The pelagic environment includes those waters associated with the nearshore zone, the continental shelf and the deep water basins of the Arctic.  The distribution of pelagic fish and invertebrates is defined largely by salinity, temperature, and light.  In the extensive shelf waters of North America and the Russian Arctic there are two distinctly different bodies of water, the nearshore brackish waters and the Arctic Surface Water (ASW) or Atlantic Water (AW; Craig 1984).  The occurrence of a shore-parallel band of turbid, warmer (5-10°C) brackish (10 - 25‰) water is a characteristic feature of the Arctic coastlines (Craig 1984).  The ASW includes all open waters, extending a depth of approximately 100 m.  The ASW is typically colder (-1° to 10° C) and more saline (28‰ – 34‰) than the nearshore waters.  Underlying the ASW is the Arctic Intermediate Water (AIW), layers of denser and colder waters that extend from approximately 75 to 450 m (lower).  The shallower portions of the AIW tend to have temperatures <2°C and salinities ranging from 34.7 to 34.9‰.  The deeper portions of AIW have temperatures that are 0-3°C and salinities greater than 34.9‰.  During certain portions of the year, the deeper water may be slightly warmer than the shallower waters, with pelagic species using the deeper water as refuge from colder waters.  AW on the other hand, pushes northward into the Arctic during the spring and summer months.  Temperatures in the Eurasian Arctic increase as AW pushes northward in the spring and summer months.  This push of AW is an important transport mechanism for larval dispersal and for boreal species to enter the Arctic.  The Arctic Deep Water (ADW) occupy the abyssal basins of the Arctic, with stable environmental conditions with temperatures ranging from -2°C to -10°C and a salinity of 34.9‰.

The phytoplankton population and primary production in Arctic waters are controlled primarily by light and nutrient availability.  Ice cover limits light availability in the water column throughout the winter season. Phytoplankton blooms develop as the ice opens, with propagation of the algal bloom following along a latitudinal gradient from the southern Barents Sea in March – April to August – September in the Fram Strait and Arctic Ocean (Figure 2-5).  Once the ice recedes, there is a gradual increase in phytoplankton abundance (Falk-Peterson et al. 2007). 

Figure 2-5. Timing of Plankton Blooms in the Arctic Oceans. From Falk-Petersen et al. 2005

While light controls the timing and seasonality of primary production in the Arctic, the bloom duration and seasonal production are controlled by nutrient availability (Tremblay et al. 2012).  Nutrient limitations in the Arctic are exacerbated by a persistent stratified water column, caused by salinity and temperature differences resulting from riverine input and melting pack ice.  The nearly permanent stratification limits replenishment of nutrients from deeper, nutrient rich waters (Hsiao 1978). Chlorophyll-aestimates indicate that phytoplankton abundance is highest in the nearshore band, gradually decreasing with distance from shore.  Unlike population cycles in more temperate waters, conditions in the Arctic do not allow for a second phytoplankton bloom during the late summer.  Although nutrients in the water column slowly recover with periodic upwelling events, solar radiation begins to decline rapidly following the equinox.  With the combination of short periods of light and nutrient limitations, primary production in the high Arctic is among the lowest in the world.  The highest primary production is found in areas with energetic mixing with deep waters (Figure 2-6; Bering Strait, Fram Strait and the Atlantic sector).  Nearshore and shelf areas having variable production depending upon the extent of stratification and import of nutrients from terrestrial sources (Tremblay et al. 2012; Carmack et al. 2006). 

Phytoplankton in the Arctic includes 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).  Recent field investigations have documented the distribution and potentially changing functional roles of picoplankton (<2µm) and nanoplankton (2-20 µm) due to changes in sea ice coverage in the Arctic Ocean (e.g. Coupel et al. 2012).  The Sea of Okhotsk and western Barents Sea are the most speciose regions, with the deep Arctic Ocean having the fewest species (Melnikov 1997; Sakshaug 2006).  Diatoms and flagellates are most abundant, followed by lower densities of dinoflagellates and chrysophytes.  Centric diatoms are the most common planktonic diatoms, withChaetoceros spp. and Thalassiosira spp. associated with spring blooms.  Pennate diatoms are less common in the phytoplankton, but are dominant in the ice-algal community.  The microplankton Phaeocystis is also common throughout the sub-Arctic coastal basins and can form blooms of several billion cells per m³ (Sakshaug 2006)  Flagellates were more predominant at offshore stations in colder, less turbid water and lower nutrient concentrations (Hsiao 1978).  Horner (1984) found that in the western Beaufort both diatoms and flagellates were found in the open waters, with distribution controlled on a vertical rather than horizontal scale.  Li et al. (2009) found that the nearshore phytoplankton community shifted from larger nanoplankton towards picoplankton and bacterioplankton with salinity and distance.  The picoplankton community was dominated by the unique pan-Arctic, cold-adapted Micromonas.

