icefree study period and once after ice-melt. The trap samples were freezedried, homogenized and analyzed for C and N contents on an elemental analyzer (ECS 4010 Elemental Combustion System, CHNS-O).
Sub-bottom profiling
The morphometry of two study lakes (Papers II and IV) was mapped with a
sub bottom profiler as described in Ferland et al. (2012). Briefly, the instrument simultaneously measures the interfaces of water-sediment and sediment bedrock. The difference between these two layers equals the sediment
thickness.
19
For measurements, a triple beam sub-bottom profiler (BSS+3, Specialty
Devices Inc.), was used that was suspended next to the boat in the surface
water. Measurements were performed in about 5 m distances on transects
across the study lakes and the data points were spatially interpolated.
Monitoring of lakes and streams
Water column monitoring and the diel DO technique
Temperature, dissolved oxygen (DO), pH, and conductivity were automatically and continuously monitored in the water column of two study lakes at
the deepest point (Papers II and IV). Temperature sensors were deployed at
0.5-1.0 m increments throughout the water column, while DO, pH, and conductivity were measured in surface and bottom waters. Logging was performed over at least one annual cycle including the ice-cover period.
Surface water DO concentration was logged at 20 min intervals, and the
diurnal patterns of variability in DO were used to model gross primary production (GPP), net ecosystem production (NEP), and respiration (R) following Staehr et al. (2010).
Stream monitoring of discharge and CO2
To calculate the load of C to and from one study lake (Paper III), discharge
(Q), and pCO2 were automatically monitored in the inlet- and in the outletstream. Q was measured using the salt dilution method (Day 1975) and correlated to water level that was automatically monitored with a pressure sensor. For pCO2 a non-dispersive infrared sensor was used, which was covered
with a polytetrafluoroethylene membrane that is impermeable to water but
permeable to CO2. Logging was performed during the ice-free season of one
annual cycle. Automated measurements were complemented by repeated
manual measurements of pCO2, DOC, DIC, and POC at various locations in
the headwaters of the study lake.
Lake warming simulations
In Paper IV, lake warming was simulated for two lakes to test the effect of
higher water temperatures and altered stratification patterns on the OCBE of
sediments. Using water column monitoring data of temperature and oxygen,
we first assessed present-day conditions in the study lakes and quantified the
basin-wide, annual OCBE. Lake warming simulation were performed according to regional climate change scenarios (Kjellström et al. 2014). These
20
scenarios predict changes in seasonal air temperatures for different regions
in Sweden.
Photochemical modeling
In Paper II, the DOC photomineralization in lake water was simulated. Briefly, the DOC photomineralization was simulated as daily DIC production
following the method and calculations described in detail in Koehler et al.
(2014). For simulations the chromophoric dissolved organic matter (CDOM)
absorption coefficient (m-1) was determined for lake water from absorbance
measurements. The apparent quantum yield of DIC photoproduction Φ (mol
C mol photons-1) of the study was determined earlier in Koehler et al. (2014)
and used here for simulations.
Gas flux determination
General approach
The diffusive flux F of CO2 and CH4 from water to the atmosphere can be
determined by concentration measurements of the respective gas in water
and in air and by the determination of the gas exchange coefficient k according to:
! = !(!!" − !!" )
where F is the diffusive flux of CO2 or CH4 in mg C m-2 d-1, k is the gas exchange coefficient, also termed as the piston velocity, in m d-1, Caq is the
concentration of the respective gas in water, and Ceq is the theoretical concentration of the respective gas in water if it was in equilibration with air.
For lakes, k was derived using wind speed data and the empirical relationship after Cole and Caraco (1998). For streams, k was derived by tracer injections as described below.
Headspace equilibration method
The concentration of CO2 in lake and stream water (Papers II and III) was
determined by measurements of the partial pressure of CO2 (pCO2) using an
infrared gas analyzer (IRGA, EGM-3) and the headspace equilibration
method according to Sobek et al. (2003). Briefly, water was sampled in polyethylene syringes and equilibrated with ambient air by shaking. After shaking the equilibrated air was analyzed on an IRGA. The pCO2 of ambient air
21
was also measured during sampling campaigns to correct for the pCO2 in
equilibrated air samples. The pCO2 and CO2 concentration in water was calculated according to Weiss (1947) and using Henry´s constant.
Floating chamber method
CO2 and CH4 fluxes from lakes (Papers II and IV) were measured by floating chambers, which were placed in transects across the lake surface to cover
different depth zones. Briefly, the flux of the respective gas was determined
as the change in gas concentration in the chamber air over time. CO2 chambers were equipped with CO2 mini-loggers (CO2 Engine® ELG, SenseAir
AB), and the flux was determined as the rate of change in CO2 concentration
over 30 minute measuring periods. CH4 chambers were placed on the lakes
for 24 h periods and the change in CH4 concentration was determined from
initial and final air samples that were extracted from the chambers by syringes and measured on a gas chromatograph (GC-FID; Shimadzu GC-8,
PoropackN column).
