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Progress in Oceanography 71 (2006) 446–477
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Food webs and physical–biological coupling on
pan-Arctic shelves: Unifying concepts and
comprehensive perspectives q
Eddy Carmack
a
a,*
, Paul Wassmann
b
Department of Fisheries and Oceans, Institute of Ocean Sciences, 9860 West Saanich Road, Sidney, BC, Canada V8L 4B2
b
Norwegian College of Fishery Science, University of Tromsø, N-9037 Tromsø, Norway
Abstract
Perhaps more than in any other ocean, our understanding of the continental shelves of the Arctic Mediterranean is
decidedly disciplinary, regional and fractured, and this shortcoming must be addressed if we are to face and prepare
for climate change. A fundamental ﬂaw is that while excellent process studies exist, and while recent ship-based expeditions
have added greatly to our collective body of knowledge, an integrated and fully pan-Arctic perspective on the structure and
function of food webs on Arctic shelves is lacking. Based on the collective overviews given in Progress in Oceanography xx,
xx–xx, we attempt to address this issue. To build a perspective that inter-connects the various shelf regions we suggest
three unifying typologies aﬀecting food webs that will hopefully allow inter-comparison of regional investigations. The ﬁrst
is for shelf geography, wherein shelves are classiﬁed according to their role in the Arctic throughﬂow. The second is for ice
climate, wherein the various ice regimes are examined for their speciﬁc impacts on food web dynamics. The third is for
stratiﬁcation where it is argued that the source of buoyancy, thermal or haline, impacts production and the vertical carbon
ﬂux. We then address the connection between physical habitat and biota on pan-Arctic (and global climate) scales. This
discussion begins with the recognition that the Arctic Ocean is integral to the World Ocean via its thermohaline (‘‘estuarine’’) exchanges with the Atlantic and Paciﬁc. As such the Arctic and its shelves act as a double estuary, wherein incoming
waters become both lighter (positive estuary), by mixing with freshwater sources, and heavier (negative estuary) by cooling
and brine release. Shelves are central to such transformations. This complex interconnectivity coupling of the Arctic Ocean
to its sub-Arctic (and more productive) neighbors demands that food webs be considered through a macroecological view
that includes an ecology of advection. We argue that the macroecological view is required if we are to understand and
model food webs under forcing along climate gradients. To aid this eﬀort we introduce the concept of contiguous domains,
wherein physical habitats are joined by common features that will allow inter-comparisons of existing and future food
webs over large scales and climatic gradients. Finally, we speculate on the range of possible futures for Arctic shelves based
on the palaeo-record.
2006 Elsevier Ltd. All rights reserved.
q
Wahrlich, es wu¨rde euch bange werden wenn die ganze Welt, wie ihr es fordert, einmal im Ernst durchaus versta¨ndlich wa¨re (Friederich
Schlegel) The rational mind is a faithful servant. The imaginative mind is a sacred gift (Albert Einstein).
*
Corresponding author.
E-mail addresses: [email protected] (E. Carmack), [email protected] (P. Wassmann).
0079-6611/$ - see front matter 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.pocean.2006.10.004
E. Carmack, P. Wassmann / Progress in Oceanography 71 (2006) 446–477
447
1. Introduction
The current state of the Arctic is deﬁned by the permanent presence of frozen water in three forms: sea ice,
glacial ice and permafrost (Fig. 1). The interplay between the shrinkage and expanse of these three ice forms
aﬀects the structure and function of food webs in the Arctic Ocean (AO). In this perspective we adopt the consensus opinion of climate scientists that the earth is entering a prolonged period of warming associated with
greenhouse gas build-up (IPCC, 2001; ACIA, 2004). These forecasts may or may not stand the test of time;
nonetheless, the concepts we discuss are aimed at understanding the general principles of physical–biological
connection and phasing under all scenarios of climate variability, natural or anthropogenic; the warming scenario is chosen for consistency and for its current social relevance.
The structure and function of pan-Arctic ecosystems are intimately related to regional geography, to
wind, ocean and ice dynamics, and to biogeochemical exchange processes (Carmack et al., this volume).
In contrast to the deep AO basins, the surrounding marginal seas of the Arctic contain some of the most
dynamic ecosystems of the world, and are characterized by signiﬁcant spatial and interannual diﬀerences in
ice cover and primary production. The pan-Arctic shelves establish an estuarine-like domain wherein Atlantic (AW), Paciﬁc (PW) and inﬂowing river (RW) waters meet and mix to form Arctic water (ArW) masses
(Fig. 2). Their ecosystems are characterized by tightly connected physical–biological systems and, as such,
are particularly sensitive to spatial and temporal variations in hydrographic properties. The ﬂow of AW
into the AO through Fram Strait and the Barents Sea is variable, but is on average >5 times larger than
inﬂow of Paciﬁc Water (PW) through Bering Strait (e.g. Schauer et al., 2002; Woodgate et al., 2005). This
is reﬂected by the dominance of Atlantic-derived species over wide sections of the Russian shelves (e.g.
Zenkevich, 1963). Some shelves, such as those of the Chukchi and Barents Seas, support food webs that
culminate in large populations of seabirds, mammals and species targeted by regional ﬁsheries. Harvesting
practices, combined with a changing physical environment, will have important consequences for system
sustainability and for northern people, particularly in light of new industries and increased economic
activities.
Climate deﬁnes the prime forcing of Arctic ecosystems, and both observations and models suggest that climate is changing (cf. Sorteberg et al., 2005). Arctic shelf ecosystems are likely to be more sensitive to climatic
perturbations than those of temperate shelf areas because (a) disproportionate warming is expected (e.g.
ACIA, 2004) and (b) these ecosystems are characterized by comparatively few trophic links and low biodiversity (e.g. Sakshaug et al., 1994). Indeed, recent studies have revealed signiﬁcant reductions in Arctic ice cover
at both pan-Arctic (Johannessen et al., 2002) and regional (Shimada et al., 2006) scales, and we may already be
witnessing the early stages of ecosystems on the verge of dramatic change (cf. Grebmeier et al., 2006). Reductions in ice cover thickness, extent and duration, and changes in current patterns and fronts will likely have
Fig. 1. The state of the Arctic is deﬁned by the permanent presence of frozen water in three forms: sea ice, glacial ice and permafrost.
Warming (W) will reduce each of these forms: freshwater discharge, stratiﬁcation and freshwater outﬂow to adjacent oceans will increase
while sea level will rise.
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Fig. 2. The Arctic Ocean with its shelves and basins. The blue arrows indicate places and strength for the inﬂows of Atlantic and Paciﬁc
Waters and the red arrows indicate the outﬂows of Arctic Water.
both gradual (predictable) and catastrophic (surprise) consequences: (a) bottom-up controls (e.g. stratiﬁcation, mixing, and upwelling) will certainly change; (b) keystone predators within a given region may be
recruited, relocated or made extinct; and (c) ecosystems may shift from tight to weak pelagic–benthic coupling.
Changes in the cryosphere will have cascading eﬀects throughout the ecosystem, from altered patterns of primary production, to changes in trophic structure and elemental cycling pathways (e.g. Grebmeier et al., 2006),
to introduction of boreal and displacement of Arctic species (e.g. Berge et al., 2005) to modiﬁcations in oceanic
and atmospheric transport mechanisms (e.g. Olsen et al., 2003; Dickson et al., 2003; Karcher et al., 2003). System perturbations brought on by climate change will interact with human activities, such as ﬁshing, mineral
extraction, oil and gas exploitation and shipping, which will grow signiﬁcantly in the near future. Because
change may be rapid and sweeping, extraordinary and novel measures of conservation may be required to
allow marine animals the resilience to relocate as existing biomes are altered by climate forcing, be it natural,
anthropogenic or both. And as physical and biological systems are altered, we must ask how human systems
will prepare for and respond to such change?
In one of the most inﬂuential books of the 19th century Schopenhauer (1819) shows how humankind’s
vision of the world depends upon perception and will. How consistent and unbiased is our current perception
of the AO? Do our views about the Arctic in general and the Arctic shelves in particular rest upon adequate
perceptions? Below we argue that this is not the case: Arctic marine research is suﬀering from a priori perceptions that obstruct satisfactory pan-Arctic understanding. A more balanced view is needed and this requires
unifying schemes and concepts, which are the main objectives of this contribution.
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2. The need for a pan-Arctic perspective
The traditional view of the world ocean through projections, such as of the Mercator type, gave us the
impression that the polar regions are remote and vast, and that the Paciﬁc and Atlantic Oceans are decidedly
distant and separated from each other (Fig. 3a). However, a projection that shows the mediterranean nature of
the AO suggests that it is relatively small, that it occupies a central world region with a considerable hinterland
and that the distance between the Paciﬁc and Atlantic Oceans is remarkably small (Fig. 3b). Even this simple
comparison of perspectives indicates that our perception of the AO is shaped by the projection we apply.
Research teams investigating the AO typically implement a strategy of south-north ship surveys across the
shelves. A European team will be aware of the signiﬁcant ﬂows of AW and ArW through the Fram Strait and
across the central Barents Sea and of the relatively deep, highly productive and only partly ice-covered waters
in that region (Fig. 4a). They would thus view the AO as strongly inﬂuenced by advection into and out of the
central AO, would study its balance, and would focus biological research on pelagic food webs and exploitable
resources such as ﬁsh, shrimps and marine mammals. Continuing counter-clockwise, a Russian team would
cross the, shallow, wide and highly ice-covered Siberian shelf, strongly inﬂuenced by rivers draining their hinterland, and would study ice formation and the spread of the Atlantic biogeographic province eastward
(Fig. 4b). The focus of biological research would emphasize sediments, the benthos and the fate of terrestrial
matter rather than plankton and ﬁsh. Research teams entering the AO from the Paciﬁc through the Bering
Strait across the Chukchi Sea would naturally pay attention to the eﬀect of the nutrient- and biomass-rich
PW, its eﬀect upon the productivity of this very shallow shelf and the impact of this productivity upon the
Fig. 3. The traditional, Mercator projection view of the World (a) and the space-oblique Mercator projection with the Arctic
Mediterranean at the center (b). In (a) the high latitudes are distorted, to the extent that the poles become as wide as the equator. The
Atlantic and Paciﬁc Oceans are distant and hardly connected. In (b) the Arctic Ocean appears as a node for hemispheric circulation. It
further indicates that there is only short distance at high latitudes between the Paciﬁc and Atlantic Oceans.
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Fig. 4. Maps illustrating how our perception of the Arctic Ocean depends upon our geographic location and view. The European/
Greenland (a), Russian (b), Russian/USA (c) and Canadian/Greenland views. Also indicated are the present limits of summer ice cover
(blue) and that projected for 2050 (red) by Comiso (2003).
most isolated of the Arctic basins, the Canada Basin (Fig. 4c). But, because the inﬂow from the Paciﬁc is
roughly 5 times smaller than that of the Atlantic, and because Paciﬁc waters cover only about a quarter of
the AO’s surface area, the emphasis given to this perspective (Fig. 4c) does not provide a fully holistic understanding of the AO. Finally, Canadian or Greenlandic teams would study the ﬂow of ArW and PW through
the Canadian Archipelago and into the Labrador Sea and North Atlantic (Fig. 4d).
From the above examples, any generic idea about the food webs and ecosystem dynamics of pan-Arctic
shelves would reﬂect the sector through which the AO is examined. The diﬀerent perspectives in Fig. 4 are
the unavoidable consequence of sector-based teams attempting to explore the AO by traversing the bordering
shelves. We obviously have a priori perceptions of the AO that obstruct satisfactory pan-Arctic
understanding.