Figure 2-6. Annual Primary Production in Arctic (gC/m²/year) [Source: Carmack et al. 2006]

Sea-ice algae represent an important component of primary production, particularly in regions with permanent ice.  The ice algae community is discussed more fully in later sections.  However, it is important to understand that the spring blooms of ice algae precede the phytoplankton community and provide a critical food source for zooplankton.

Copepods and euphausiids represent the most important zooplankton group in terms of energy transfer to upper trophic levels (i.e. Arctic cod, birds, baleen whales).  Among the large copepods (1-5 mm adult size), the Calanus species Calanus hyperboreus and C. glacialis are dominant throughout the Arctic region.  The subarctic species Calanus finmarchicus and Oithona atlantica are also common in Atlantic Water and adjacent coastal waters (Sakshaug 2004).  While not considered to be an Arctic species, C. finmarchicus, it is widely distributed across the Arctic by the northern Atlantic current that moves northward along the Norwegian coast through Fram Strait and into the Barents Sea and onto the Arctic Ocean (Falk-Petersen et al. 2007).  Other pan-arctic copepods include O. similis and Metridia longa.  In Pacific waters Neocalanus spp., Eucalanus bungii, Pseudocalanus spp. and M. pacifica are also common in the Pacific Water (Sakshaug 2004; Griffiths and Thomson 2002).  Smaller copepods dominate the brackish shelf waters Russian Arctic including Limnocalanus macrurus and Drepanopus bungei (Peters et al. 2004).

Herbivorous krill are also an important component of the pelagic food web throughout the Arctic.  The euphausiids Thysanoessa raschii and T. inermis are common in the Pacific and Atlantic arctic, with T. rachii also numerically important in the middle and Coastal Bering shelf.  In the subarctic Atlantic, T. longicauda can be common; whereas T. longipes and Euphausia pacifica are more common in the Pacific Waters.  Both T. longipes and E. pacifica are important prey items associated with bowhead whales in the western Beaufort Sea (Brinton 1962).

The diatom – Calanus food chain is considered to be critical to the overall production in the Arctic (Falk-Petersen et al. 2007).  Calanoid copepods comprise 50% to 80% of the mesozooplankton biomass in the Arctic.  Calanus spp. take advantage of early ice-algae blooms and feed through planktonic diatom blooms, converting low energy sugars into a high energy lipid reserves.  Calanoid copepods create lipids that are rich in longer-chained fatty acids and alcohols; a characteristic not shared by other smaller copepod species (Peters et al. 2004).  The combination of rich lipid reserves and their large size makeCalanus spp. a key prey item for a number of higher level consumers, in particular Arctic cod. 

While the three dominant calanoid copepods co-occur, they differ in their life histories.  Calanus hyperboreus is the most polar species and is most common in the deep sea areas of the Arctic, including the Arctic Ocean, Fram Strait, and the Greenland Sea.  It is a large copepod (4.5-7 mm) that can overwinter at depths of 500 m to 2000 m (Falk-Peterson et al. 2007).  C. hyperboreus has a 2-yr life span during periods of high productivity which can extend to 3 to 5 years during periods of extensive ice cover.  Calanus hyperboreus reproduce in deep waters in the winter/spring prior to the phytoplankton blooms (Niehoff 2007) with the naupliar forms developing just under the ice surface, feeding on sympagic (ice-associated) algae.  Egg releases in early March appear to provide a lipid-rich food source prior to significant primary production (Darnis et al. 2012).