Determination of k in streams by propane injections
For determinations of k in streams (Paper III) we used propane (C3H8) as a
volatile tracer gas. Briefly, gas injections were performed at three stream
reaches at different discharge (Q) conditions for which the reach travel time
(τ) was determined by the salt dilution method according to Day (1975).
C3H4 was injected to the streams through an air curtain upstream of the sampling points and 10-15 min prior to sampling to achieve steady state conditions. Stream water samples were taken from upper and lower reach ends in
polypropylene syringes and according to the travel time τ. The headspace
equilibration method described above, was applied to transfer C3H8 from
water samples into air samples, which were measured on a gas chromatograph (7890A GC system, Agilent Technologies). The gas transfer coefficient k for C3H8 was calculated according to Genereux and Hemond (1990)
modified by Wallin et al. (2011), and the kCO2 was calculated following
Jones and Mulholand (1998) and Wanninkhof et al. (1990).
22
Results and Discussion
Role of sediments for C cycling in a lake and catchment
perspective
Burial and mineralization of OC in lake sediments
In Papers II and IV we investigated two small lakes with respect to OC burial and OC mineralization in sediments. The quantification of burial and mineralization in these lakes was performed on a whole-basin scale and over an
entire year, in order to account for spatio-temporal dynamics of the C fluxes.
The results of these studies illustrate, that OC mineralization in lake sediments dominates over OC burial, if accounting for the spatio-temporal variability of C fluxes over the year. OC burial in the study lakes (0.3 and 0.5 t C
yr-1, respectively) was found to be about one third of sediment OC mineralization (1.0 and 1.2 t C yr-1, respectively), which implies that about 25% of
the OC that reaches the lake bottom is buried in the sediments. The results
were remarkably similar for these two lakes and suggest that sediments of
small boreal lakes are a stronger source than sink of C. However, it is important to state that even the small of OC burial flux in these lakes represents
a permanent removal of C from the short-term C cycle.
Sediment C fluxes vs. emission C flux from lakes
In addition to sediment C fluxes, the total annual emission of CO2 and CH4
was quantified for the two study lakes (Papers II and IV). Annual CO2
emission equaled 6.4 and 6.9 t C yr-1, and the CH4 emission was estimated at
0.08 and 0.03 t C yr-1, respectively. Hence, the total C emission was 21- and
13-times higher than the respective annual OC burial flux of each lake,
which agrees with values (range, 4-86; mean, 30), reported in a large scale
study of boreal and arctic Finnish lakes (Kortelainen et al. 2013).
Our results also demonstrate that sediment OC mineralization, i.e., the
production of CO2 and CH4 in sediments, can in total account for a significant share of the annual C emission (16 and 17%, respectively). In this perspective, sediments are considered to be the major source of the CH4 flux to
the atmosphere, as the production of CH4 in lakes is mostly restricted to the
anoxic environments found in the sediments (Bastviken 2009). However, the
23
sediment CO2 production could not account for the 6 times larger amount of
CO2 emitted to the atmosphere on an annual scale. This finding was surprising since previous studies have pointed out sediments as a major source of
CO2 emission from boreal lakes (Kortelainen et al. 2006), and given the shallow basin morphometry of the study lakes with a high share of sediments
located in warm and oxygen-rich epilimnetic waters, which stimulate sediment OC mineralization (Sobek et al. 2009; Gudasz et al. 2010).
a)!
-present -!
Lake Erssjön!
Lake Gäddtjärn!
6.4 !
6.9 !
1.2 !
mineralization!
SEDIMENT!
burial!
1.0 !
SEDIMENT!
0.5 ! 0.3!
b)!
-future- !
% change!
Figure 3. Comparison of OC burial and mineralization in lake sediment to total C emission
-1
for two small boreal
40! lakes (Papers II and IV). Numbers express C flux in t C yr .
H
20!
L!
M
H
L!
M
Role of sediments in an annual lake C budget
0!
In Paper II, we investigated the full annual C budget of a small boreal lake
(Figure 2) in-20!
order to put OC burial and mineralization in sediments into
perspective to other C fluxes in the lake. In doing so, the importance of different CO2 sources
-40! for the annual lake CO2 emission could be assessed.
!"#$%&'()#
We found that OC mineralization in the
water column of the lake (2.4 t C
-1
!"#*'+,&()'-(./+##
than
twice
the
annual
sediment
OC mineralization and acyr ) was more
-60!
"#,*'00'/+#
counted for about 40% of the annual CO
emission.
About 16% of the OC
2
mineralization in water was ascribed to photochemical OC mineralization,
which amounted to about 0.4 t C yr-1, and hence was alone larger than annual
OC burial in the lake. Most OC mineralization in water was however attributed to the net heterotrophic character of the lake, with low primary production but high respiration of OC. The dominance of water column DOC
mineralization over sediment OC mineralization in this lake can be explained by similar mineralization rates in 1 m2 of sediment (mean, 69 mg C
24
m-2 d-1 at 15°C) as in 1 m3 of water (48 mg C m-3 d-1, respectively) in combination with the bathymetric properties of the lake with 3.8 m3 of water per 1
m2 sediment area.
