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2.2.2 Arctic Food Webs

Marine communities of the Arctic can be divided into three compartments or realms:  the pelagic, benthic and sea-ice.  While each of these compartments have a closely associated assemblage of organisms, they are linked to each other with many overlapping species.

Our understanding of food webs and trophic linkages within the food web are built upon several different types of data.  Direct observations of feeding behavior and stomach contents analysis are the most common types of information used to determine prey items and linkages.  However, the ability to collect this type of data in certain habitats is difficult, particularly in Arctic and deep water environments.  More recent methods have been developed that evaluate stable isotope ratios in tissues and in lipids.  In the Arctic, the ratio of N13 and N15 has been an effective method for evaluating trophic levels.  The presence of long-chained C20:1(n-9) and C22:1(n-11) fatty acids and alcohols has also been used as an indication of whether Calanoid copepods are a component of the diet.

In this section we briefly summarize the food webs for each of these different realms and identify VECs for the Arctic marine ecosystems (Figure 2-4).  Following this section, the components of each of these different food webs is discussed in more detail. Pelagic Communities

The pelagic food web is controlled by light and ice cover, altering the growth conditions for phytoplankton and the ability of surface-oriented predators to access prey.  In early spring, increasing light and ice melt result in the release of ice algae into the water column and a dramatic increase in phytoplankton growth (Falk-Peterson et al. 2005).  Some of the highest rates of primary production occur in these marginal ice zones (MIZ).  While phytoplankton blooms are initiated by an increase in light, the magnitude and duration of the blooms are controlled by the nutrient concentrations (Tremblay et al. 2012). Nutrients increase in surface waters during the low production of the polar night and allow for the high production (approximately 50% of the total annual production) in the MIZ.  The duration of the phytoplankton blooms is limited by nutrient availability and the decrease in light availability during the fall results in only one significant bloom event in the Arctic Ocean.  The increase in open water and phytoplankton blooms begins in the boreal waters and progresses northwards towards higher latitudes over the spring and summer (Falk-Peterson et al. 2005). The predominant groups of phytoplankton include prasinophytes, diatoms, haptophytes, green flagellates, dinoflagellates, and chrysophytes, with blue-green algae (cyanobacteria) in the southern regions (Hsiao 1978, Sakshaug 2004; Li et al. 2009, Fujiwara et al. 2014).

The zooplankton community in the Arctic is dominated by copepods, particularly the Calanoid copepods Calanus glacialis and C. hyperboreus.  Both species are endemic to Arctic waters, with all life stages found in the Arctic (Sakshaug 2004). Calanus finmarchicus is a subarctic species that is found in the Atlantic domain, but does not reproduce in Arctic waters (Falk-Petersen et al. 2007).  The diatom –Calanus food chain is considered to be critical to the overall production in the Arctic.  Calanus spp. take advantage of early ice-algae blooms, continue feeding through the planktonic diatom blooms, converting low energy sugars into a high energy lipid reserves (Niehoff 2007).  They create lipid stores that are rich in longer-chained fatty acids and alcohols; a characteristic that allows them to over-winter in a non-feeding state.  The combination of rich lipid reserves and their large size make C. glacialis and C. hyperboreus a key prey item for higher level consumers throughout the Arctic.  While less lipid rich and smaller in size, C. finmarchicus is a valuable food resource in the Atlantic sector, particularly the Barents Sea.  Other pan-Arctic copepods include Oithona similis and Metridia longa.  The subarctic species Neocalanus spp., Eucalanus bungii, Pseudocalanus spp. and M. pacifica are found in the Pacific domain (Sakshaug 2004; Griffiths and Thomson 2002).

Figure 2-4. Arctic Food Webs
Figure 2-4. Arctic Food Webs

Euphausiids (krill) are a subarctic herbivorous species that is abundant in portions of the Atlantic domain (Thysanoessa inermis) and in the Bering Strait-Chukchi-Beaufort region (T. longicauda and T. raschii; Suydam and Moore 2004; Letessier et al. 2009).  While not as common as in the Antarctic, euphausiids are a key prey resource for higher-level consumers, in particularly the Bowhead whale (Brinton 1962).  Other pelagic invertebrates that act as secondary consumers include amphipods, squid, and jellyfish.  Hyperiid amphipods are large, free-swimming amphipods that feed on both smaller zooplankton and Calanoid copepods (Auel et al. 2002).  The species Themisto libellula is a pan-arctic species found associated with sea-ice and shelf waters, whereas the species T. abyssorum and Cyclocaris guilelmi are more closely associated with outer shelf and deep waters of the Arctic (Auel et al. 2002; Kraft 2012).  Second to copepods, hyperiid amphipods are a common food item for fish, seals, and birds.  The squid,Gonatus fabricii is abundant in the Arctic and subarctic waters of the North Atlantic, with the squid Berryteuthis magister more commonly found in the Pacific Siberian-Chukchi waters (Gardiner and Dick 2010; Roper and Young 1975).  Squid are agressive predators and can move easily from the surface to deeper waters of the Arctic, being an important vertical integrator of marine food webs (Navarro et al.2013).  Arctic squid are an important prey item for narwhals (Monodon monoceros), White whales (Delphinapterus leucas), porpoise, and some seals.  Jellyfish are common in Arctic waters and can occur in abundance, representing a important consumer of zooplankton (Gardiner and Dick 2010).