Calanus glacialis is a smaller copepod (3-4.6 mm) that is found primarily on shelf waters of the Arctic.  C. glacialis has a life span of 1-3 years, spawning throughout the Arctic prior to the yearly spring bloom.  Most larvae reach the C1 copepod stage (early adult form) by ice break up allowing the nauplii to feed on ice algae in the relative protection of the pack ice (Falk-Petersen et al. 2007; Niehoff 2007).  The C1 copepods are then able to take advantage of the phytoplankton blooms in early summer.  It descends to deep areas on the shelf (200-300 m) to enter diapause and over winter (Falk-Petersen et al. 2007).

Calanus finmarchicus is the smallest of the three Calanus species (2-3.2 mm) and has less lipid-rich reserves.   While nauplii and copepodites may be advected into the Arctic Ocean and other northerly waters, C. finmarchicus does not appear to reproduce in Arctic waters (Niehoff 2007) but relies more heavily on reliable ice-algal blooms for reproduction.  As with other calanoid copepods, C. finmarchicusrelease eggs prior to the phytoplankton blooms, which develop during the Arctic summer prior to over wintering at depths of 500 to 2,000 m (Falk-Petersen et al. 2007).

In late-spring/early summer, reproduction rates for other copepods and euphausiids increase following blooms of phytoplankton, microzooplankton, and small copepods (Pseudocalanus spp.).  Omnivorous zooplankton, such as Metridia longa feed on both living plankton, as well as the organic carbon spikes following blooms.  Reproduction in such species appears to be more independent of the plankton blooms.

Neuston refers to a community of microbial, plant (phytoneuston) and animal (zooneuston) species that live at the water’s surface.  Surface communities include obligate species that are in the surface layer throughout their life cycle, as well as facultative species that occupy the surface only during larval stages.  Arctic neuston have a number of adaptations that allow them to live in the harsh surface environment, including oil droplets to shield them from ultra violet (UV) exposure and keep them at the surface, and diel migrations that help them compensate for the temperature extremes (Zaitsev 1971).

While large temperature fluctuations and extreme UV exposure limit species diversity, marine phytoneuston abundance per unit volume may be a thousand times higher than the underlying bulk water populations (Zaitsev 1971; Word et al. 1986).  Marine phytoneustonic communities are dominated by small diatoms and flagellates.  As with abundance, the production per unit volume can be many times that of the underlying water.  The zooneustonic community in Arctic waters include chaetognathans, copepods, euphausiids, hyperiids, and in smaller numbers of rotifers, pontellids, cladocerans, decapods, and fish larvae and eggs (Zaitsev 1971).  Microbial populations, including bacterioneuston, may be 10,000 times denser than bacterioplankton populations (Hardy 1982; Sieberth 1971).

The neuston community is somewhat self-contained, with zooneuston feeding on phytoneuston.  Neuston is an important component of the pelagic food web, with zooplankton, fish, birds, and marine mammals feeding at the surface, particularly along convergence zones where the surface populations are highest (Zaitsev 1971).  Bowhead whales, as well as other baleen whales, have been observed skim-feeding at the surface (Koski and Miller 2002).

The pelagic invertebrate community includes a wide variety of other species, including amphipods, mysids, shrimp, squid, jellyfish, pteropods, chaetognaths, and ichthyoplankton.  There are only two Arctic species of pelagic shrimp and they do not appear to be a common component of the nekton (Hopcroft et al. 2008).  This section will focus on certain elements of the pelagic invertebrate community that are important food web components.

Krill

Unlike the Antarctic, euphausiids are not abundant in Arctic waters.  However they can be difficult to accurately assess in ice-covered waters as they seek shelter in the pockets and fissures in the ice (Percy and Fife 1985).  Euphausiids are not considered to be a truly Arctic species, rather they are advected from the Bering Sea Water inflow (Suydam and Moore 2004) or from the Atlantic (Letessier et al. 2009).    The euphausiids Thysanoessa raschii and T. inermis were important prey items associated with bowhead whales in the western Beaufort Sea (Brinton 1962).  Euphausiids are found from the surface to the deep midwater pelagic areas, in depths greater than 500 m. 