The contribution of the different OC mineralization processes to the gain
in lake CO2, however, varied over the annual cycle. OC mineralization in
water dominated over OC mineralization in sediments during the ice-free
season, whereas during months of ice cover, sediment and water contributed
about equally to the overall OC mineralization in the lakes.
OC mineralization in sediment and in water accounted together for about
50% of the annual CO2 emission from the study lake. To identify and relate
the remaining CO2 sources in the lake C budget, we investigated the supply
of C to the lake from the surrounding catchment (Papers II and III). The
import and export of C via fluvial stream transport (13.5 and 14.5 t C yr-1,
respectively) was found to dominate quantitatively over all other C fluxes in
the lake. Furthermore, the inflow of shallow groundwater supplied a substantial amount of C to the lake (5.1 t C yr-1), which illustrates the strong influence of the catchment for the C balance in the lake.
emission!
6.5!
stream import!
stream export!
14.5!
0.4!PM!
2.0! WM!
GW import!
1.0!
SM!
13.5!
5.1!
burial!
0.3!
Figure 2. A simplified annual C budget of Lake Gäddtjärn (Paper II) showing the means,
expressed in t C yr-1, of OC burial, sediment OC mineralization (SM), net water OC mineralization (WM), photochemical OC mineralization (PM), C emissions, C import from the
catchment via surface water and groundwater, and C export from the lake via the outlet
stream.
25
Comparison of lake- and catchment-scale C fluxes
from headwaters (9.8 g C m!2) was
44% higher than the export of organ
and inorganic C from GD outlet (tota
5.4 g C m!2), and more than 3 times
greater than CO2 evasion from Lake
Gäddtjärn (2.7 g C m!2).
In Paper III, the headwater systems and the outlet stream of the study
lake in
4. Discussion
Paper II were explored with respect to fluvial C transport downstream
and
In this study, we show that CO2
CO2 emission to the atmosphere. This study was carried out in order
to
comevasion from headwater systems an
pare C losses from the aquatic network upstream of the lake with especially
C lossesstreams
of can be the
dominant pathway of C loss within th
the lake itself.
continuum of a boreal lake
C loss by CO2 emission from headwater systems was found toaquatic
be higher
Figure 5. Distribution of C flux pathways of the different C species for
catchment, even at small spatial
than the total
loss
of reaches
the study
via water-sampling
fluvial downstream
export and
the twoCinlet
stream
duringlake
the open
period,
scales. Streams covered only about
standardized
to
upstream
drainage
area.
Error
bars
represent
the
minimum
atmospheric emission (Figure 3). The largest source of aquatic CO0.1%
2 emission
of the Lake Gäddtjärn catchme
maximum estimate obtained from cumulative standard error of
from the and
entire
catchment
to
the
atmosphere
was
the
headwater
areastreams,
yet were responsible for a C los
discharge and concentration measurements for DOC and DIC, and
despite their
smallstandard
areal errors
coverage
the catchment
(<0.1%). Theto high
CO2 greater than the
cumulative
for tracerof
injection
replicates and concentration
the atmosphere
for CO
2 evasion.
sum of in
all Cthe
losses from Lake
emission measurements
from streams
was
mostly explained by the large difference
Gäddtjärn
(including
CO2 evasion
gas transfer velocity k, which was on average 30 times higher in streams
from
the
lake
and
DOC
and
DIC
export
via
the
outlet)
(Figure
6).
At
larger
spatial
scales,
several
studies have
than in the lakes.
shown that stream evasion dominates total aquatic evasion and downstream export of carbon [Huotari et a
Hence, in a catchment perspective lake C fluxes played a secondary role,
2013; Lundin et al., 2013; Wallin et al., 2013]. At the global scale, Raymond et al. [2013] estimated that stream
when comparing
the overall C loss from the system, which illustrates the
and rivers evade 6 times more CO2 to the atmosphere compared to lakes and reservoirs. Our study adds to
importance
of addressing
fluxes
on integrated
scales.
Including
upstream
current
knowledge byCusing
measurements
of all relevant
parameters
in both
streams and lakes within one
aquatic emissions
further
illustrates
that onlyspatial
a very
small variability
fractioninof
the
catchment, and
by accounting
for pronounced
and temporal
C concentrations
and fluxes,
show
that stream
dominates
aquatic
C loss even
a small
spatial scale. The only comparable stud
total C that
enters
borealCOsurface
will
be buried
in atthe
sediments.
2 evasion waters
was reported from a small boreal
lake catchment in northern Finland,
similar to our lake catchment
but with a proportionally larger
wetland coverage, and that study
showed that downstream C export v
streams was larger than stream C
evasion [Juutinen et al., 2013]. The
lower importance of stream CO2
evasion in the Finnish catchment ma
be related to higher share of wetland
pointing toward the role wetland
systems might have on C export. In
our study catchment we have a
wetland coverage (14%) closer to th
national wetland coverage in Swede
(15%), thus making our study
catchment more representative for
Figure 3. C
losses
the
catchment
of Lake Gäddtjärn
(Paper
III) waterduring the
open-water
Sweden.