Throughout the Arctic, Arctic cod (Boreogadus saida) and Polar cod (Arctogadus glacialis) represent a critical link between the zooplankton community and higher trophic levels (e.g. seals, toothed whales).  Both species are truly pan-Arctic occurring in all marine waters of the Arctic and are widely distributed throughout the Arctic, occupying nearshore, pelagic, and sea-ice habitats, residing both at depth and near the surface waters, depending upon age and season (Breines et al. 2008; Madsen et al. 2009).  Both B. saida and A. glacialis can be found in small numbers or in large, densely packed schools.  The primary prey for the Arctic gadids is Calanoid copepods and hyperiid amphipods (Sufke et al. 1998; Lonne and Gulleksen 1989; Bradstreet and Cross 1982; Frost and Lowry 1984).  Capelin (Mallotus villosus) are also an important secondary consumer throughout the Arctic, particularly in the Barents Sea where they are the primary link between C. finmarchicus and Atlantic cod (Gadus morhua; Blanchard et al. 2002; Hamre 1994; Mehl and Yaragina 1992; Titov et al. 2006).  Capelin are energy dense fish that move throughout the more estuarine nearshore waters and the Arctic waters.  Arctic and polar cod, capelin, and herring are important food resources for higher trophic levels including larger fish (e.g. Atlantic cod), marine mammals, and birds.

Anadromous fish are primarily found in the sub-adult and adult stages in the nearshore brackish waters of the Arctic.  Arctic char are circumpolar and can be numerous in waters of the Russian Arctic, where lower salinity waters extend across much of the shelf (Craig 1984; Mecklenburg et al. 2011; Sherman and Hempel 2008).  Cisco (Coregonus spp.) and salmonids will also use the nearshore zone during periods of high river flow and ice melt.  Anadromous fish feed on a variety of nearshore resources including benthic copepods and amphipods, herring and capelin (Dunton et al. 2012).  They represent an important prey resource for marine mammals and humans while in their freshwater habitats.

Pelagic fishes of the Arctic deep water regions are not as well understood as nearshore and shelf species.  Studies that have focused on deep waters have found that Arctic cod are numerically important throughout the year, with some stratification by water properties and by age group (Geoffrey et al. 2013; Parker-Stettner et al. 2011). Other midwater and deep water fishes common in other oceanic basins have been observed in the Arctic, including Myctophids and Gonostomidae (Reist and Majewski 2013; Dolgov et al. 2009; Jorgensen et al. 2005).  The diel vertical migration patterns observed in other systems may not occur throughout the year in the Arctic due to the polar day and night. Benthic and Demersal Communities

Benthic communities in the Arctic are influenced by substrate type, presence and interaction with seasonal or permanent ice, salinity and temperature.  Intertidal and nearshore subtidal communities are often limited by direct contact with ice, seasonal freezing of soft substrates, and scouring and scraping behavior of melting ice.  Additional limitations in community diversity and abundance in the nearshore environments are highly variable salinities in areas influenced by large river systems in the Beaufort, Barents, Kara, and Laptev seas.  Soft substrates dominate the Arctic seas, with silts and clays occupying the deep water basins and the outer shelf; fine sands and silts are common across the shelf, while coarser sands and cobbles limited to isolated portions of the shelf and the nearshore zone.