To overcome the constraints of the sector perception that at present characterizes Arctic marine research,
and to prepare for developing a deeper understanding of the least known, but climatically probably most signiﬁcant region of the World Ocean, we here propose some necessary ﬁrst steps toward a pan-Arctic perspective. Such a perspective must reﬂect the mediterranean nature and structure of the AO. In a pan-Arctic
perspective the diﬀerent shelves form the inter-connected periphery of the deep Arctic basins, and the distinction between the eastern and western Arctic is discarded. Another characteristic of the pan-Arctic shelves is
that they span a large range of latitudes and thus of climate forcing: the Bering/Chukchi from 55 to 75N,
the Barents from 70 to 82N and the shelves of Greenland from 59 to 84 (cf. Rysgaard et al., 1999).
Our goal here is thus to summarize and systematize existing knowledge and to present unifying concepts of
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Fig. 5. The nine pan-Arctic shelf regions where the structure and function of contemporary food webs are reasonably well known and
which are summarized in the following review publications: (1) Fram Strait/western Spitsbergen (Hop et al., this volume); (2) Barents Sea
(Wassmann et al., this volume); (3) Kara Sea (Hirche et al., this volume); (4) Laptev Sea (Schmid et al., this volume); (5) East Siberian and
Chukchi Seas (Grebmeier et al., this volume); (6) Beaufort Sea (Dunton et al., this volume); (7) Canadian Archipelago (Michel et al., this
volume); (8) Baﬃn Bay and North Water Polynya (Tremblay et al., this volume); (9) North-eastern Greenland and Young Sound
(Rysgaard and Nielsen, this volume).
a non-sector based understanding for the pan-Arctic Mediterranean; the regional reviews on pan-Arctic marine food webs as presented in Progress in Oceanography xx, xx–xx (cf. Fig. 5) create the basis for this
approach. We will then step even further back into the connectivity of the AO to the adjacent Atlantic and
Paciﬁc oceans and associated physical forcing in order to ﬁnd an adequate perspective for the dynamics of
the AO. At the end, we wish to describe the contiguous domains of the AO that, in eﬀect, constitute major
biogeographical provinces, explore the role of the seasonal ice zone and shelf break and the seasonal ice zone,
and then discuss the response of the pan-Arctic shelves to diminishing ice cover.
3. A comprehensive perspective of pan-Arctic shelves: three typologies
3.1. Ice typology: a generic model for seasonal and marginal ice zones
It is important ﬁrst to distinguish between the marginal ice zone (MIZ) and the seasonal ice zone (SIZ).
The marginal ice zone (MIZ) is where the open waters meet the ice cover at any given time. The seasonal ice
zone (SIZ) lies between maximum (winter) and minimum (summer) ice cover that freezes and melts annually/This distinction is important because the MIZ upon its retreat across the SIZ leaves behind open and
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strongly stratiﬁed water, which strongly impacts the ecological function in the open water. The functional
modes of such water, either as part of the SIZ or lying outside the SIZ (as in the southern and south-western Barents Sea and parts of the Nordic Seas), diﬀer substantially from those in ice-covered locations due
to changes in stratiﬁcation forced by ice melt and brine formation, altered underwater light climate, constrained vertical mixing and primary production, distinct phytoplankton blooms, intense herbivory and tight
pelagic–benthic coupling.
To allow inter-regional comparisons, we propose a generic conceptual model of the SIZ for the AO that is
drawn from the work of Williams et al. (2006). The main elements are land-fast ice (LFI), pack ice, ﬁrst-year
ice (FYI), multi-year ice (MYI) and the MIZ (Fig. 6a). During the freezing period LFI advances from the land
and eventually joins the pack ice that consists of FYI that has been formed south of the MYI. A shear zone
between the drifting FYI and MYI and the stationary LFI typically develops into ﬂaw leads during late spring
and summer. Pressure ridges (stamukhi) are frequently formed at the border between the LFI and FYI during
episodic periods of compression. The particularly prominent stamukhi that form oﬀ the Mackenzie River form
an inverted ice barrier that retains incoming river water and eventually forms an ice-dammed ‘lake’ beneath
the LFI. Along this shear zone a system of recurrent ﬂaw leads collectively engirdles much of the entire AO, in
particular in spring (Fig. 6). During the melting period (June–October) the ﬂaw leads open more and more to
eventually establish the MIZ. The MIZ then advances northward towards the MYI and southward as the LFI
melts. During the freezing period (November–March/April) the FYI and LFI move south and north, respectively, colliding at the shear zone.
In parts of the Bering Sea, the Nordic Seas and the Barents Sea, pack ice is not met by LFI, but instead by
open water. While in most regions the SIZ and MIZ described in Fig. 6 are oriented east-west, in some regions,
such the Greenland Sea, Fram Strait, the Barents Sea and the Kara Sea the ice typology lies south-north
(Fig. 6). If the current warming of the AO and the concurrent reduction in ice cover continue, several of
the regions that currently can be described by the generic ice model (Fig. 6a) will develop into a Barents
Sea-like system without LFI (Fig. 6b). This could, for example, occur in the Bering Strait-Chukchi Sea region.
Fig. 6. A general model (a) of the Seasonal (SIZ) and Marginal Ice Zone (MIZ). The growth and melting of Land Fast Ice (LFI), First
Year Ice (FYI) and Multi Year Ice (MYI) are indicated as a function time (i.e. months). In the shear zone between LFI and FYI, an initial
ﬂaw polynya develops into the highly stratiﬁed MIZ. Also shown (b) is a general model for regions such as the Barents Sea or future
climate warming scenarios when LFI disappears. Redrawn and modiﬁed from Williams et al. (2006).
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A particular phenomenon of the SIZ is the occurrence of large areas of open water called polynyas (Fig. 7).
Williams et al. (2006) distinguish among (a) mechanically forced polynyas (wind-driven and current-driven
polynyas, including ﬂaw polynyas), (b) convectively forced polynyas (through free or forced convection, including tidally-forced polynyas and upwelling polynyas), and (c) hybrid or other polynyas. Flaw leads are included
in this classiﬁcation, but while most other polynyas can appear at various places in the ice cover, these are a
speciﬁc feature of the SIZ, occurring in association with LFI, and should be dealt with in a SIZ context.
Flaw leads are also a site of brine release and dense water formation on pan-Arctic shelves (cf. Aagaard
et al., 1985). Dense water drainage from these regions is a key ventilation mechanism for the Arctic Basin
(Aagaard et al., 1981), and this mechanism may be qualitatively and quantitatively perturbed by an altered
ice climate. A special occurrence of ﬂaw polynyas currently exists along the south-facing shorelines and islands
of the northern Bering Sea, in that cold, dense waters sink and drain southwards to form a persistent ‘cold
pool’ in the central Bering sea that limits the northward expansion of sub-Arctic ﬁsh species that are not
adapted to polar waters (Grebmeier et al., 2006). Should sea ice retreat farther north, such leads and the attendant cold pool barrier may cease to exist, thus allowing northward immigration of sub-Arctic pelagic species.
Over the course of the summer and after the disappearance of the LFI the central AO is engirdled by the
narrow MIZ and wide SIZ bands (Fig. 7). This girdling stretches counter-clockwise all the way from eastern
Greenland to the Canadian Archipelago and in early autumn even to northern Greenland. It is along this variably expanding and shrinking continuum of the MIZ and SIZ bands that most of the AO production takes
place and where key climate-induced changes will most likely take place. Expected ecological changes along
these bands support contentions that polar ecosystems will be bellwethers of a global climatic change (ACIA,
2004).
Fig. 7. Ice-covered water of the Arctic Ocean with its marginal ice zone extending toward permanently open water (purple) and various
types of polynyas. In concert these turn into the quasi uninterrupted marginal ice zone system that during summer and autumn girdles the
inner, multi-year ice core of the Arctic Ocean. Clearly visible is the contiguous Seasonal Ice Zone domain (SIZ) that covers most of the
pan-Arctic shelves and the shelf break, while the quasi-continuous Riverine Coastal Domain (RCD) covers the innermost shelves (see
4.2.1).
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3.2. A functional pelagic typology: the a and b ocean distinction
As in most ocean domains, biological production rests on a complex interplay of light and nutrient
availability, and both ﬁelds are strongly regulated by stratiﬁcation. As such, stratiﬁcation is perhaps the
most important attribute of oceans regarding climate and biology. Stratiﬁcation is typically expressed by
the buoyancy frequency, N2, which derives from the vertical gradients of temperature, dT/dz, and salinity,
dS/dz, so that N 2 ¼ N 2T þ N 2S ; where N 2T ¼ gaðdT =dzÞ; N 2S ¼ gbðdS=dzÞ, g is gravity, a is the thermal expansion coeﬃcient and b is the haline contraction coeﬃcient. Temperature stratiﬁcation is of primary signiﬁcance in the open oceans to the south, while salinity stratiﬁcation predominates in the AO (Carmack,
2006).
Two aspects of the ocean’s climate system regulate stratiﬁcation in the AO. First, the Paciﬁc is fresher than
the Atlantic, due in part to atmospheric vapor transport across the Isthmus of Panama by westerly winds. This
requires freshwater pathways back to the Atlantic, one of which is the throughﬂow of low salinity PW entering
the AO via Bering Strait. The resulting thermohaline distributions thus establish a ‘downhill journey’ of low
salinity waters from the North Paciﬁc to the Arctic and then into the North Atlantic, which we come back to
in Section 4.1. Second, a substantial portion of the excess heat absorbed at low latitudes is carried poleward as
latent heat (moisture) by the atmosphere. Simply stated, because water warms and evaporates at low latitudes
and water vapor cools and condenses at high latitudes, the upper layers of subtropical seas are permanently
stratiﬁed mainly by temperature (termed alpha oceans after the thermal expansion coeﬃcient), while the upper
layers of high-latitude seas are permanently stratiﬁed mainly by salinity (termed beta oceans after the haline
contraction coeﬃcient). The physical basis for the boundary separating a and b oceans is unclear, but the
boundary is found to the south on both the Paciﬁc and Atlantic side of the AO. A schematic of the a/b distinction is shown in Fig. 8. Distinctive patterns of stratiﬁcation, response to atmospheric forcing, nutrient
availability and biogeographical boundaries persist throughout the northern oceans, but this simple distinction – a versus b oceans – provides a useful framework for interpretating linkages among physical and biological processes and rates, including the impacts of climate variability. Superimposed on the broader,
climate-scale a and b domains are the seasonal and regional patterns due to surfaces ﬂuxes of heat and freshwater, leading to superposition of seasonal and permanent stratiﬁcation. We then ask: Are there ecological
elements that complement the physical diﬀerences between a and b oceans? Can we deﬁne a functional typology of a and b oceans? We suggest that we can and present here a few elements for which veriﬁcation will have
to await more detailed observation and scrutiny.
The deeper extent of vertical mixing and convection occurring in a compared to b oceans will result in phytoplankton concentrations much lower than those in the highly transient blooms typical blooms of b oceans
Fig. 8. The integration of the physical oceanography of the Arctic Ocean into that of the North Atlantic and Paciﬁc, deﬁning a- and boceans; see text for explanation. P, precipitation; R, run-oﬀ; B, buoyancy ﬂux; LSW, Labrador sea water; NADW, North Atlantic deep
water; NPIW, North paciﬁc intermediate water; PW, paciﬁc water; AW, Atlantic water; SW, surface water; DW, deep water.
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455
with seasonal ice cover (e.g. Fig. 15). For example, Backhaus et al. (2003) present data suggesting far deeper
convection occurring in the a compared to the b regimes of the North Atlantic. Major phytoplankton blooms
with high biomass accumulation and chlorophyll a concentrations (Chl a) > 5 mg m2 (and away from both
eutrophication and upwelling) are a typical phenomenon of b oceans. In a oceans the growing phytoplankton
can easily be mixed below the euphotic zone and primary production is thus retarded by decreased light availability to phytoplankton cells while nutrient availability is increased (e.g. Open Ocean II, Fig. 15). The
increased vertical mixing extant in a oceans results in a far higher integrated primary production compared
to that in b oceans (which are notoriously nutrient limited after ice retreat). In the Barents Sea this is clearly
illustrated by a comparison between the south-eastern and northern regions, where respective maximum Chl a
concentrations are 5 versus 20 mg m2 and primary production rates are 200 versus 30–60 g C m2 y1 (Wassmann et al., 2006).