In addition, our study
Figure
6. Cfrom
losses
from
the Lake Gäddtjärn
catchment for
the open
sampling period.
Error
bars
representto the
minimum
estimate obtained
sampling
period,
standardized
catchment
area forand
lakemaximum
Gäddtjärn and
catchmentfrom
has a different topograph
area.
Error bars represent
the minimum
and maximum for DOC
cumulative upstream
standarddrainage
errors of
discharge
and concentration
measurements
DIC,
with and
steeper
slopes than the Finnish
estimate
obtained
from of
cumulative
standard errors
of discharge
and wind speed modeland cumulative
standard
errors
tracer injection
replicates
(streams),
catchment,
resulting
in higher k
measurements for
DOC and DIC, for
andCO
cumulative
standard
derived kCO2concentration
(lakes) and concentration
measurements
2 evasion.
[Wallin et al., 2011] and hence more
errors of tracer injection replicates (streams), wind speed model-derived
CO2 evasion from streams.
kCO2 (lakes), and concentration measurements for CO2 evasion.
©2014. American Geophysical Union. All Rights Reserved.
KIC ET AL.
26
1
Temporal variability of the lake sediment C sink
Variability in OC accumulation over the Holocene
In Paper I, we analyzed vertical sediment profiles of seven Swedish boreal
lakes for centennial- and millennial-scale OC accumulation rates, i.e.,
CMARs, to indicate changes in OC accumulation over time.
Both centennial and millennial CMARs exhibited variability over time,
i.e., over sediment core depth (Figure 4), which was mostly attributed to the
variability in sedimentation rate. However, while the millennial variability in
CMARs seemed to be related to lake size, with smaller lakes showing higher
variability (regression of lake area against standard deviations of CMARs;
R2=0.63, p<0.01, n=7), there was no such pattern obvious for CMARs on a
centennial time scale. Furthermore, there was no common temporal trend
observed for millennial CMARs across lakes, however, in six out of seven
lakes centennial-scale CMARs increased towards recent time.
The larger variability in millennial CMARs in the smallest lakes was explained by the more dominant impact of local-scale changes, while the larger
lakes exhibit more resilience towards changes in the local environment.
Changes in OC accumulation in the larger lakes might therefore reflect rather regional-scale changes, such as climatic shifts over the Holocene (Seppä
et al. 2005).
The trend of increasing CMARs over the last century has been observed
by several other studies investigating OC accumulation over time
(Kastowski et al. 2011; Anderson et al. 2013). These studies concluded that
recent changes in CMARs are caused by changes in land use and associated
soil erosion. For the investigated lakes of this study this explanation is, however, unlikely since no intensified agriculture took place in their catchments
over the past century. Instead, the effect of re-forestation during the twentieth century, after a long history of mining activities in central Sweden
(Eriksson 1960), might be reflected here in elevated CMARs. Also, increased leaching of OC from soils as a consequence of recovery from acidification might be responsible for a larger quantity of OC supply to lakes in
the more recent past (Monteith et al. 2007; Bragée et al. 2015).
27
SV
0
1
MSV_rate
SV_rate
GD
0
1
1
year_DA
year_OV
year_OP
GR
0
LS
0
1
time_DA
time_DA
time_SV
time_DA
time_SV
time_OV
time_OV
time_SV
time_OV
time_SV
time_OP
time_OP
time_SV
time_OP
time_SV
time_GR
time_GR
time_SV
time_GR
time_SV
time_GD
time_GD
time_SV
time_GD
time_SV
year_LS
year_GR
year_GD
0
8000
2000
6000
4000 time_LS
time_SV
time_LS
time_SV
time_LS
OP
0
1
time_SV
time_SV
time_SV
year_SV
OV
0
1
MOV_rate
MGD_rate
GD_rate MGR_rate
GR_rate
MLS_rate
LS_rate MOP_rate
OP_rate
OV_rate
DA
0
MDA_rate
DA_rate
Figure 3. Short-term (a-g) and long-term (h-n) CMARs in sediments profiles of seven study
lakes in central Sweden (Paper I). Lakes are in order of increasing surface area from left to
right panel.
Moreover, it can be discussed whether the temporal variability in OC accumulation reflects a changing strengths of the sediment C sink over time, or
if periods of higher OC accumulation indicate periods of higher OC input
and therefore also higher C emission if assuming that the OC burial efficiency remained stable.
Sources and stability of terrestrial OC in boreal lake sediments
The vertical sediment profiles in Paper I were investigated for terrestrial
sources and the degradation state of OC by analyzing the molecular composition of lignin phenols in the sediments.
The analysis revealed that wood-containing material from gymnosperms
was a constant and dominating source of OC to the lakes over the Holocene.
This was indicated by different lignin phenol parameters, such as low S/V
and C/V values throughout the entire sediment core profiles. Moreover, the
lignin phenol indices Ad/Al and P/(V+S), which are indicative of the degradation state of the OC, reflected that no consistent degradation of the organic
material was detectable even though the organic matter has resided in the
sediment for thousands of years. Together with the CMARs, these results
indicate that OC is buried in sediments within the first century of deposition
and stabilized on a permanent basis.