Benthic macrofauna in the Arctic, like most oceanic basins, are dominated by polychaetes, bivalve mollusks, crustaceans (e.g. amphipods and isopods), and echinoderms.  In the nearshore and inner shelf communities, Arctic mollusks often define the benthic communities and are a key prey item for higher trophic levels (e.g. walrus, bearded seals).  The salinity tolerant bivalve clam Portlandia arctica is a dominant species in nearshore zones and is able to rapidly repopulate areas of disturbance.  The Greenland cockle, Serripes groenlandicus, is a common circumpolar bivalve found up to 100 m depth on a variety of substrates and is a main component of the diet for walrus and bearded seals.  Macoma calcarea is also a common component in Siberian-Chukchi-Beaufort and Barents-Kara-Laptev shelf communities, as well as in the fjords of northern Canada and Baffin Island (Grebmeier and Cooper 2012; Denisenko 2007; Filatova and Zenevich 1957).  Other dominant clams in the shelf include Astarte sp.,Mya truncata, Tellina sp. and Yoldiella solidula.  In deep water benthic habitats, bivalves are smaller and less common, with species that are common to other oceanic regions (Axinopsida, Nucula, andNuculana.

As with clams, amphipods play an amplified role in the benthic communities of the Arctic, relative to other oceanic regions.  Arctic amphipods include both infaunal and epifaunal species.  The most widely distributed species across the Arctic are Ampelisca eschrichti, Anonyx nugax, Arrhis phyllonyx, Gammarus setosus, and Byblis gaimardi (Piepenburg et al. 2011; Dunton et al. 2012).  Of particular interest are the Lysianassid amphipods; a species rich group of epibenthic omnivorous amphipods that are key scavengers in the Arctic and deep sea waters.  Many are especially adapted to scavenging with specialized mouthparts and extendable guts for food storage.  In shallower waters, Lysianassid amphipods may have a more diverse diet.  Lysianassid amphipods are domenate invetebrate macrofauna in certain environments.  On tidal flats, Onisimus litoralis constitutes up to 95% of the macrofaunal density (Weslawski et al. 2000).

Polychaetes are among the most abundant infaunal species of the shelf and deep water benthos throughout much of the Arctic (Bluhm et al. 2011).  Species that represent the Arctic shelf are species observed in temperate habitats and include Maldane sarsi, Spiochaetopterus sp., Chone sp., Lumbrineris sp., Capitella capitata, and Eteona longa (MacDonald et al. 2010; Renaud et al. 2007).  Echinoderms in the Arctic are the dominant epibenthic megafauna, with ophiuroid brittle stars occurring throughout the shelf.  Dense aggregations of brittle stars can be found in areas of organic enrichment, such as polynyas (Piepenburg et al. 1997).  The sea urchin, Strongylocentrotus droebachiensis is also found in Arctic waters and is an epibenthic omnivore, often grazing along the bottom of rocky or soft substrates.  Decapod crustaceans are less common in the Arctic and are primarily represented by the shrimp Pandalus borealis and Pandalopsis dispar and the crab Chionoecetes spp. and the Red king crab (Paralithodes camtschaticus; Bluhm et al. 2009; Iken et al. 2010; Orlov and Ivanovo 1978).

Epibenthic or demersal fish communities in the Arctic are dominated by sculpins (Cottids) and eelpouts (Zoarcids); taxa that are speciose, eurybathic, and pan-arctic (Mecklenberg et al. 2011).  Arctic cod are also an important demersal fish found associated with the benthic zone at all depth ranges (Majewski and Riest 2013).  The genera Myoxocephalus and Lycodes are genera that well represented in the Arctic (Mecklenburg et al. 2011).  The Arctic flounder (Pleuronectes glacialis) and the Greenland halibut (Reinhardtius hippoglossoides) are also important epibenthic predators.  Demersal fish generally feed on benthic infauna, as well as small or juvenile fish.  Common predators include seals, birds, and some larger fish.  As such demersal fish act as a link between benthic infauna and epifauna and higher trophic levels (Dunton et al. 2012). Sea-ice Communities

Sympagic communities include those organisms that live in close association with sea-ice.  In some cases, species are obligate to the sea ice, but many are representatives of the pelagic or benthic community that have a portion or all of their life cycle in the ice (Melnikov 1997).  There appear to be differences between the seasonal ice communities and those that inhabit the permanent ice.  However, to a great extent the species assemblage appears to be similar with differences in abundance and biomass.  Brine channels that form in the ice-water interface create a protected environment for an algal community dominated by pennate diatoms (Melnikov 1997; Sakshaug et al. 2009).  Harpacticoid and cyclopoid copepods are the primary consumers of the ice-algae moving within the brine channels as well as along the ice bottom (Kramer 2010).  The sympagic amphipods (Gammarus wilkitzkii, Onismus spp. and Apherusa glacialis) represent a critical link between the sympagic algal and copepod communities and higher trophic levels (Hop et al. 2000; Melnikov 1997; Arndt and Swadling 2006).  The herbivorous amphipod, A. glacialis, is a primary food source for the Little auk, Alle alle.  Both G. wilkitzkii and Onismus spp. are motile predators that feed on the sympagic copepods and are important prey items for fish (e.g. cod) and sea birds.  The amphipod G. wilkitzkii has a life span of up to six years and generally prefers multi-year ice (Arndt and Swadling 2006). 