The diﬀering mixing regimes of a and b oceans also appear to support contrasting feeding regimes for
mesozooplankton. The dilute nature of phytoplankton in a oceans requires mesozooplankton to select a feeding strategy that optimizes energy acquisition. Mesozooplankton species, such as calanoid copepods, grazing
in the typical MIZ bloom of the b ocean, dominate the biomass and appear to utilize a feeding strategy for
which they seem specialized: suspension feeding and herbivory with incidental coprorhexy (breaking of fecal
pellets without ingestion). If the food concentration is more dilute, as will be the case in a oceans, these copepods would probably use too much energy for suspension feeding and may ﬁnally switch over to raptorial
feeding of living (small zooplankton, carnivory) or sinking particles (e.g. fecal pellets, coprophagy). As a consequence of this feeding strategy, the vertical export attenuation eﬃciency in b oceans will be high in the upper
part of the aphotic zone. In contrast, grazing and vertical mixing in a oceans will be distributed over a greater
section of the water column and consequently vertical ﬂux attenuation will be lower. Also, the retention eﬃciency of mesozooplankton fecal pellets will be low in b oceans because the coprophagic food acquirement will
be reduced, opposite to that of a oceans.
An interesting perspective regarding our functional pelagic typology in respect to climate change derives
from the fact that the ice cover extent is decreasing and that ice thickness is thinning. The width of the SIZ
will increase, reaching farther into the AO basins in later summer (e.g. Fig. 7). Two opposing eﬀects on light
and nutrients thus come into play. On the one hand, the greater width of the SIZ will result in stronger
stratiﬁcation due to more meltwater, yet also stronger mixing due to increased winds and longer exposure
to the passage of atmospheric low-pressure centers. On the other hand, thinning of ice would reduce the
annual burden of meltwater, and thus weaken stratiﬁcation. Thus, it is not clear whether or not the overall
response of the AO, now characterized as a b ocean, will be to become more ‘a-like’ in its ecological
function.
3.3. Arctic Shelf typology: physical–biological forcing and food webs
The shelves of the Arctic Mediterranean are strikingly diﬀerent from those of the rest of the world ocean.
About 50% of the AO surface is comprised by shelves (Fig. 9a), and thus an emphasis on these shallow
domains is needed to understand its functional dynamics. However, there are also signiﬁcant diﬀerences
among the pan-Arctic shelves (Fig. 9b). Those oﬀ North America are typically narrow, while those oﬀ Eurasia
are wide and have very steep slopes. The shallowest shelves are the Chukchi, East Siberian and Kara Seas,
while the Barents Sea and the Canadian Archipelago are relatively deep.
In order to obtain a more adequate perspective of the pan-Arctic shelves, we expand on the typology proposed by Carmack et al. (this volume). We distinguish between inﬂow, interior and outﬂow shelves, which represent entirely diﬀerent functional types (Figs. 10 and 11). These three basic shelf types comprise
approximately similar areas (2.4, 2.4 and 2.1 · 106 km2, respectively; Table 1). Among the three basic shelf
types we diﬀerentiate additional functional types: (a) the shallow and deep inﬂow shelves (Bering Strait/Chukchi Sea and Barents Sea, respectively); (b) the narrow and wide interior shelves (Beaufort Sea and Kara/Laptev/East Siberian Seas, respectively); and (c) the network and longitudinal outﬂow shelves (Canadian
Archipelago and east-Greenland shelf, respectively).
The following characterization of the six pan-Arctic shelf types is based in principle upon the nine reviews
that are citied in Fig. 5 and only evidence not referenced in these reviews will be cited.
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Fig. 9. Hypsographic curve of the world ocean and the Arctic Ocean (left). Speciﬁc hypsographic curves from the various Arctic shelves
(right). Note the considerable diﬀerence between the inﬂow shelves such as the Barents and interiors shelves such as the Beaufort Seas.
3.3.1. Inﬂow shelves
During transit, in-bound waters from the North Atlantic and North Paciﬁc are strongly modiﬁed by biogeochemical and physical processes over inﬂow shelves (western Spitsbergen, Barents, Bering and Chukchi
seas (Figs. 10 and 11)). The transformation during transit depends on the width and depth of the shelves that
in turn determine the water residence times, in particular in the biogeochemically active layers such as the
euphotic zone and the benthic boundary layer. These waters subsequently subduct at or along the shelf break,
and thus inﬂuence property distributions within the whole of the Arctic basin. This is the reason that much
recent research has focused on shelf/basin interaction. Canyons, trenches and deep plateaus such as Harold
Canyon, the St. Anna Trough and Yermak Plateau in the Chukchi, Barents Sea and north of western Spitsbergen, strongly inﬂuence exchanges across inﬂow shelves. Inﬂow shelves play an important role in the transfer of water masses of Paciﬁc or Atlantic origin as well as pelagic organisms (such as phyto-, zoo- and
meroplankton). The direct supply of freshwater from rivers and ice through inﬂow shelves is relatively small,
particularly in the Barents Sea. Consequently, stratiﬁcation of surface waters is weak in the southern Barents
Sea, but relatively strong in the SIZ. Oﬀ west Spitsbergen stratiﬁcation is enhanced by freshwater additions
from fjords and from the West Spitsbergen Coastal Current as well as thermal stratiﬁcation of the North
Atlantic Current. Freshwater supply through the Bering Strait and ice melt support a much stronger seasonal
stratiﬁcation in the Chukchi Sea. The seasonal ice cover of inﬂow shelves is characterized by their limited
extent of LFI and dominance of FYI. Some limited and variable transport of MYI, with its associated biota,
over the northernmost regions of inﬂow shelves takes place. Inﬂow shelves respond strongly to larger-scale
atmospheric forcing, and thus reﬂect North Atlantic and Arctic Oscillation (NAO/ArO) events.
Inﬂow shelves are the boundary domains connecting the AO to the world ocean. On both Atlantic and
Paciﬁc sides shelf primary productivity is high, and combined they comprise about two-thirds of the total primary production of the AO (Sakshaug, 2004). The introduction of nutrients and advection of suspended biomass are of signiﬁcance, in particular through the shallow Bering Strait into the Chukchi Sea where advected
E. Carmack, P. Wassmann / Progress in Oceanography 71 (2006) 446–477
457
Fig. 10. The spatial distribution of shelf types in the Arctic Mediterranean. Green: inﬂow shelves. Blue: interior shelves. Yellow: outﬂow
shelves. Red: shelf break. Grey: the deep basins.
organic matter fuels a signiﬁcant benthic community. Also, the advection of larger zooplankton from sub-Arctic or boreal regions onto and across the inﬂow shelves is an essential aspect of their speciﬁc functionality.
Advection introduces expatriates across the shelf break and into the Arctic basins. This most likely results
in a distorted balance between primary and secondary producers oﬀ the shelf break. Some advection of detritus and ice biota by MYI advection into the northernmost regions of inﬂow shelves has to be considered. The
relative contribution of ice algae to the total primary production on inﬂow shelves is generally low, but
increases with increasing latitude.
3.3.1.1. Shallow inﬂow shelves (northern Bering Strait and Chukchi Sea). Being nearly three times larger than
the Beaufort shelf, the most important characteristic of the shallow inﬂow shelves of the northern Bering Strait
and Chukchi Sea (V/A = 12.4; Table 1) is their high productivity. This is partly based upon continuous biomass advection and the high nutrient content of PW through the Bering Strait. The supply of nutrient rich PW
to the deep Polar Ocean is about one-ﬁfth that of AW. The SIZ derives from a mixture of LFI and MYI melt
and covers the entire region to variable degrees. Also the supply of freshwater from rivers such as the Yukon
via the Bering Sea is signiﬁcant. As a consequence, stratiﬁcation is well-developed throughout the region and
vertical mixing through the passage of low-pressure systems is of less signiﬁcance. A bloom follows the MIZ
during its northward withdrawal. Suspended biomass is signiﬁcant in the upper layers, but also the Benthic
Boundary Layer (BBL) supplies in substantial amounts a mixture of allocthonous and autochthonous biogenic matter along the slope and through the major canyons to a rich benthic fauna. Subduction of PW takes
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Fig. 11. Three diﬀerent Arctic Ocean shelf types: inﬂow (1), internal (2) and outﬂow (3) shelves. Slightly modiﬁed from Carmack et al.
(this volume).
Table 1
Areas, volumes and mean depths of Arctic Shelves (after Jacobson et al., 2004)
% Total shelf
area
Volume
(103 km3)
Arctic Sea
Area
(103 km2)
Barents Sea
Kara Sea
Laptev Sea
East Siberian Sea
Chukchi Sea
Beaufort Sea
Canadian Arctic
Archipelago (CAA)
Northern CAA
1597
926
498
987
620
178
1032
210
3
65
Total shelf
6048
100%
829
27
15
8
16
10
3
18
307
121
24
57
50
22
183
% Total shelf
volume
Mean depth
(m)
Volume/area
ratio
200
56
131
48
58
80
124
5.20
7.65
20.75
17.30
12.40
8.10
5.65
7
310
3.10
100%
140
7.30
37
15
3
7
6
3
22
Also shown are the volume/area ratios.
place particularly through canyons. However, the supply of expatriate zooplankton through the Bering Strait
and over the Chukchi shelf is relatively small because of the shallowness. As compared to nearshore waters or
to almost any other place in the Arctic, zooplankton grazing and the microbial loop play less signiﬁcant regulatory roles in these nutrient-rich waters.
The high primary production, strong advection of biogenic matter from the Bering Strait and the shallow
depth translate into a tight pelagic–benthic coupling and high standing stocks of benthic communities. Signiﬁcant stocks of benthic feeding marine mammals have annual feeding migrations through the Bering
Strait, and benthos rather then pelagic ﬁsh is the base of their subsistence. The high productivity does
not – under current climatic conditions – translate into an extensive commercial ﬁshery. In these zooplankton-poor waters diatoms dominate during the start of the bloom (high silicate concentrations of PW and
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459
continuous advection), in particular following the northward migration of the MIZ. Smaller forms and
lower primary production are encountered under the deep, ice-covered waters towards the Canadian Basin,
where the typical top-down regulation by zooplankton in the AO is prominent. The rich supply of biogenic
matter and eﬃcient pelagic–benthic coupling are clearly reﬂected by some of the highest faunal biomass of
benthos in the Arctic. As a consequence of the large vertical carbon ﬂux, the shelf is also the site of substantial denitriﬁcation (Devol et al., 1997). The northern Chukchi Sea and western portion of the Beaufort
Sea, hydrographically downstream of the northward ﬂowing PW, receives advective and locally produced
carbon, allowing for locally high benthic biomass in oﬀshore areas compared to the more river-inﬂuenced
central and eastern Beaufort Sea (narrow interior shelf, 3.3.2.1). Both benthic carbon cycling and carbon
incorporation in long-lived benthic fauna are dominant aspects of the productive Paciﬁc-inﬂuenced inﬂow
shelves. These short food chains be vulnerable to rapid change under modiﬁed environmental forcing. Also,
the tight pelagic–benthic coupling in this region is due, in part, to the presence of a bottom ‘cold pool’ of
near-freezing water, apparently formed during winter in polynyas along the south-facing shores of the
northern Bering Sea, which creates a barrier to ﬁsh living in the southern Bering Sea. This suggests a possible ‘tipping’ point’ in pelagic/benthic coupling should the SIZ migrate fully into the Chukchi Sea, thus
eliminating the south-facing polynyas and the presumed source of cold pool waters.