28
OC burial efficiency of lake sediments: now and in the future
In Papers II and IV, the basin-wide OC burial efficiency (OCBE) of sediment was quantified for two small lakes with differing basin bathymetry.
Additionally, in Paper IV, the effect of lake warming on the OCBE was simulated for both lakes following different climate change scenarios.
The whole-basin OCBE was found to be low under present-day conditions, and of similar magnitude in both study lakes (28 and 25%, respectively). Different lake warming simulations caused consistent decreases in the
OCBE of on average 5-16% and 3-11% in the two study lakes respectively
(Figure 4). The more shallow lake exhibited in general higher declines in the
OCBE, related to more sediment area being affected to changes in epilimnetic water temperatures. The simulated extent of hypolimnetic anoxia during summer stratification counteracted the effect of increased OC mineralization in epilimnetic sediments only to a minor degree.
Hence, this study illustrates that the single effect of warmer water temperatures in lakes may reduce the efficiency of sediments in storing OC in
the future, however the magnitude in OCBE reduction will, among other
factors, depend on morphometric properties of lakes.
whole-basin OCBE (%)!
40!
PD!
30!
Lake Erssjön!
Lake Gäddtjärn!
L!
M!
H!
20!
10!
0!
0!
1!
2!
3!
4!
temperature change (°C)!
5!
Figure 4. Decline of the OC burial efficiency (OCBE) in Lake Erssjön and Lake Gäddtjärn in
response to lake warming, calculated following climate change scenarios for each lake region.
Temperature on the x-axis is predicted increase in mean annual temperature. PD=present-day
conditions, L=low-, M=medium-, H=high-case scenario. Future projections assume no change
in OC deposition onto the sediment surface. Error bars indicate minimum and maximum
estimates of 95% confidence intervals.
29
Conclusions and Perspective
This thesis provides an insight into the role of lake sediments in the C cycle
of boreal lakes on integrated spatial and temporal scales. By exploring sediment processes and C fluxes within lakes and across the aquatic network,
and over long and short time periods, we put sediments into a larger perspective and elucidate their function for C cycling from the past to the future.
The major conclusions of this thesis are that:
1) OC burial in sediments represents one of the smallest aquatic C
fluxes at both the lake and the catchment scale, but is the only flux
that continuously withdraws C from the active cycling loop.
2) Sediment OC mineralization is not the dominating CO2 source for
CO2 emission, even in a shallow lake, given the strong heterotrophic
character of the water column and the dominating influence from the
lake catchment.
3) At a catchment scale, C emission in headwaters dominates greatly
over the entire C loss from a small boreal lake via emission and fluvial export.
4) OC accumulation in lake sediments varied differently in boreal lakes
on a millennial time scale, however, similar patterns in OC accumulation are visible in most investigated lakes on a centennial scale.
5) Allochthonous OC is buried and stabilized in sediments within the
first century of deposition.
6) The OC burial efficiency of sediments is low if integrated on a
whole-basin scale and is likely to decrease in warming lakes in response to future climate change.
Future studies that address the role of sediments for C cycling in boreal lakes
may focus even more on the spatio-temporal dynamics of CH4 in lake C
budgets given its strong greenhouse gas effect, despite the comparatively
small C flux. Moreover, studies that investigate OC storage could operate
more on a catchment-scale and investigate the role of headwaters as first OC
storage sites. Finally, studies could explore the temporal variability of the
OC burial efficiency in sediments over the past, by combining investigations
of OC burial rates with paleolimnological proxies that are used to reconstruct
the past environment of lakes. This would further help to better project and
assess the role of lake sediments as C source and C sink in the future.
30
Sammanfattning på Svenska
Det finns ungefär 120 miljoner sjöar på jorden. Trots att de sammanlagt
täcker bara drygt 3% av den del av kontinenterna som inte är täckt av inlandsisar, har de en viktig funktion i det globala kolkretsloppet. Grundämnet
kol finns i allt organiskt material, t ex växtdelar och humus från skog och
mark. Mycket av detta organiska material sköljs ner till sjöar och vattendrag,
där det antingen sjunker ner och lagras som sediment på sjöbotten, eller bryts
ner av mikroorganismer. Denna nedbrytning av organiskt material leder slutligen till att växthusgaserna koldioxid och metan bildas. En del av den koldioxid och metan som bildas i sjöarna når atmosfären vilket medför att sjöar
påverkar klimatet. Å andra sidan kan det organiskt materialet i sedimenten
bevaras där under mycket lång tid – sjösedimenten har kontinuerligt byggts
upp sedan sjöarna bildades. I svenska sjöar har detta pågått sedan landskapet
blev isfritt efter den senaste istiden, dvs ungefär tio tusen år, på andra håll
betydligt längre – i Tanganyikasjön är till exempel de äldsta sedimenten
ungefär 10 miljoner år gamla. Genom att sjöar avger växthusgaser till atmosfären samtidigt som de utgör ett förvar av kol i sedimenten, utgör de samtidigt både en kolkälla och en kolsänka. Summan av de dessa båda processer,
dvs. inlandsvattnens kolutsläpp till atmosfären och deras kolinlagring i sedimenten, är en omsättning av kol lika stor som landväxternas samlade nettoupptag av koldioxid. Det är uppenbart att inlandsvatten spelar en viktig roll
i hela landskapets och planetens omsättning av kol.