Amphipods

Free-swimming amphipods are a key component of the Arctic food web, representing a link between zooplankton and higher level consumers such as fish, marine mammals, and birds.  Hyperiid and Lysianassid amphipods are among the common taxa found in Arctic waters and are a dominant component of the zooplankton abundance and biomass (Auel et al. 2002).  The hyperiid amphipod, Themisto libellula is a pan-Arctic species that can occur in high numbers in the ice free waters of the Arctic.  Based on the high amounts of C20:1 (n-9) and C22:1 (n-11) fatty acids and alcohols found in T. libellulatissues, their diet was domianted by Calanoid copepods.  Further analysis indicated a close association with ice-algal production.  This confirms field sampling data showing Themisto sp. associated wtih sympagic communities.

Themisto abyssorum is more closely associated with deepwater communties.  Themisto abyssorum is  considered to be a boreal species and is found in close association with the Atlantic Water that moves northward through Fram Strait.  Abundance generally decreases from east to west, dropping from >200 ind/m² over the contiental slope north of Spitsbergen to <40 ind/m² in the central Arctic (Auel et al. 2002).  It does not show a similar lipid signature seen in T. libellula, indicating that copepods are not a dominnant component of the diet.  Rather, the deepwater species is more likely an omnivore and scavenger.

Cyclocaris guilelmi, is also an epipelagic species, occurring in the deep-waters of the Arctic.  As wtih T. abyssorum, C. guilelmi appears to be Atlantic in origin but is found throughout the Arctic ocean.  Kraft (2012) found C. guilelmi to be a dominant component found in deepwater traps in Fram Strait.  The population appeared to be stable with peaks from August to February.

Cephalopods

Cephalopods are a predator of and a key prey item for many of the VECs in the Arctic.  Squid move vertically through the water column, integrating marine resources throughout the pelagic environment.  The squid Gonatus fabricii is the most commonly reported species in the Arctic, with numerous records in the Atlantic sector, the Barents Sea and the high Canadian Arctic (Gardiner and Dick 2010). Berrytheuthis magister is the predominant squid species found in the Pacific domain.  Squid exhibit all manners of vertical distribution: near-surface dwellers (to 50 m), vertical migrators that either move into the surface waters at night or just move higher in the water column, and near-bottom dwellers.  In addition, some species exhibit ontogenetic descent moving progressively deeper as they age (Roper and Young 1975).  Cephalopods are voracious predators feeding on crustaceans, fish, other squids, and zooplankton.  Based on isotope analysis, squid are occupying different trophic levels in different regions (Navarro et al. 2013) indicating that squid are able to shift their diet based on availability.  The isotope ratios for Arctic squid indicated a trophic level of 3 to 5.  Squid are an important component of marine mammal and sea bird diets, including narwhals, White whales, walrus, murres and fulmars (Gardiner and Dick 2010).

Jellyfish

Gelatinous zooplankton are poorly understood in the Arctic, largely due to the difficulty of capturing them with traditional sampling methods.  However, they are considered to be a substantial sink for primary and secondary production (Purcell et al. 2009).  In the Beaufort Sea, the scyphomedusa, Chrysaora melanaster is among the most common gelatinous zooplankton in shelf waters 25 to 75 m in depth (Purcell et al. 2009).  In shallower waters, there was a more diverse community dominated by the delicate medusa Bolinopsis infundibulum, other small cnidarians, and ctenophore species occurred immediately underneath the sea ice (Purcell et al. 2009; Raskoff et al. 2010b).  Other common species include Sminthea arctica in the midwater depths, and Atolla tenella which was found in high abundance in the deep Canadian Basin (Raskoff et al. 2010b). 

Large populations of gelatinous zooplankton have been observed throughout the Arctic, particularly in at convergences, fronts, and polynyas (Ashjian et al. 1997).  In such areas, medusa and ctenophores can have a substantial grazing impact.  In the eastern Canadian high Arctic, the ctenophore Mertensia ovum was estimated to consume up to 9% per day of the C. hyperboreus population and 3-4% of the C. glacialis population.   Other prey items included hyperiid amphipods, pteropods, and smaller copepods.  Other food resources for gelatinous zooplankton include detritus and algal cells and in some cases small or juvenile fish (e.g. larval capelin and herring; Raskoff et al. 2010b).