3.3.1.2. Deep inﬂow shelves (western Spitsbergen and Barents Sea). The deep inﬂow shelves of western Spitsbergen and the Barents Sea (2.7 times larger then the Chukchi; V/A = 5.2; Table 1) are characterized by high
primary production (average 90 g C m2 y1) and signiﬁcant spatial (30–200 g C m2 y1) and interannual
variability (30%). The supply of AW moving along and through the Barents into the deep AO is about 5
Sv. The SIZ does not extend to the southern Barents Sea and, thus, stratiﬁcation is weak and vertical mixing
is strong, particularly during the frequent passage of low-pressure systems (e.g. Open Ocean I, Fig. 14). This
supports a high primary production rate that is based (due to nutrient supply) upon a relatively low and dilute
phytoplankton concentration (due to mixing), but a deep euphotic zone. In contrast, the primary production
in the MIZ/SIZ is moderate and the suspended biomass of phytoplankton is high above the seasonal halocline.
One of the most characteristic features of the of western Spitsbergen shelf and the Barents Sea is the transformation of Atlantic-derived waters as ﬁrst the upper layers encounter and melt sea ice and the deeper layers
continue as boundary currents along the shelf break and ridges of the AO. Substantial quantities of long-lived
calanoid copepods ride these currents and connect the productivity of this inﬂow shelf with the AO. Other
important speciﬁc features of the Barents Sea are little LFI and the lack of riverine and terrestrial supplies
of biogenic matter, except for the Pechora Sea.
The high primary production, relatively great depth and advection from the Norwegian Sea translate into
high standing stocks of zooplankton, shrimp and small pelagic ﬁsh (such as capelin) that support one of the
principal ﬁsheries of the world. Above a consistent background of heterotrophic and autotrophic ﬂagellates,
diatoms dominate during the start of the bloom, in particular along the northward migration of the MIZ. The
relatively low silicate concentrations in this region of the AO limit the production of diatoms. Lipid-rich zooplankton and pelagic ﬁsh such as capelin and herring play key roles in the pelagic food web, supporting abundant stocks of cod, seals and whales. Primary production is positively correlated with benthic standing stocks.
Benthos is dominated by polychaetes, and relatively high biomass is encountered, especially in the shallow
eastern parts of the Barents Sea. The relatively deep shelves of the Barents Sea (about 200 m) and oﬀ western
Spitsbergen, together with the eﬃciency and dominance of the pelagic food web, suggest that the ﬂux through
the hyperbenthic and benthic food web is smaller than that in the Chukchi Sea.
3.3.2. Interior shelves
The Eurasian interior shelves are several hundred km wide, while those of North America are relatively
narrow (Figs. 10 and 11). All interior shelves are very shallow and characterized by strong impact of major
Arctic rivers, e.g. the Yenise, Ob, Lena and McKenzie Rivers. Interior shelves exhibit a positive estuarine ﬂow
(plume spreading) in summer and a negative estuarine circulation (brine release) in winter. During periods of
plume spreading, ﬂocculation of estuarine and marine matter (both particulate and dissolved) is high at variable distances from the river deltas. This is the so-called marginal ﬁlter (Lisitsyn, 1995) that results in enrichment of sediments with speciﬁc elements. Tidal ﬂows are signiﬁcant and enhance the spread of the estuarine
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plume towards the sea, while full strength seawater is transported towards the littoral zone. The horizontal
exchange of water masses is thus considerable and variation in salinity signiﬁcant. Outside the shelves eastward-directed currents of transformed ArW, partly derived from Atlantic and/or Paciﬁc sources girdle the
shelves as boundary currents. Large and voluminous troughs and canyons are not especially prominent,
although the beds of some rivers continue as small sea valleys across the shelf and into the AO. The load
of terrigenous matter of the rivers is great, and thus the turbidity is high. The interior shelves are characterized
by a predominance of LFI that melts during summer. The pack ice collides against the LFI, and between these
two ice types is the band of pan-Arctic ﬂaw polynyas (Fig. 6).
Compared to the inﬂow shelves, the biogeochemical transformations taking place on interior shelves are
trivial. The primary production and the general biological activity are low, and much of the allochthonous
matter is of a refractory nature. High turbidity and export of surface waters below the ice cover, followed
by nutrient limitation due to stratiﬁcation are the main causes for the low primary production. Suspended biomass of planktonic organisms is thus low, but that of benthic organisms is relatively high. Some of the food for
the benthic organisms is of marine origin and derives from the estuarine counter current, some is locally produced and a signiﬁcant amount derives from littoral and riverine sources. Natural transport and erosion (permafrost reduction) are signiﬁcant. The role of terrestrially derived carbon in Arctic estuarine food webs is
especially important in view of the current warming trend in the Arctic environment. The impact of LFI often
eradicates benthic fauna during winter and spring and new populations must reestablish annually.
3.3.2.1. Narrow interior shelves (Beaufort Sea). The near-shore shelf of the Beaufort Sea, which comprises
only 8% of the interior shelf area (V/A = 8.1; Table 1), is deﬁned by extreme physical and biological gradients
that have a distinctive inﬂuence on its productivity and trophic structure. The narrowness of this shelf adds to
the signiﬁcance of these gradients. The primary production in some of the coastal lagoon ecosystems of the
Alaskan Beaufort Shelf is <10 g C m2 y1, as light penetration in spreading and meandering river plumes
is extremely limited. Ice cover by LFI is persistently strong, and this severely limits primary production (cf.
Carmack et al., 2004a). Primary production on the adjacent Canadian Beaufort Shelf with its mixture of
LFI, MYI, ﬂaw polynyas, stamukhi and higher light penetration outside the river plumes is higher, in the
range of 30–70 g C m2 y1 (Sakshaug, 2004). As for all interior shelves, high freshwater inputs from numerous rivers and streams produce an environment that is decidedly estuarine in character, especially during the
late spring and the summer months. Coastal erosion and river discharge are largely responsible for introducing
high concentrations of suspended sediment from upland regions into the near-shore zone, sediment often
trapped in the nearshore lagoons that are characteristic of the Beaufort Sea. The lack of direct grazing pathways among Arctic biota on narrow interior shelves results in the wide range in 13C values of eastern Beaufort
Sea benthic fauna (nearly 8&) compared to the same species collected in the northeastern Chukchi (3&). The
wider spread in stable isotope values in the eastern Beaufort Sea also reﬂects a decoupling between benthic and
pelagic components.
A relatively low autochthonous production and a rich terrestrial, allochthonous supply mean that the latter
material is to a larger extent incorporated compared to more productive regions. Benthic biomass is relatively
rich in terrestrial carbon. Zooplankton populations, however, are poorly developed in the inner coastal
regions. Access to fully marine water outside of the lagoons allows the Beaufort coast to provide a critical
habitat for several species of amphidromous ﬁshes (e.g. Char, Cisco), some of which are essential to the subsistence lifestyle of Arctic native populations.
3.3.2.2. Wide interior shelves (Kara, Laptev, and East Siberian Seas). The wide interior shelves extend up to
800 km before they reach the shelf break at the start of the deep AO basins (V/A = 15.2; Table 1). Most of
the trophic dynamics remain to be resolved, since the wide interior shelves, in particular the Laptev Sea,
belong to the least investigated regions of the world. Much of this area is heavily covered by LFI, very shallow
(<50 m) and exposed to most of the discharge of freshwater to the AO (in total about 10% of the total discharge to the world ocean). As a consequence of these constraints, total primary production is low, and
the sympagic primary production probably comprises a large share. Stratiﬁcation is strong during the productive season and the biota is dominated by benthos. The shelf-typology of the Kara Sea is complex. The southwestern sector, inﬂuenced by modiﬁed AW from the Barents and eastern part of the Kara Seas (aﬀected by the
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461
Ob and Yenisei Rivers), belongs to the wide interior shelves. In contrast, the central and northern regions are
under the inﬂuence of surface AO and boundary currents.
High chlorophyll a (Chl a) concentrations in the sediments indicate a tight coupling between sympagic and
pelagic primary production and nutrient supply to the benthos throughout the entire Laptev Sea. However,
pronounced regional diﬀerences exist in the magnitude of primary production. The shallow nature of these
ecosystems implies that the impact of zooplankton on carbon ﬂux is limited, increasing towards the shelf
break.
Preliminary carbon budgets indicate that a high proportion of primary production is channeled through the
benthic foodweb in the Laptev Sea. Primary production in the northeastern and western Laptev Sea is apparently not suﬃcient to fuel both pelagic and benthic secondary production. As in the case of the narrow interior
shelves input of allochthonous organic carbon seems to be required to balance the overall carbon demand. In
the Kara Sea estimates of the carbon requirements from zooplankton and benthos suggest an underestimation
of primary production. This points at signiﬁcant gaps in knowledge, in particular in the scale of seasonal variations. Despite its refractory nature, the supply of riverine DOC and particulate matter of terrestrial origin,
mediated by microbial food webs, may be signiﬁcant for all interior shelves.
3.3.3. Outﬂow shelves
The outﬂow shelves allow ArW back into the North Atlantic (i.e. the Nordic and Labrador Seas) via the
Canadian Archipelago and east coast of Greenland (Figs. 10 and 11). The outﬂow shelves are not simple gates
or channels (McLaughlin et al., in press). Transit times of out-ﬂow shelves are long enough for thermohaline
and biogeochemical changes to occur en route. The Canadian Archipelago in particular has long and highly
variable throughﬂow and residence times. On the whole, the Archipelago is a complex network of channels,
sub-basins and sills, while the east Greenland shelf is deeper and has a depth range similar to that oﬀ west
Spitsbergen. The archipelago is ice-covered by LFI during most of the year with extensive, but variable icemelt and stratiﬁcation observed during summer and early autumn. A matrix of MYI, FYI and heavy LFI covers the northern-most outﬂow shelves. The current direction above the longitudinal outﬂow shelves of eastern
Greenland is characteristically parallel to the ice edge, in contrast to the inﬂow shelves where it is most often
perpendicular to the ice edge. Also, the longitudinal outﬂow shelves of the Fram Strait and eastern Greenland
are, to various degrees, perpetually covered by pack ice transported from the Transpolar Drift. Most of the ice
produced in the AO melts along the longitudinal outﬂow shelves. This results in signiﬁcant stratiﬁcation and
reduced salinity of the East Greenland Current (EGC).
Primary production on outﬂow shelves is spatially variable. Over the southern parts of outﬂow shelves the
primary production can be signiﬁcant. It is highly seasonal, quickly nutrient limited and highly variable
between years. The zooplankton dynamics are even more variable, probably due to irregular advection episodes through the Archipelago. Ice fauna and ﬂora are rich in the Archipelago and close pelagic–benthic coupling results in rich benthic communities in the shallow sections. The ecosystems along the entire length of the
longitudinal outﬂow shelves are not adequately investigated so far. However, primary production is generally
low and the transport of organic matter (mostly of terrestrial origin) and ice biota by the MYI of the Transpolar Drift is signiﬁcant along longitudinal outﬂow shelves.
3.3.3.1. Network outﬂow shelves (Canadian Archipelago and North Water Polynya). The Canadian Archipelago represents a complicated network of channels and sounds with extreme topographic complexity (V/
A = 5.6; Table 1) where surface water leaves the Arctic (McLaughlin et al., in press). They cover 23% of
the pan-Arctic shelves (Fig. 9). Larger rivers are missing, but moderately sized rivers enter from the Canadian mainland, and there is other, local freshwater run-oﬀ. Stratiﬁcation derives primarily from ice melt.
LFI in the region is regular, but strongly variable with regard to depth, time of melt and snow-cover.