Speciellt i den nordliga barrskogsregionen, som sträcker sig genom stora
delar av Sverige, är inlandsvatten viktiga. Jämfört med andra regioner på
jorden finns här osedvanligt många sjöar, och dessutom exceptionellt mycket
organiskt material i skogsmarkens och myrarnas humuslager. I den nordliga
barrskogen har det exempelvis visats att mer kol lagras i sjösediment än i
växter och mark i den omkringliggande skogen. Vidare svarar alla miljontals
sjöar som ligger insprängda i barrskogslandskapet för en viktig del av koldioxidusläppet från jordens inlandsvatten.
I denna avhandling har jag undersökt sedimenten i olika svenska skogssjöar för att utröna följande frågor:
1)
Hur stor andel av det kol i organiskt material som sjunker ner till
sjöbotten inlagras där, och hur stor andel bryts ner av mikroorganismer till koldioxid och metan?
31
2)
3)
4)
Hur mycket av en sjös utsläpp av växthusgaser till atmosfären
härstammar från sedimenten? Vilka andra processer bidrar till en
sjös utsläpp av växthusgaser?
Hur mycket har inlagringen av kol i sjösedimenten varierat sedan
den senaste istiden? Hur stabil är kolsänkan i sjösediment?
Hur kommer ett varmare klimat påverka sjösedimentens effektivitet som kolsänka?
För att kunna besvara dessa frågor studerade vi ett antal olika skogssjöar i
centrala och sydvästra Sverige. Samtliga sjöar är typiska för det svenska
barrskogsområdet och kännetecknas av att vattnet är brunt av humusämnen
från omgivningarna.
I två av sjöarna placerade vi sensorer i vattnet som gav oss kontinuerliga
mätvärden under ett års tid som är viktiga i samband med kolomsättningen,
bland annat temperatur och syrgas löst vattnet. Koldioxid och metan mättes
även direkt i sjöarna, och vi beräknade hur mycket av dessa växthusgaser
avges till atmosfären. Vid en av sjöarna följde vi även halten av koldioxid
med liknande sensorer, placerade i bäckar och i grundvatten. På så vis kunde
vi undersöka betydelsen av tillförseln av koldioxid, jämfört med hur mycket
som produceras i sjön.
Vi undersökte sedimenten med den särskild provtagare som tar upp ett
ostört prov på hela sedimentet i ett långt plaströr. I dessa prover mätte vi kol
och andra organiska ämnen, och bestämma sedimentens ålder bland annat
med hjälp av kol-14 metoden. Vi kunde sedan beräkna i vilken takt materialet har lagrats på botten, och hur takten varierat över tiden. Vi gjorde också
experiment med både sediment och sjövatten för att se hur mycket koldioxid
och metan som bildas. På så sätt kunde vi jämföra sjöarnas produktion av
växthusgaser med deras inlagring av kol i sediment – det vill säga jämföra
sjöarnas roll som kolkälla och som kolsänka.
Slutligen använde vi regionala klimatmodeller för att simulera hur en
varmare lufttemperatur påverkar vattnets temperatur och syrehalt, för att
därifrån göra en prognos över hur inlagring av kol i sjösediment kommer att
förändras i framtiden.
Denna avhandling visar att sjösedimenten i små skogssjöar avger ungefär
tre gånger så mycket kol i form av koldioxid och metan till atmosfären än de
lagrar in i sedimenten. Trots att kolinlagringen är förhållandevis liten så pågår den kontinuerligt, och utgör därmed en viktig kolsänka över längre tidsperioder. Även om sjösedimentens växthusgasproduktion var större än deras
kolinlagring, bidrog sedimenten bara med en liten andel (~15%) av sjöns
utsläpp av koldioxid till atmosfären. Andra processer, såsom nedbrytning av
löst humus i vattnet och inflödet av löst koldioxid via bäckar och grundvatten hade större påverkan på sjöarnas utsläpp av koldioxid. Metan utgjorde
bara en liten del av sjöarnas kolomsättning, men eftersom den är en mycket
32
kraftigare växthusgas än koldioxid är den ändå viktig för sjöarnas effekt på
klimatet.