Sea ice provides a productive substrate and protective shelter.  As such there are a number of pelagic or benthic species that spend a portion of their life in close association with sea ice.  Pelagic copepods will perform diel migrations to feed at the ice-water interface, particularly during the ice melt when the ice algae and early season phytoplankton blooms can account for more than 50% of the copepods lipid reserves (Melnikov 1997; Arndt and Swadling 2006).  One and two-year-old Arctic and polar cod find shelter in the shelves and fissures of the sea-ice, feeding on sympagic amphipods and pelagic copepods (Lonne and Guilliksen 1989; Gradinger and Bluhm 2005).  Adult fish are seldom observed in close association with the sea ice (Hop et al. 2000).  Ice amphipods and cod are subject to strong predation by top carnivores including seals and sea birds. Mammals and Birds

Arctic mammals include species associated with the sea ice and pelagic species.  Ice seals include Ringed seals (Phoca hispeda) that feed primarily on cod and hyperiid and sea-ice amphipods when young, Bearded seals (Erignatus barbatus) that feed on clams, and Hooded (Cystophora cristata) and Harp (P. groenlandica) seals that feed on cod, capelin, squid and pelagic amphipods (Bradstreet and Cross 1982; NAMMCO 2005a).  Walrus are found in the Bering-Chukchi, the Laptev Sea, and in the Barents-Greenland-High Canadian Seas (NAMMCO 2005b).  Walrus feed primarily on clams of the inner continental shelves (Outrides et al. 2003; Bluhm and Gradinger 2008).  Whales of the Arctic include both baleen and toothed whales.  Bowhead whales (Balaena mysticetus) feed on copepods and euphausiids in the open Pacific and Atlantic waters (Rice 1998; NAMMCO 2005c).  The White, narwhal, and Orca (Orcinas orca) whales are the dominant toothed whales of the Arctic.  White whales and narwhals feed primarily on cod and capelin.

Arctic seabirds are dependent on Arctic marine resources for all or most of their energy requirements while they are in the region.  Most seabirds are migratory arriving as spring blooms and breakup begins. Arctic birds that forage in the open pelagic are mostly alcids, gulls, skuas, and terns (Huettmann et al. 2011).  Other taxa tied to marine food webs are sea ducks, most notably eider ducks. Communities of Special Significance

Polynyas, estuarine lagoons, and rocky substrate habitats represent important but less common habitats in the Arctic.  The species that occur in these areas are not necessarily unique in Arctic or boreal waters, but they are found either in great abundance or in an assemblage of species that are well adapted to that habitat.

Polynyas are openings or leads in the sea-ice that form due to currents or water temperatures generally in nearshore areas.  They are a seasonal feature that allows light to penetrate into the water column and allows for direct access of the water surface in the absence of ice.  Narwhals remain in close association with pack ice and congregate in great abundance in polynyas using this time for most of their annual feeding.  Pelagic copepods and fish also congregate in these areas.  Sediments associated with polynyas are highly organically enriched during the polar winter, and the abundance and biomass of the more motile components of the benthic community respond with increased abundance and biomass; strong evidence for pelagic-benthic coupling.

Estuarine lagoons also represent another productive ecosystem, receiving organic input from terrigenous sources (e.g. riverine systems).  Estuarine benthos (including amphipods, polychaetes, benthic copepods, clams, and snails) in lagoons are protected from ice or ice scour, enabling them to take advantage of organically enriched sediments.  In response to the protected habitat and enriched benthic communities, epibenthic fish use the lagoons as feeding and nursery grounds.  Anadromous fish, particularly char, cisco, and salmonids will move along the brackish nearshore zone as young adult and adult fish, taking advantage of the availability of food resources.  As salinities increase in the lagoons, pelagic invertebrates and small fish will move into the protection and production of the lagoons.  Finally, the productive lagoons are an important feeding ground for sea birds, in particular Eider ducks.

Rocky substrate is less common in the Arctic, predominantly occurring in the Atlantic region of the Arctic.  The communities associated with rocky intertidal and subtidal habitats are generally similar to those that occur in the northern Atlantic, with brown macroalgae (e.g. Fucus and Laminariales), barnacles, serpulid worms, mussels, and motile scavengers such as amphipods.  Hard substrate in the deep sea occurs along the ridges, however, there is currently little data associated with the fauna on the Arctic ridges.