The comparatively great depth implies that much of the carbon ﬂux is channeled though plankton, with
additional supplies from sympagic biota (cf. Welch et al., 1992). Existing measurements of total primary
production in the Archipelago suggest that it may support up to 32% of the total primary production of
pan-Arctic shelves. High year-to-year variability in nutrient inventories and ratios, the magnitude of the
ice algae and phytoplankton bloom, the timing of ice algae sedimentation in the spring, and the composition
of the zooplankton community all imply that one of the foremost characteristics of this archipelago is its
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interannual variability. This high interannual variability in food web composition, ﬂuxes and pelagic–benthic coupling complicates predictions of the impact of climate change on the Canadian Archipelago. Interannual comparisons of ice algae production in Barrow Strait reveal strong year-to-year variability, ranging
from 2 to 23 g C m2 y1 (Smith et al., 1988). Thus, this outﬂow shelf experiences signiﬁcant variability in
its planktonic/sympagic primary production. The high year-to-year variability in production and carbon
transfer pathways (e.g. pelagic versus benthic) in the Archipelago suggest that the system might be resilient
to the increased variability in climatic conditions that occurred in the past decade. However, this increased
variability, combined with directional change in climatic and oceanographic conditions, might modify the
existing range of ecological processes. Shifts in the timing of events appear to have occurred already in
the past decade, with potential cascading eﬀects throughout the ecosystem.
The North Water Polynya receives surface waters both from the AO through the northern Archipelago and
from Baﬃn Bay. The physical forcing that supports the polynya also creates the setting for a highly productive, but spatially variable ecosystem. Primary production is among the highest so far recorded in the AO, and
the richness of the biota suggests recurringly high productivity. The interannual variability is thought to be
signiﬁcant as the early opening of the polynya in a basically ice-covered region depends on formation of an
ice bridge in the Lincoln Sea. Harvestable production in this region is high, and it is the resource base of
the northern-most Inuit settlement at Thule. Recycling in the pelagic system is eﬃcient, and so the supply
of fresh biogenic material to the benthos is relatively low.
3.3.3.2. Longitudinal outﬂow shelves (Fram Strait, east Greenland coast). The dense and continuous ice-cover
of the Fram Strait and east Greenland coast has limited observational studies in this important region where
the AO discharges its low salinity surface water and biomass into the Nordic Seas and the North Atlantic
Ocean. The supply of ice by the Transpolar Drift gives a signiﬁcant potential for stratiﬁcation, but brine formation during the freeze-up period also plays a role. Rivers and glaciers in the Greenland hinterland add to
the freshet. In concert this results in signiﬁcant stratiﬁcation along the entire east-Greenland coast and shelf,
and some limited spreading into the Greenland and Iceland seas. Substantial amounts of DOC from rivers and
the particulate biogenic matter derived from ice ﬂora and ice-melt add to a signiﬁcant supply of allochthonous
matter. Primary production is moderate and phytoplankton blooms of the MIZ and the Northeast Water
Polynya (Deming et al., 2002) frequently meandering again under the ice cover, reducing phytoplankton
growth and prolonging bloom development. In addition to biogenic matter, Arctic-derived populations
(e.g. Calanus glacialis and C. hyperboreus) represent an important connection between the AO and the North
Atlantic biota. The terrestrial biogenic matter that is released along the East Greenland outﬂow shelves represents a substantial quantity of potential food for coastal zone benthos. The initiation of ice-free periods in
the fjords advances from spring in Denmark Strait to July or later in the northern Fram Strait. The total period at the northern extreme is 2 months or less. Following the break-up of ice, the immediate increase in light
penetration to the water column causes a steep increase in pelagic primary production. Usually, the bloom
lasts until August–September when nutrients begin to limit production in surface waters and sea ice starts
to form. The grazer community, dominated by copepods, soon takes advantage of the increased phytoplankton production. The carbon demand of the heterotrophic plankton has been reported to be approximately
twice the estimated pelagic primary production, illustrating the importance of carbon advected along the outﬂow shelf and carbon from land to fuelling of the ecosystem. In the shallow parts of the fjords (<40 m) benthic
primary producers can dominate primary production. A high and diverse benthic infauna dominated by polychaetes and bivalves exists in these shallow-water sediments (<40 m), which are colonized by benthic primary
producers and in direct contact with the pelagic phytoplankton bloom.
4. Unifying concepts coupling physical–biological scales on pan-Arctic shelves
4.1. The connection to adjacent oceans and biogeochemical constrains
We have thus far discussed the pan-Arctic shelves – not by their geopolitical settings – but rather by their
physical geography and function within the climate system. To advance general concepts of physical–biological coupling from a macroecological perspective (cf. Li, 2002) and to examine the joint roles of stratiﬁcation
E. Carmack, P. Wassmann / Progress in Oceanography 71 (2006) 446–477
463
and advection, we next give a hemispheric context for the Arctic as a double estuary coupled to the meridional
thermohaline circulation, and then review concepts of shelf/basin and shelf/land interaction. This is followed
by thoughts on an ‘ecology of advection’ that is applicable to the AO.
4.1.1. Action, ecosystems and the thermohaline circulation
Johnson (2002) applied the concept of ‘action’ – physically deﬁned as the product of energy and time – to
the structure of biological systems. But whereas physical systems act on the principle of ‘least action’, with
energy transport processes working rapidly so as to minimize action, Johnson argued that biological systems
evolve means to slow (store) ﬂuxes of energy (carbon) as much as possible; he termed this ‘most action’. Johnson also noted that on a global scale, there is a net transport of carbon-based energy from high latitudes to
low, carried out by migrations. We think that Johnson’s concept oﬀers a useful window to view both physical
(abiotic) and biological (biotic) systems on a variety of interactive scales.
At the hemispheric scale one can argue that the abiotic component of the climate system operates so as to
redress the global heat imbalance as eﬃciently as possible, for anything less would result in the build-up of
excessive gradients and compensating instabilities (Fig. 12). Hence the integrated eﬀect of the oceans and
atmosphere is to follow the principle of least action in transporting heat from the low latitudes where it is
gained in excess to the high latitudes where the excess is radiated back into space. At the same time, many
animal species follow the principle of most action and move carbon equatorward by feeding at high latitudes
and respiring at low (Johnson, 2002). This latter tendency may be counter-acted by thermohaline transport
processes (e.g. the inﬂowing Atlantic and Paciﬁc waters) which advect carbon from the highly productive
sub-Arctic domains, including the SIZ, into the less productive polar basins (Fig. 12). Seasonality, as measured by any number of physical parameters (e.g. ﬂuctuations in temperature, solar insolation, etc.), increases
from low to high latitudes.
4.1.2. Double estuary and the thermohaline circulation
The Arctic Ocean is the northernmost extension of the North Atlantic. Atlantic waters crossing the Greenland/Scotland Ridge, passing through the Norwegian and Iceland seas, and entering the AO through Fram
Strait and across the Barents Sea (including those that are transformed in the Greenland Sea and enter at
Fig. 12. Schematic showing tropho–halo–thermo-dynamic processes in the northern hemisphere. ‘Least action’ is required by climate to
redress the global heat imbalance and to ‘‘close’’ the freshwater loop; while most action is required in trophic dynamics to redress
seasonality and to satisfy evolutionary pressure. The amplitude of seasonality increases from low to high latitudes.
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Fig. 13. The Arctic Ocean functions like a double (+ ) estuary. AW, Atlantic water; FW, freshwater.
depth) comprise the core of the Arctic’s thermohaline circulation. Upon entering the AO the AWs underﬂow
fresher waters derived from ice-melt, river inﬂow and the inﬂowing PWs, and generally move as topographically-steered boundary currents following the continental margin and trans-basin ridges (Fig. 13). Episodic
events may draw AW onto the pan-Arctic shelves, particularly along the axis of canyons, where it may be further modiﬁed. Such on shelf ﬂow also delivers nutrients to the shelf system, and an analogous mechanism has
been described on the shelves of the Western Antarctic Peninsula by Pre´zelin et al. (2004). Along this pathway
the original source waters are rendered denser, primarily by cooling (a negative estuary) and lighter, by mixing
with fresher waters (a positive estuary). This complex, double estuary function (Fig. 13) forms the dominant
physical backdrop of for Arctic marine ecology.
Modiﬁcation of AW along this pathway also sets the stage for major shifts in physical habitat. Aagaard and
Carmack (1994) hypothesized that changes in ice extent and freshwater export from the AO might alter the
sites of deep thermohaline overturning currently occurring in the Nordic seas. Suppose, for example, that
the main site of deep convection were to shift from the Greenland Sea into the western Eurasian Basin. Is
it then not reasonable to assume that as the structure and function of the physical habitat changes, so will
the structure and function of ecosystems? A similar argument can be made for the coupled Bering/Chukchi
system should sites of dense water formation change.
4.1.3. Arctic Ocean ecology necessarily entails advection
Biomass concentrations vary spatially on a variety of scales. Physical processes may serve both to produce
biomass locally by bringing nutrients into the euphotic zone or enhancing light conditions through stratiﬁcation, and to accumulate biomass locally by advection. In turn, advection aﬀects Arctic marine ecology in two
fundamental ways. On the global scale, biomass produced elsewhere may be carried by currents to a new location where it may prey upon (top-down) or be consumed by (bottom-up) other organisms. On the regional
scale, biomass may be concentrated by ﬂow structures (e.g. eddies, convergent fronts) to increase concentration locally and thus their availability to predators. Higher-level predators then exploit local concentrations,
but again for a variety of reasons. For example, marine mammals in the Barents Sea follow the capelin front,
which is the result of local production, while those in the North Water polynya are likely attracted by the open
waters, not production. The Grey whales in the Chukchi Sea feed on amphipods that reﬂect both advection
from the Bering Strait and local production. Complex intertwining of physics and biology of course occurs
outside the Arctic as well. For example, Right Whales oﬀ Georges Bank forage near frontal features that concentrate zooplankton (Beardsley et al., 1996). This demands that we distinguish such causal inﬂuences and
their combined eﬀects.
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One of the most prominent features of the AO is the signiﬁcance of advection of AW that enters the AO
north of Spitsbergen and the Barents Sea, and PW that enters through Bering Strait (Fig. 14). The advected
water carries not only signiﬁcant amounts of allochthonous nutrients and suspended biomass into the AO, but
also large quantities of zooplankton. This is particularly the case for the Eurasian section of the AO (e.g.
Wassmann, 2001; Olli et al., 2006), while on the Paciﬁc side the advection of zooplankton may to be less signiﬁcant (but see Springer et al., 1989) and suspended biogenic matter and nutrient supply into the shallow
Chukchi Sea are vital (Ashjian et al., 2005). Among the mesozooplankton the larger Calanus species play
an important role. Adapted to foraging conditions in the sub-Arctic and Arctic, they have extended their life
cycles to spread over more then one year. The boreal to sub-Arctic species C. ﬁnmarchicus increases its life
span up to 1.5 years in the Barents Sea (Tande, 1991). The fully arctic species C. glacialis and C. hyperboreus
have 2–3 and 3–5 year life cycles, respectively (Kosobokova, 1999; Kosobokova and Hirche, 2001). The main
mesozooplankton species in the Bering Sea is C. marshalliae that, like C. ﬁnmarchicus, resides outside the AO.
They can be expatriated to the AO and are capable to survive, but not to reproduce there (Falk-Petersen et al.,
2000; R.J. Nelson, pers. comm.). Both species are advected into the arctic regions west, east and north of Spitsbergen as well into the Chukchi Sea (Springer et al., 1989; Falk-Petersen et al., 2000; Ashjian et al., 2005)
where these expatriates locally comprise at times the major fraction of zooplankton biomass.
Not all advection is northward. For example, C. glacialis and C. hyperboreus are advected southwards from
the AO through Fram Strait and into the Greenland Sea by the EGC where they encounter good conditions
Fig. 14. The Arctic Ocean communicates with the North Atlantic and North Paciﬁc via sills. There is signiﬁcant advection of Paciﬁc and
Atlantic waters. The shelf break and ridges steer the boundary currents. Currents are basically cyclonic around the basin and ridges in the
European sector, but anticyclonic in the Canadian Basin. Allochthonous biomass, in particular long-lived mesozooplankton, follows along
with the currents throughout the AO.