Inlagringen av kol i skogssjöars sediment varierade över tid. Sedan den
senaste istiden (upp till 9000 år sedan) var variationen större i små sjöar
jämfört med större sjöar. Under de senaste 100 åren uppvisade de flesta sjöarna liknande trender. Detta tyder på en ökad kolinlagring, möjligen som en
följd av ökad urlakning av humus från skogsmark. Analys av sammansättningen av molekyler på olika djup i sedimenten visade inga tydliga tecken på
nedbrytning, oberoende av hur mer i sedimentet proverna togs – det vill säga
hur gammalt det är. Detta pekar på att skogssjöars sediment utgör en stabil
kolsänka. För att förutspå effekterna av ett framtida varmare klimat på sedimentens kolinlagring simulerade vi temperatur och syresättning i två små
skogssjöar utifrån klimatmodellernas prognoser. Dessa simuleringar visade
att kolinlagringen i skogssjöars sediment sannolikt kommer att bli mindre
effektiv i framtiden.
Sammantaget visar denna avhandling att sedimenten spelar en förhållandevis
liten roll i skogssjöarnas omsättning av kol, och bidrar med bara en liten
andel till sjöarnas utsläpp av kol till atmosfären. Å andra sidan är de en stabil
kolsänka, och är därmed av stor betydelse för barrskogslandskapets långsiktiga kolbalans. Ett varmare klimat kommer däremot sannolikt göra sedimentens kolinlagring mindre effektiv.
33
Acknowledgements
Many people have contributed to this work in one or the other way, and even
more people are responsible for making these last 4.5 years in Uppsala a
wonderful and memorable time for me. This thesis would certainly not have
been possible without all the great help, support, and the friendship that have
accompanied me on this PhD journey, and I would like to express my greatest thanks to all of you.
First of all, I would like to thank my supervisor Sebastian for being the
initiator of this thesis, and for giving me endless support and help during the
entire process. Thank you Sebastian, for making all this happen. For letting
me come to Sweden, and not sending me back again after one week, when
you maybe realized that I only knew stones. Instead, you supported and
helped me in learning about limnology, and I truly appreciate all the time
you took for explaining the world of carbon, lakes, and science as itself to
me. Thank you for believing in me when I doubted myself sometimes, and
for always having an open ear for my questions and thoughts. You deserve
some price for supervision in my mind (and certainly not the golden cod,
that one we both shared anyway ;-). Thank you so much for everything during this PhD time, I really appreciate it.
Secondly, I would like to thank my co-supervisors Lars and David for
their great support and inspiring thoughts to this work. Lars, thank you also
for giving me the opportunity to participate in COW. It has been an inspiring
environment, from which I have learned a lot. David, thank you for your
support within and beyond the LAGGE projects, for all your help with the
methane parts, and also for the amount of pancakes that you provides once to
everyone in Skogaryd.
Thirdly, lots of thanks go to Jutta and Thorsten, and the entire lab crew in
Oldenburg, for their great support and help in this work, and to all the coauthors without whom this thesis would not have reached this level .
Furthermore, there are many people at the Limnology department, whom
I would like to thank for their help and friendship during the past years, and
for creating this warm environment, where I have always enjoyed working.
Many thanks to the scientifically older generation in this corridor, Gesa,
Silke, Eva, Anna, Don, Peter, Alex, Stefan, and Janne, you all have always
been very kind to me. Thank you for making this department a great place to
be. Janne thank you as well you for your help and support during field work
out on the snowy lakes, and the great minced moose with mashed potatoes.
34
Eva, thank you for the great time in the fat-camp, it was truly fun every year.
Though I am glad to be done with the fish. And thanks for the star!
Jovana, thank you for being such a great friend, and that right from the
beginning. Without you, I would not have survived my first year in Sweden,
and you probably saved me many more times from getting lost in the forest
or in the lake than I realize. Torsten, thank you for so many things, for being
a good friend, for helping me to survive at Limno as a fish teacher, for all the
great B-movie nights, and the fun time in Flogsta. Monica, thank you for
your great friendship, for all your creative and inspiring ideas, and for all the
fun time we spend in Uppsala. Valerie, you are an amazing person and sharing an office with you, Jingying, and Brad during the last year was a wonderful insanity, which kept me sane somehow. Thank you for your friendship
and all the brain-food you shared with me. Jingying, thank you as well for
the last year and for the fun time at Central Badet. I wish you all the best for
the next PhD years. Blaize, it was a very very very very very very good decision from you that summer to move to Uppsala and I will never forget the
great time we had together in this funny flat full of random stuff. You are
such a good friend, so full of energy, and have all these great ideas. Brussles
can truly be proud of you! Anne, I miss your laughter and your colourfulness
here already. You are a great person, a good friend, and this PhD time was
much fun with you. Please remind me to bring you some liver pate when I
come to visit the Kingdom of Tallahassee. Francois, hopefully we will meet
there again. It was always nice having you around here at Limno, with your
positive attitude and this great laughter. Alina, you are a great friend. The
time with you and Jovana in New Orleans and New York is one of my favorite memories of the last years. And if we ever in the future get to go back to
the Empire State Building, let's bring a bottle of hair spray J. Roger, you
have always been a great help to me, in many ways, and beyond the universe
of GIS. Thanks for being a good friend, for sharing this never ending stock
of chocolate in your office with me, and for being someone that understands
how to distribute keys. Maria, thank you for being a good friend and for the
activation energy you always gave me to go to Yoga. I am sure your cats
miss me a lot, so I will try to find a way some day to visit you in Australia.