Unlike the Antarctic, euphausiids are not abundant in Arctic waters.  However they can be difficult to accurately assess in ice-covered waters as they seek shelter in the pockets and fissures in the ice (Percy and Fife 1985).  Euphausiids are not considered to be a truly Arctic species, rather they are advected from the Bering Sea Water inflow (Suydam and Moore 2004) or from the Atlantic (Letessier et al. 2009).    The euphausiids Thysanoessa raschii and T. inermis were important prey items associated with bowhead whales in the western Beaufort Sea (Brinton 1962).  Euphausiids are found from the surface to the deep midwater pelagic areas, in depths greater than 500 m. 

Free-swimming amphipods are a key component of the Arctic food web, representing a link between zooplankton and higher level consumers such as fish, marine mammals, and birds.  Hyperiid and Lysianassid amphipods are among the common taxa found in Arctic waters and are a dominant component of the zooplankton abundance and biomass (Auel et al. 2002).  The hyperiid amphipod, Themisto libellula is a pan-Arctic species that can occur in high numbers in the ice free waters of the Arctic.  Based on the high amounts of C20:1 (n-9) and C22:1 (n-11) fatty acids and alcohols found in T. libellulatissues, their diet was domianted by Calanoid copepods.  Further analysis indicated a close association with ice-algal production.  This confirms field sampling data showing Themisto sp. associated wtih sympagic communities.

Themisto abyssorum is more closely associated with deepwater communties.  Themisto abyssorum is  considered to be a boreal species and is found in close association with the Atlantic Water that moves northward through Fram Strait.  Abundance generally decreases from east to west, dropping from >200 ind/m² over the contiental slope north of Spitsbergen to <40 ind/m² in the central Arctic (Auel et al. 2002).  It does not show a similar lipid signature seen in T. libellula, indicating that copepods are not a dominnant component of the diet.  Rather, the deepwater species is more likely an omnivore and scavenger.

Cyclocaris guilelmi, is also an epipelagic species, occurring in the deep-waters of the Arctic.  As wtih T. abyssorum, C. guilelmi appears to be Atlantic in origin but is found throughout the Arctic ocean.  Kraft (2012) found C. guilelmi to be a dominant component found in deepwater traps in Fram Strait.  The population appeared to be stable with peaks from August to February.

Cephalopods are a predator of and a key prey item for many of the VECs in the Arctic.  Squid move vertically through the water column, integrating marine resources throughout the pelagic environment.  The squid Gonatus fabricii is the most commonly reported species in the Arctic, with numerous records in the Atlantic sector, the Barents Sea and the high Canadian Arctic (Gardiner and Dick 2010). Berrytheuthis magister is the predominant squid species found in the Pacific domain.  Squid exhibit all manners of vertical distribution: near-surface dwellers (to 50 m), vertical migrators that either move into the surface waters at night or just move higher in the water column, and near-bottom dwellers.  In addition, some species exhibit ontogenetic descent moving progressively deeper as they age (Roper and Young 1975).  Cephalopods are voracious predators feeding on crustaceans, fish, other squids, and zooplankton.  Based on isotope analysis, squid are occupying different trophic levels in different regions (Navarro et al. 2013) indicating that squid are able to shift their diet based on availability.  The isotope ratios for Arctic squid indicated a trophic level of 3 to 5.  Squid are an important component of marine mammal and sea bird diets, including narwhals, White whales, walrus, murres and fulmars (Gardiner and Dick 2010).

Gelatinous zooplankton are poorly understood in the Arctic, largely due to the difficulty of capturing them with traditional sampling methods.  However, they are considered to be a substantial sink for primary and secondary production (Purcell et al. 2009).  In the Beaufort Sea, the scyphomedusa, Chrysaora melanaster is among the most common gelatinous zooplankton in shelf waters 25 to 75 m in depth (Purcell et al. 2009).  In shallower waters, there was a more diverse community dominated by the delicate medusa Bolinopsis infundibulum, other small cnidarians, and ctenophore species occurred immediately underneath the sea ice (Purcell et al. 2009; Raskoff et al. 2010b).  Other common species include Sminthea arctica in the midwater depths, and Atolla tenella which was found in high abundance in the deep Canadian Basin (Raskoff et al. 2010b). 