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for feeding and reproduction. Recirculation of ArW from the EGC back into the northward ﬂowing West
Spitsbergen Current (WSC), or eastward ﬂowing North Atlantic Current may return arctic populations back
to the polar basin. The subduction of AW-type water, with its accompanying mesozooplankton community,
under the ArW north-east of Spitsbergen and in the northern Kara Sea appears to set up a band of increased
mesozooplankton along the boundary currents that follow the shelf break and ridges (Fig. 14), as discussed in
Olli et al. (2006).
We suggest that the allochthonous mesozooplankton biomass that is transported into and through the AO
comprises a larger zooplankton fraction than that supplied by local production in these low-production
waters. Zooplankton move with the currents for several years along the pan-Arctic shelf break and transarctic
ridges (Fig. 13) and thus increase grazing pressures there. While small forms such as Oithona sp. and Oncaea
sp. derive from local populations and must obtain suﬃcient food from the low autochthonous primary production for their entire life cycle, we suggest that the dominant mass of mesozooplankton probably obtains
too little food to raise new generations every year, and that their populations remain dominated by biomass
and lipids that were produced and accumulated in the highly productive regions outside the AO. Mesozooplankton can extend their life cycles such that growth occurs in the productive seasons of several years, which
allows overwintering stages to match the sparse phytoplankton or ice algae blooms whenever such blooms
develop. There is an extreme mismatch between the autotrophic production and peaks of allochthonous mesozooplankton biomass. As a consequence a phytoplankton bloom should be impossible in the deep AO, where
mesozooplankton can overwinter at depth and where primary production may be as low as 20 g C m2 y1
(Sakshaug, 2004). Basically boreal and sub-arctic expatriate species with ‘one-foot-in-the-grave’, such as C.
ﬁnmarchicus and C. marshallae, increase substantially the food resources of carnivorous planktivores north
of Spitsbergen, the northern Barents Sea and the northern parts of the Chukchi Sea.
The mismatch between the autotrophic production and the allochthonous mesozooplankton that cannot
successfully reproduce in the central AO may be a key food web characteristic of pan-Arctic shelf breaks
and ridges. It may support an ecosystem where the consumption of biomass (autotrophic, heterotrophic
and terrestrial supplies, as well as stored energy reserves of the larger zooplankton forms) is greater than
the local primary production. That is, the AO should be net-heterotrophic, as suggested by Olli et al.
(2006). This would indeed make the AO fundamentally diﬀerent from any other part of the World Ocean.
A quasi landlocked, deep and low-production ocean, dominated by cyclonic gyres and exposed to signiﬁcant advection of AW and PW from outside has crucial consequences for ecosystem function. Climate change
in an ocean whose circulation is dominated by quasi-closed gyres relates to their orbital period or how fast the
gyres spin. In turn, this orbital period inﬂuences the water mass residence times and potential for biogeochemical change. In general, when advection governs a system, moving fronts and changing currents will have disproportionate inﬂuences on ecosystems. Species distributions change and new species may be introduced by
altered ﬂows. Orbital water movements, long residence times, large amounts of advectively supplied organic
matter relative to local primary production and substantial quantities of mesozooplankton originally grown
outside contribute to a strong and climatically variable top-down regulation. The net eﬀects are that phytoplankton biomass, particularly that of large cells, will be low throughout the AO, that all organisms will be
continuously starved, that terrestrial matter (to the extent digestible or available through microbial cycling)
will be heavily grazed and that most of the autotrophic biomass will be encountered as ice algae, either in
unpalatable forms (e.g. Melosira arctica) or inside the ice.
4.2. Contiguous domains
Climate change implies forcing over large spatial and temporal scales. To link biota to climate strictly
requires viewing all scales from that of climate to that of molecular processes. This view is supported by
the ‘success’ of teleconnection indices, such as the NAO or the ArO, to explain some fraction of the variance
observed in ecosystems, wherein the variance of physical forcing cascades through coupled physical–biological
systems to smaller scales aﬀecting biogeochemical cycling and biota. Still, the teleconnection approach does
not speciﬁcally address causal mechanisms. An alternate approach is that of macroecology, which examines
patterns in species distribution and abundance to determine relationships between abiotic and biotic factors
(Li, 2002). The full set of scales linking climate to biota, however, is seldom accommodated. It is thus useful
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to inquire as to what biogeographical scales come closest to matching those of the climate system (cf. Carmack
and McLaughlin, 2002). Here it is useful to think in terms of contiguous domains; that is, physical habitats
with common salient characteristics – by geography, state or transport processes - that are internally linked
in space, time or both, and within deﬁned contiguous domains the principles of macroecology can then be
applied. Functions within contiguous domains are likely to share a domain-wide response to variance in climate. This approach is similar in intent to that expressed by Sarmiento et al. (2004), who argue that a key
impact of climate change will to shift the boundaries of world ocean biomes. We suggest that the perception
gained from this view will prove valuable in the design of research programs and the management of marine
arctic resources, and that a key to understanding the ecological impacts of climate change lies in understanding the functions within each domain.
The primary contiguous domain of importance is the Arctic Ocean’s b-type stratiﬁcation, already discussed
above, as it deﬁnes connections (through stratiﬁcation similarities) between the AO and the subarctic Atlantic
and Paciﬁc. Within the AO at least four additional climate-linked, contiguous domains can be identiﬁed
wherein global warming may impact ecosystems from the bottom-up and the top-down; these are the seasonal
ice zone, the riverine coastal domain, Paciﬁc-Arctic domain and the pan-Arctic shelf-break and margin
domain.
4.2.1. The seasonal ice zone (SIZ)
At present about half (7 · 106 km2) of the sea-ice cover freezes and melts each year. The top of this solid
cover is habitat for large mammals while the underside provides a habitat for sympagic biota, plankton and
ﬁsh. Global warming is expected to shrink and thin the seasonal sea-ice (e.g. Fig. 7), and thus aﬀects the range
and survival of many arctic species. Warming has already resulted in a retreat of the MIZ during summer, thus
supporting a widening of the SIZ, and this will almost certainly progress. Over decades to come global warming will change the physical features of SIZ and the MYI will shrink (e.g. Fig. 6). Shorter duration of the seasonal ice cover, by earlier break-up and later freeze-up, will impact on the timing of annual cycles critical to
the arctic biota. The seaward extent of LFI will decrease; the permanently LFI-free regions around the AO (at
present mostly the southern Barents Sea) and its functional mode (mainly the lack of or limited stratiﬁcation)
will expand. The present day open water in the southern and central Barents Sea may represents the functional
mode of the future pan-Arctic inﬂow and interior shelves. The latter will be modiﬁed by rivers (stratiﬁcation,
terrigenous matter, and turbidity). It is assumed that outﬂow shelves (network shelves that have a continued
LFI cover during winter) will continue as at present with a smaller proportion of sea ice cover and higher
ﬂuxes of freshwater.
How will the functional modes of various regions change in light of the global warming-induced shrinkage
of the SIZ? From the bottom-up perspective, a number of consequences may be anticipated from longer icefree periods. Earlier break-up will signiﬁcantly increase underwater light availability; at present, ice lingers
through May and June, months of high insolation. A delayed freeze-up will also expose more open water
to forcing by autumn storms. Retreat of the SIZ into the central Arctic basins will impact nutrient availability
on shelves during summer and autumn through increased mixing and upwelling (Carmack and Chapman,
2003). There may be increased productivity if the SIZ routinely retreats past the shelf break and allows coastal
upwelling of nutrient-rich water from the basin interior. We will also experience changes in bottom-up regulation associated with ice algae (less MYI, less ice on shelves) and altered advection of resources. From the
top-down regulation point of view we will face increases in mesozooplankton and krill, changes in top predator populations (fewer seals, more whales) and altered advection of predators (such as boreal ﬁsh). Major
changes are thus expected with regard to primary production, match versus mismatch of autotrophs/heterotrophs, retention versus export of biogenic material and pelagic–benthic coupling.
4.2.2. The Riverine coastal domain (RCD)
A key process in the transport of freshwater through and around the perimeter of the AO is via the formation of buoyancy-driven coastal currents (cf. Chapman and Beardsley, 1989; Weingartner et al., 1999; Bacon
et al., 2002). Such ﬂows form when freshwater of low-density is discharged into the more saline, higher-density
ocean, and are deﬂected to the right by the Coriolis eﬀect. In the Northern Hemisphere, buoyancy-boundary
currents travel in a clockwise sense, with landmasses always to the right in the direction of ﬂow. The dimension
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(width) of buoyancy-boundary currents scales with the Rossby radius RR = (g*h)1/2/f, where g* is the reduced
gravity, h is depth, and f the Coriolis term; typically, in the Arctic RR 10 km. At the pan-Arctic scale, the
multiple sources of freshwater discharge allow the formation of a network of quasi-continuous ﬂows, here
termed the Riverine Coastal Domain (RCD; e.g. Fig. 7). At any given time the RCD is neither stationary
nor continuous; also it is highly idealized in that it ignores wind and tidal forcing. However, it is (a) contiguous
– more or less continuous around the pan-Arctic shelves of Eurasian and the North American continent, and
(b) terrigenous – transporting a strong signal of land–ocean interaction. The RCD may provide an important
migration and dispersal corridor for biota, and is of suﬃcient size and extent to be strongly linked to the climate system. Taking a bottom-up view, some consequences of climate variability on the RCD are evident.
Climate models predict increased precipitation in high-latitudes under greenhouse gas warming. This may
result in altered runoﬀ and export of terrestrial carbon (POC, DOC) to coastal ecosystems. Increased runoﬀ
may also alter erosion, suspended load transport, and thus the turbidity and light climate of coastal waters.
Top-down impacts may also be important as density currents carry macroplanktonic grazers and many anadromous ﬁsh species (salmon, Cisco and Char) use the RCD as a migration (and perhaps navigation) pathway.
4.2.3. The Paciﬁc-Arctic domain (PAD)
Paciﬁc waters entering the AO through Bering Strait circulate almost entirely within the anticyclonic Beaufort Gyre of the Canada Basin at depths between 40 and 280 m, and then exit into the North Atlantic through
Fram Strait and the Canadian Arctic Archipelago (Jones et al., 2003; Kawai-Yamamoto et al., 2006). Such
waters are diﬀerent from their Atlantic-origin counterparts by virtue of their lower salinities, higher nutrients
and distinct biological communities. The storage of PW within the gyre considerably increases the volume of
freshwater contained within the Canada Basin and increases stratiﬁcation. Expatriate zooplankton from the
Bering Sea are transported across the wide Chukchi shelf and into the central Beaufort Gyre (R.J. Nelson,
pers. comm.). The summer (warmer) variety of incoming PW is suﬃciently shallow to aﬀect ice cover (Shimada et al., 2006) and, to a limited extent, supply suﬃcient nutrients to the base of the euphotic zone to support a ubiquitous chlorophyll maximum layer (E. Carmack, unpublished data). Evidence exists that a front
separates the anticyclonic PW and cyclonic AW waters, and that its location varies on decadal time-scales
due to large-scale atmospheric forcing (McLaughlin et al., 1996); such shifts would impact stratiﬁcation, ice
cover and the range and productivity of species originating in the Paciﬁc.
4.2.4. The pan-Arctic margin domain (PMD)
The ﬁnal contiguous domain considered here is the shelf-break and margin that extends around the AO
from Spitsbergen to West Greenland. Material exchange across the shelf-basin boundary has been a focused
priority in Arctic research for over a decade, but has led in part to the ‘sector perspective’ discussed in Section
2. We argue that an ‘azimuthal perspective’ of the contiguous domain is also required, because climateinduced changes associated with light, nutrients, productivity and ice cover likely will be greatest at the
shelf-break. Indeed, the fundamental question of arctic climate research is how the PMD will respond to
the inexorable retreat of sea ice.