Yinghua, you have saved me many times with chocolate. Thanks for being
such a nice and positive character. Dolly, Andrea, Moritz, Inga, Nuria, and
Annika, thanks for the many nice evenings at Fyrishov, and the great philosophical and non-philosophical talks in the hot tap. Annika, thank you a lot
for many fun times since you arrived here in Uppsala. And for the milk
foamer J. Dolly, thank you for just being such a nice person. I miss you at
the department. Hopefully we can meet more often now again. Nuria, thanks
for being a great friend, and a very inspiring person, and also for the nice
time in Barcelona. I will not forget the fun beach time at the Costa Brava,
and the wonderful evening in Girona! Andrea, thanks for being such a kind
and caring person. Anastasija, I have to thank you for la lot of things, because you were of amazing help in the field and in the lab, and thank you
35
also for the many hugs! Frederike and Lorena, you are both so nice and lovely characters, I am glad I got to know you. Simone, Karolína, you are a great
party organizing team, and I wish you both luck for the next steps in life.
Marcus, thanks for being a very kind person and for your great help out in
the field. Sari, Pilar, Heli, Kristin, Raquel, Omneya, Leyden, I am glad all
you found your way to Uppsala. Omneya, thank you also for the nice evenings with Egyptian food. Martin, thank you for being the initiator of the
Swedish Fikas, and for your endless patients with all the non-native ones.
Birgit, thank you for the very first weeks here in Uppsala, you were the first
one that provided me with a place to stay. Cristian, thanks for paving the
way, much of my work is based on yours, and I got a lot of inspiration from
your thesis. Nice to have you back here in town! Anna, it is nice to have you
back here too, after a long time. I am glad to see how people return, and are
still the same nice person. Christoffer, great to have you here too, and you
know, I share your passion for Mozzarella.
Many thanks go to all the former Limnos, that gave me a great start and
many wonderful memories here in Uppsala. Thank you Jason, Mercè, Inga,
Ina, Jerôme, Hannes, Pia, Phillip, Jürg, and Charles. Mercè and Anne again,
thank you as well for many nice times at your Flogsta place with pancakes,
pa amb tomàquet, and lots of more food and fun. Thanks Jason, for being a
great friend. I will always remember the great Cocktail party at your place,
when suddenly, the red carpet was on fire! Good luck for your PhD, and
hopefully we meet soon again! Thanks also to the many master students that
haven been around and contributed to make this department a fun place. To
all the other people, that arrived this year at the department, and will continue or start their PhD/postdoc/etc.: Great luck, and most of all, enjoy!
I would also like to thank everyone outside the Limnology department
that I got to know here in Uppsala: Thank you Eva, Anna, and Rob for all
the fun time, midsommar celebrations, and beach volley ball matches. Eva,
thank you too, for great road trips and evening swims in Skogaryd. Andrea,
Sara, and Ieva, you all were great flatmates and I really enjoyed the time
living with you in Flogsta. Johan, thank you as well for the fun party nights
next door. Brian, thanks for many fun nights out with lots of beers and also
for your help with all the sediment stuff. Will we ever write that paper?
Christina, thank you (and Roger of course again) for many fun times at your
place with cheese fondue and poker nights. Johnny, thank you for being a
great friend, the many fun nights out, and the amazing trip to Härjedalen.
Salar, thanks for what you managed to start in Granada, and for lots of fun
parties at your place. Axel and Kat, thank you for all the great evenings,
celebration times, and summer days by the lakes. Mirco and Rebecca, thank
you for bringing someone to Uppsala, and also for the nice time with wine,
whisky and crêpes in Gottsunda!
Alex, thanks you for your love, the great support, and patience with me
during the last six month, I do not know how I would have managed this
time without you. So glad you found your way up north. Thank you!
36
Und nun noch ein paar Zeilen auf deutsch: Ich möchte mich bei allen bedanken, die mich aus der Heimat unterstützt und an mich geglaubt haben.
Ein Dank an alle Freunde, wo auch immer ihr euch momentan in der Welt
befinden mögt. Liebe Münster/Texel-Crew, liebe Steinis, und liebe Oma,
liebe Doina, Viorel, und Anca, danke euch für all die Jahre in denen ihr euch
um mich gekümmert habt, und für die vielen schönen Erinnerungen aus
meine Kindheit. Doina, danke dir besonders auch für all die Jahre in denen
du mit so viel Liebe und guter rumänischer Küche versorgt hast.
Mein allergrößter Dank zum Schluss geht and Mama, Papa, und Max.
Danke für eure Liebe und Unterstützung seit ich mich erinnern kann, ihr habt
mich zu dem gemacht was ich bin. Ein bisschen verschusselt manchmal vielleicht, aber sehr sehr glücklich.
Thank you, tack så mycket, und dankeschön to everyone for all the great
memories.
37
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42
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urn:nbn:se:uu:diva-261157
ACTA
UNIVERSITATIS
UPSALIENSIS
UPPSALA
2015