The PMD is the domain of the circum-polar boundary currents of both Atlantic and Paciﬁc origin and the
pathways along which the material properties of the global ocean and the pan-Arctic shelves are carried, and it
is the site where both positive and negative estuarine exchange takes place (Fig. 12). But along this azimuthal
pathway large diﬀerences can be expected in both space, for example, proximity to source waters and rivers,
shelf typology, depth and width, local topographic features (e.g. canyons and sea valleys), seabed material and
ﬁnally time: synoptic events, seasonality, interannual variation and climate trends. Retreat of sea ice beyond
the PMD, both seasonal and permanent, will have large and likely non-linear eﬀects on exchange process,
water mass formation and biota. From a bottom-up perspective the exposure of the PMD to changed atmospheric forcing is expected to greatly enhance shelf-break upwelling and the supply of nutrients to the panArctic shelves; but the net impact of this will vary from shelf to shelf (Carmack et al., 2004b). Because of their
salt (b) stratiﬁcation the pan-Arctic shelves rapidly become nutrient limited as soon as light becomes available,
but the longer periods of exposure of both the PMD and shelves to wind mixing will facilitate greater nutrient
resupply. From a top-down perspective, the retreat of sea ice beyond the PMD has obvious consequences to
marine mammals that utilize the ice cover as a habitat: bears will be isolated from land, walrus from shallow
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feeding sites, seals from stable food sources at critical times of the year. Also, retreating ice and the associated
decrease in internal ice stress, which opposes wind forcing, will lead to stronger coupling of the wind ﬁeld and
the ocean, thus changing currents and the advection of expatriate communities.
4.3. The functional signiﬁcance of the marginal ice zone (MIZ)
The quasi-uninterrupted MIZ system engirdling the AO, that seasonally advances and retreats over almost
the entire extent of the pan-Arctic shelves, is a prominent physical–biological characteristic that deserves particular attention. It is commonly assumed that the MIZ is highly productive and more so than open waters
(e.g. Ferreyra et al., 2004). This assumption derives from the notion that phytoplankton and zooplankton
stocks are high, that primary production rates are elevated, that sediments are rich in biogenic matter and diatoms and that benthos is rich in the MIZ. The high accumulation rates of diatoms in sediment below the MIZ
are traditionally interpreted as signs of increased vertical export (e.g. Smetacek, 1985) and increased productivity (Andersen et al., 2004). In fact, on an areal and time-integrated basis, the MIZ has only moderate productivity. In the Barents Sea, for example, primary production in the MIZ is 1/3 to 1/2 that in the AWdominated regions south of the SIZ (Reigstad et al., 2002; Wassmann et al., 2006).
What is the basis of this apparent contradiction, and what processes in the MIZ gives rise to these opposing
conclusions? The answer is to found in the functional mode of the MIZ. The noticeable and frequently spectacular phytoplankton blooms in the MIZ are the consequence of the wintertime renewal of nutrients by convection, the onset of stratiﬁcation and the sudden intensiﬁcation of photosynthetically active radiation.
Stratiﬁcation and limited nutrient availability constrains the system to a transient bloom of short duration.
However, because of the strong stratiﬁcation and rapid phytoplankton growth, the accumulation of Chl a
is signiﬁcant, often in the range of 10–20 mg m3 (e.g. Wassmann et al., 1999; Matrai et al., 2006). It is during
the MIZ bloom that the stocks of diatoms are highest (Ratkova and Wassmann, 2005) and that substantial
quantities of herbivorous zooplankton, often copepods, migrate to the resulting abundant feeding grounds
in the euphotic zone (Arashkevich et al., 2002). Also the vertical export of biogenic matter, fecal pellets
and phytoplankton out of the euphotic zone is extraordinarily high (up to 2000 mg C m2 d1; Olli et al.,
2001).
These examples illustrate why the MIZ is commonly assumed to be highly productive. One has to distinguish, however, between stocks and rates per volume (e.g. m3) or area (e.g. m2). From an integrated point of
view the MIZ has high primary production rates per m3, within a shallow euphotic zone, but not per m2.
The MIZ is thus only moderately productive, and its most prominent feature is the high accumulation of various stocks in the euphotic zone that (in the case of phytoplankton) can be perceived with the bare eye
(Fig. 15). The stocks of phytoplankton and suspended biomass in the waters outside the SIZ (e.g. the southern
Barents) or in more mixed waters (e.g. the southern Chukchi) do not increase so much, but remain persistently
high over lengthy periods of time during the productive season. The actual productivity of the MIZ is thus a
question of deﬁning it adequately with regard to volume, area and time. Depth and time integrations show
that the MIZ has low to moderate primary production, that ice-free waters with increased vertical mixing have
high primary production, and that the deeply mixed regions (e.g. the central Norwegian Sea) have decreased
primary production (Fig. 15).
The current misinterpretation of the relative productivity of the MIZ also derives from the notion of
increased diatom accumulation rates in sediments and maxima in benthic macrofaunal biomass (e.g. Wassmann et al., this volume). This phenomenon does not, however, mirror the productivity of the MIZ, but
instead reﬂects its function. The favorable phytoplankton growth conditions result in a signiﬁcant biomass
accumulation, favor diatoms and the prymnesiophyte Phaeocystis pouchetii (Ratkova and Wassmann, 2005;
Wassmann et al., 2005) and generate excellent feeding grounds for zooplankton grazing. High accumulation,
in turn, results in rapidly sinking phytoplankton aggregates, phytoplankton cells and fecal pellets (from suspension feeding). Ice algae can, to various degrees, add to the increased vertical export. Retention of sinking
material in the upper 30–50 m below the stratiﬁed euphotic zone is signiﬁcant, resulting in relatively high vertical ﬂux attenuation in the MIZ (Wassmann et al., 2003; Fig. 15). Nevertheless, there is a distinct and rapid
pelagic–benthic coupling in the MIZ that translates into increased, fresh biomass transfer to the benthic nepheloid layer, the sediment and benthos. The high accumulation rates of diatoms in sediments and increased
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Fig. 15. Schematic depiction of the functional signiﬁcance of the Marginal Ice Zone and its adjacent permanently ice-covered and open
waters. The transition between the permanent ice cover (MYI), the seasonal ice zone (SIZ), the MIZ and two open water scenarios are
indicated. The abundance of ice algae, phytoplankton, suspended biomass, primary production and the depth of the euphotic and mixed
layer are indicated. The vertical arrows indicate the vertical export of biogenic matter during the most prominent growth phase of
phytoplankton. The extensive export of fresh suspended matter during the MIZ bloom is illustrated by green arrows, while the
degradation in the water column at other sites results in export of detritus, depicted by brown arrows. The primary production is highest in
stratiﬁed a oceans with moderate vertical mixing, while the MIZ as well as deeply mixed a oceans experience decreased productivity. The
vertical export of the MYI, SIZ and the open oceans I and II are minimal, moderate, great and small, respectively. The scheme is based
upon an assumption of dominantly bottom-up control, while top-down regulation by overwintering and advected mesozooplankton has
not been considered. Compiled from elements discussed by Ferreyra et al. (2004) and Sections 3.2 and 4.4.
macrofauna abundance are, thus, not sign, of increased primary production, but reﬂect the functionality of the
MIZ water column and a tight pelagic–benthic coupling.
4.4. The physics–biology continuum and pelagic–benthic coupling
The primary production and vertical export of biogenic matter on pan-Arctic shelves is extremely variable
on both a spatial and interannual basis (Wassmann et al., 2004). Because the shelves are typically quite shallow, the benthos and organisms feeding on them play signiﬁcant roles. How is planktonic production on panArctic shelves connected to the benthic boundary layer, benthos and sediments? The traditional segregation
between planktonic and benthic researchers demands increased focus upon an important niche of oceanographic research – i.e. pelagic–benthic coupling – that is frequently unaccounted for. This is partly also true
for the pan-Arctic shelves. In order to evaluate the approximate character of and slowly emerging concepts
about pelagic–benthic coupling, a conceptual scheme and quasi-quantitative characterization of this coupling,
is proposed based upon data from the AO.
A continuum of physical and biological forcing shapes the basic conditions for primary production and suspended matter accumulation in the upper layers (Fig. 15). Primary production in the euphotic zone is basically
bottom-up regulated through physical processes related to ice-cover, stratiﬁcation, vertical mixing, light and limitations such as nutrient availability and productivity/irradiance relationships (Fig. 16a and b). The physical–
biological continuum creates the base for new production and the potential standing stock of primary producers that can be grazed, recycled, and exported vertically. The amount of biogenic carbon that can be potentially
exported or harvested is thus limited by the system’s rate of new production (Wassmann, 1998).
Below the euphotic zone biological forcing becomes more important than physical forcing for the fate of
biogenic carbon, and top-down regulation, through various categories of grazing zooplankton takes over
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471
Fig. 16. Schematic, semi-quantitative diagram of the relationship between the production and vertical export of biogenic matter during
the productive season as a function of the physical–biological continuum. The scheme is for the most part based upon experience from the
Barents Sea shelf that is >200 m deep, but probably it is applicable at shallower or deeper sites in the pan-Arctic region. It is primarily the
physical forcing (ice cover, stratiﬁcation, vertical mixing, and light, nutrients) and to some extent the biology (phytoplankton species, P/I
relationship) that determine new production and suspended phytoplankton biomass build-up. This is indicated in a qualitative manner by
the purple arrows in the euphotic zone (blue) in A (stratiﬁed scenario) and B (vertically mixed scenario). White, straight vertical arrows:
vertical mixing. Vertical mixing is so important in B that white rotary arrows are applied in addition. The extent of new production
determines ultimately the maximum biomass that sinks into the aphotic zone (green). The variability of vertical ﬂux [yellow lines, bottom
abscissa (mg C m2 d1)] is a function of both the magnitude of export production (qualitatively indicated by the purple arrows) and a
suite of factors in the upper layers, among which grazing is most important (qualitatively indicated by red arrows in A and B). Scenario A
illustrates the basic relationship in a stratiﬁed ocean (e.g. b oceans such as the SIZ). Scenario B illustrates the vertically mixed ocean (e.g.
open ocean (a) I and II, Fig. 15). The relevance of scenarios A and B is examined in C, illustrating 4 typical vertical export proﬁles of POC
[ 40 daily measurements from the European sector of the Arctic region (Olli et al., 2006; M. Reigstad and P. Wassmann, unpubl. res.)].
(1) polar ice cap (85–87 N), north of Fram Strait (MYI); (2) northern Barents Sea MIZ (78–81N); (3) southern Barents Sea MIZ (74–
76N); (4) south of SIZ of the central Barents Sea (72–73N).
(Fig. 16a and b). Along the physics–biology continuum, biological processes now assume the lead role for the
fate of suspended and sinking biogenic matter. The food-deprived community of heterotrophs orient themselves toward the source of food, i.e. they move upwards toward the base of the euphotic zone and remain
there. Vertical migration is not important under the quasi-continuous light conditions of the high-latitudes.
Thus the greatest amount of zooplankton biomass is usually encountered just below the euphotic zone (e.g.
Olli et al., 2006; Fig. 16a). If deep vertical mixing were signiﬁcant, such in the a ocean waters outside the
SIZ, physical forcing would also be signiﬁcant below the euphotic zone, resulting in extensive mixing of suspended biomass and export production in the upper layers (Fig. 16b).
The strength of the grazing, the types of grazers and the grazing eﬃciency determine the mode by which
suspended biogenic matter is consumed, thus eﬀecting both a slowing (sinking particles removed) and acceleration (fecal pellets produced) of vertical export (Wassmann et al., 2003). However, fecal pellets are highly
nutritious, and through processes such as coprophagy and coprorhexy most rapidly sinking particles are
retained in the upper layers (Wexels Riser et al., 2001). As a result, 50% and more of the export production
leaving the euphotic zone can be recaptured and retained in the upper 100 m of the AO (e.g. Olli et al., 2001).
In the upper layer the physics–biology continuum is forced by bottom-up regulation, which regulates new production, i.e. that part of the primary production that can be exported vertically (see purple arrow in Fig. 16a).
With increasing depth, top-down regulation increasingly takes over the biology/physics continuum, thus forcing vertical export in the opposite direction (Fig. 16a). As a consequence, the connection between new production and vertical export can be explained in a curvilinear manner (Wassmann et al., 2003). The planktonic
heterotrophs thus impose a grazing pressure toward the export production region by developing a retention
ﬁlter (Wexels Riser et al., 2001) whose vertical extent and degradation eﬃciency determines the vertical ﬂux
attenuation and the shape of the vertical export proﬁles.
Based upon more than 40 proﬁles of short-term, quasi-Lagrangian sediment traps in the deep AO, the stratiﬁed SIZ (a b ecosystem) and vertically mixed open waters (an a ecosystem) of the Barents Sea (6–14 traps over the
upper 200 m, often deployed from ice ﬂoes) four typical proﬁles are depicted that mimic in principle the average
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rates and shapes of the vertical relationship (Fig. 16c). These may be characteristic as well for the pan-Arctic
region. Primary production in the central Barents Sea, just outside the SIZ, is continuously high (150–
180 g C m2 y1) due to strong vertical mixing and nutrient pumping. Vertical export from the surface layers
is thus high, but due to turbulent mixing it declines smoothly with depth. The rate of decline with depth is the
least, and even at 200 m signiﬁcant amounts of biogenic carbon are injected into the bottom layers (Figs. 16c, 4).
There seems to be a continuous development of pelagic–benthic coupling from the southern Barents Sea (an
a ocean), through the SIZ into the MYI close to the North Pole. Very high daily export comes from the upper
layers that have high, volume-based primary production in a relatively shallow euphotic zone with high Chl a
concentrations. However, this signiﬁcant export is very eﬃciently retained by the planktonic heterotrophs, and
advective supply of mesozooplankton plays an additional role. Thus as much as 80% of the export production
may be retained in the highly productive southern SIZ over inﬂow shelves (Fig. 16c, 1).
In the northern SIZ of the Barents Sea primary production is lower (60–80 g C m2 y1) due to ice cover
and strong vertical stability. Thus export production is lower. The relative reduction of the vertical export in
the upper layers is similar, but the depth where the attenuation slows is shallower. The potential top-down
regulation is suﬃciently high to minimize the unavoidable losses of biomass and nutrients through sinking.
Most of the retention of biogenic matter takes place in the upper 50 m, and vertical ﬂux attenuation is slight
in deeper layers. This scenario may be typical for the more ice-covered regions inﬂow shelves (Fig. 16c, 2).
The primary production in the permanently ice-covered AO takes place near the underside of the ice and is
strongly exposed to grazing (Olli et al., 2006). In the Nansen and Amundsen Basins and along the trans-arctic
ridges this grazing may be particularly strong due to the advection of mesozooplankton with multi-year life
cycles. Thus, any increase in new and export production would be consumed by the heterotrophs in the upper
20 m, thus eliminating any build-up of near-surface biomass and maintaining a linear vertical ﬂux based upon
degraded biogenic matter (Fig. 16c, 3).
If the currently ice-covered shelves continue to warm, and if ice-cover retreats over the coming decades,
then the vertical ﬂux proﬁles in a given region will change to resemble the proﬁles depicted in Fig. 16. For
the regions with the heaviest ice cover we suggest a progression that proceeds from the polar ice cap (1) across
the northern MIZ (2) and south to the SIZ (3). In the case that this region ﬁnally lies outside the SIZ and
freshwater stratiﬁcation eventually erodes through wind action (transforming into an a type ocean), even progression (4) is possible inside today’s pan-Arctic shelf SIZ. Reduction in ice cover involves dramatic changes
for the production and fate of biogenic matter as reﬂected by the (a) magnitude of export production and (b)
curvature of the vertical proﬁle of downward ﬂux. If the periphery of the polar ice cap region shifts to a weak
stratiﬁcation, the export production may increase by more then 20 times, while the injection into the BBL at
least triples. At an even later stage of global warming, when sea ice has been reduced permanently to the north
and the surface melt water supply is eroded, even today’s central Barents Sea scenario may become typical for
what is at present the SIZ of the AO (Fig. 16c). This implies a ‘‘borealisation’’ (or at the most extreme an
‘‘Atlantiﬁcation’’) of the present southern SIZ, i.e. a functionality now encountered in boreal regions will
in the future be exposed to an arctic light climate. Which key species and key functional ecosystem types
are favored during such a future scenario can only be a matter of speculation.
So far only the role of biogenic matter from phytoplankton has been discussed. However, a second major
source of fresh biogenic matter in the AO is ice algae. There exist two contradictory views, which state that ice
algae are (e.g. Carroll and Carroll, 2003) and are not (Wassmann et al., this volume) important for the biogeochemical cycling in the AO. In the Barents Sea SIZ, the maximum contribution of ice algae to primary
production is less then 7%. Here the phytoplankton standing stock can be more than 20 times greater then
that of ice algae (Wassmann et al., this volume). The highly variable signiﬁcance (in space and time [years])
increases toward the north and toward regions of heavy ice cover. In the Canadian Archipelago ice algae production is rather small, but vertical export can be signiﬁcant during certain years (Michel et al., this volume).
Also, other publications note the highly variable presence of ice algae in sediment traps. Ice algae can accumulate mainly in LFI and MYI. Their accumulation derives mostly from a lack of palatability (e.g. Melosira
arctica) and diﬃcult access for grazers (e.g. inside the ice), rather than from high production rates. However,
at relatively shallow sites along the pan-Arctic shelves, in connection with LFI (low planktonic primary production) and in regions where MYI drifts unto the shelf ice algae can be a signiﬁcant food source, in particular
for the benthos (Carroll and Carroll, 2003; Tamelander et al., 2005).
E. Carmack, P. Wassmann / Progress in Oceanography 71 (2006) 446–477
473
5. Physical–biological forcing of Arctic shelves: past, present, future
Is the past a window to the future, as developed by Darby et al. (this volume) or – as asked by Overpeck
et al. (2005)? – Are we moving into a new state that is outside the known palaeo-record? Will the AO continue
to warm (cf. Polyakov et al., 2005)? Current CO2 levels are already double that at the peaks of glacial–interglacial ﬂuctuations and increasing rapidly. With continued loading of greenhouse gasses into the atmosphere,
the future AO is very likely to have substantially less MYI than currently exists; perhaps, eventually, none (e.g.
Fig. 7, Johannessen et al., 2002). Because of this it behooves us to think how the AO may have functioned at
various times in the past. One can ask, for example, how is the present AO diﬀerent in structure and function it
was at the end of the last glacial period? Did it have a completely diﬀerent environmental state or has it gradually changed into the present one? Here it is useful to start with the freshwater budget and stratiﬁcation. In
the modern AO the freshwater budget is dominated by (1) river inﬂows, (2) Paciﬁc inﬂows through Bering
Strait and (3) water mass transformations on the broad pan-Arctic shelves (Fig. 17A). But, only 10 KY
BP, sea-level was over 100 m lower, massive glaciers may have blocked much of the north-draining rivers, Bering Strait was closed and shelves were basically absent. Brine injection on the shelves did not take place, the
rivers were draining – if at all – directly over the shelf break into the AO and water column modiﬁcations on
the outer shelves by the tides were missing. Going back farther in time, to the early Pliocene (5–3 MY BP),
sea level was higher than at present (25 m), surface temperatures were much higher and glaciers are believed
to have been absent from the Northern Hemisphere. The ecological response to such altered ice and hydrological conditions poses a major research question.
These ‘paleo-scenarios’ provide us with a range of possible futures within which to consider impacts on
northern food webs. For example, the expected increase of sea level of 1–2 m in decades to come and warming
will result in large-scale erosion of coastal regions, and an increase in river discharge is expected. In concert,
this will result in a higher discharge of terrestrial organic matter to the AO, decreased primary production on
Fig. 17. (a) Idealized view of the Arctic Ocean during the height of the last glacial. Notice the vast exposed shelf regions and that the
Arctic Ocean was only connected to the Nordic Seas. The Barents Sea and the shelf of west Greenland were shallow, but ice-covered. (b)
The present interglacial period has a signiﬁcantly increased Arctic Ocean surface with an inundated shelf. Also, the Bering Strait is open
and the connection of the Arctic Ocean has become hemispheric. Forthcoming sea level rise and permafrost decline will result in coastal
erosion and a further widening of the pan-Arctic shelves.
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E. Carmack, P. Wassmann / Progress in Oceanography 71 (2006) 446–477
the shallow shelves (induced by increased turbidity) and the food webs of the interior shelves may expand.
Further, Aagaard and Carmack (1994) proposed a simple conceptual model of convective renewal occurring
at various sites in the AO and adjacent seas under varying scenarios of increasing and decreasing freshwater
supply. If such physical systems were to undergo catastrophic (abrupt) change, so would their ecological functions. A logical extension of the ecology of advection is that as gyres and fronts shift, and as physical habitats
move, some signature of their present foodweb structure and function will also move.
The pan-Arctic shelves have experienced completely diﬀerent states in the geologically recent past, separated
by abrupt state shifts. Whether or not the AO is moving into a new state that is outside the known paleo-record
is thus a question of the time interval that is considered. The immense changes in climate forcing of the AO over
relatively short evolutionary time scales, suggests that its ecosystems are capable of coping with further climatic
changes even abrupt ones, but that the range and survival of individual species is less certain. Disproportionate
success of adaptation and relocation of one species relative to another within the system may lead to fundamental changes in trophodynamic phasing, with major consequences to food web structure (cf. Parsons, 1988).
Points of no return, i.e. points were climate forcing changes the state of an ecosystem irreversibly are extremely
diﬃcult to assess. Despite this we have to ask how we will cope with future changes, on the shorter or longer
time scales. To manage for the future we must insure that the biological components of these biomes are sufﬁciently robust to move with and reorganize within their moving physical habitats; such is the ecology of complexity. Few can challenge the moral strength of the so-called Precautionary Principle. Should this principle be
extrapolated into the range of possible futures; and should especial attention be given to insure resilience of
keystone species and top predators, even in the face of market pressures? We think so.
The lack of a consistent perception of the AO generated our goal to present a more balanced view of panArctic shelves. The reader may now legitimately ask, ‘why have we tried to develop unifying concepts, explore
typologies and simplify the driving forces of physical–biological coupling in the pan-Arctic domain, while at
the same time painting so many possible futures for which we as a society are obliged to prepare?’ The equally
legitimate answer is, ‘‘That’s what scientists do.’ So we ask in turn, ‘‘What do policy-makers do?’’ But whatever scientists and policy makers do, they must do it together. Working in concert, the policy-makers and scientists must recognize and understand the characteristics of the entire Arctic System, including the social
domain and its responses to changes (cf. Hamilton et al., 2003). Based upon this course of action, better adaptation and mitigation strategies to address global warming and other anthropogenic activities aﬀecting the AO
can be developed.
Acknowledgements
E.C. was supported by the Canadian Department of Fisheries and Oceans and Project Energy Research
Development, and by collaborations made possible through the Joint Western Arctic Climate Study (JWACS)
and Beaufort Gyre Exploration Project (BGEP). P.W. was supported by the Research Council of Norway’s
Norklima programme through the projects Carbon ﬂux and ecosystem feed back in the northern Barents Sea
in an era of climate change (CABANERA) and Marine Climate and Ecosystems in the Seasonal Ice Zone
(MACESIZ). Thanks to the ARCTOS network and Shelf Basin Exchange (SBE) for encouragement and helpful discussions. Crucial discussions with Fiona McLaughlin, Marit Reigstad. Kalle Olli and Jackie Grebmeier
are gratefully acknowledged. Graphics were prepared by Patricia Kimber, Frøydis Strand, Peter van Hardenberg and Bill Williams.
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