Large populations of gelatinous zooplankton have been observed throughout the Arctic, particularly in at convergences, fronts, and polynyas (Ashjian et al. 1997).  In such areas, medusa and ctenophores can have a substantial grazing impact.  In the eastern Canadian high Arctic, the ctenophore Mertensia ovum was estimated to consume up to 9% per day of the C. hyperboreus population and 3-4% of the C. glacialis population.   Other prey items included hyperiid amphipods, pteropods, and smaller copepods.  Other food resources for gelatinous zooplankton include detritus and algal cells and in some cases small or juvenile fish (e.g. larval capelin and herring; Raskoff et al. 2010b).

For the purposes of this review, fish in the Arctic can be divided into four general groups, the pelagic shallow and midwater species, anadromous species, and demersal fishes.  The bottom fish include nearshore, shelf, and deep water species.  Pelagic fish are the least diverse group representing 13% of the 242 recorded Arctic species (Bluhm et al. 2011; Mecklenberg et al. 2011).   There are 31 species considered to be anadromous.  The remaining species are marine demersal fish.

Throughout the Arctic, gadoids (cod) represent a critical link between the zooplankton community and higher trophic levels (e.g. seals, White whales).  Arctic cod (Boreogadus saida) and Polar cod (Arctogadus glacialis) are truly pan-arctic species occurring in all marine waters of the Arctic.  Cod 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.  Both B. saida and A. glacialis can be found in small numbers or in large, densely packed schools.  In the scientific literature there is confusion on the common names “Arctic cod” and “Polar cod” at times referring to either B. saida or A. glacialis.  While these species are similar in appearance and life history, gene sequencing has shown that they are genetically distinct (Breines et al. 2008; Madsen et al. 2009). 

Both B. saida and A. glacialis have a diet dominated by pelagic or sympagic components (Sufke et al. 1998; Lonne and Gulleksen 1989; Bradstreet and Cross 1982).  Stomach contents of B. saida cod sampled in the pelagic ranges of the Beaufort Sea, northern Canadian waters, and the Barents Sea were dominated by calanoid copepods and gammarid amphipods.  Other prey items included hyperiid amphipods, mysids, and shrimps (Frost and Lowry 1984; Lonne and Gulliksen 1989).  A similar pelagic diet was observed for A. glacialis in the Northeast Water Polynya near Greenland (Sufke et al. 1998).  In the nearshore zone, the diet is dominated by copepods, gammarid amphipods, and young-of-the-year Arctic cod. 

Both B. saida and A. glacialis spawn under the ice in the winter months; however recent observations indicate that there are regional differences in the hatching season of B. saida (Figure 2-7).  Bouchard and Fortier (2011) noted that cod hatching started as early as January and extended to July in areas with significant freshwater input (Laptev/East Siberian Seas, Hudson Bay, and Beaufort Sea).  In contrast, hatching was restricted to April-July in regions with little freshwater input (Canadian Archipelago, North Baffin Bay, and Northeast Water).  The authors found that the different hatching periods resulted with different length and weight classes co-occurring throughout the Arctic.  Cod larvae generally occupy a depth of 10 – 30 m, settling to the bottom in September (Craig 1984; Graham and Hop 1995). 

As noted above, the distribution of B. saida and A. glacialis includes nearly all marine waters of the Arctic depending upon the age class and the season.  Arctic cod have been found to be a dominant component of both the pelagic and demersal communities at all depths; cod have been collected from the mixed water mass (<200 m), at the sharp halocline (200-300 m), and in the deeper Atlantic water mass (>300 to 1000 m; Majewski and Riest 2013; Norcross et al. 2012). One and two year old fish also occupy fissures and gaps in the ice pack, feeding on the sympagic fauna.  In the summer months, adult cod are dispersed in habitats ranging from coastal brackish waters to the demersal and pelagic zones of the shallow shelves, including the ice-water interface (Craig et al. 1982; Lonne and Gulliksen 1989; Gradinger and Bluhm 2004).  In autumn, as nearshore salinities increase, large shoals of cod are observed in shallow (<10 m) waters (Hop et al. 1997; Welch et al. 1993), presumably following shoreward fronts of plankton.

Acoustic surveys conducted as part of the BREA program as well as others conducted in the US Beaufort have found large shoals of Arctic cod in the water column and near the bottom along the entire shelf of the Beaufort Sea (Geoffrey et al. 2013; Parker-Stettner 2011).  There was a clear segregation between the young-of-the-year (YOY) and age 1+ fish in the summer months.  Age 1+ fish were found to aggregate in the deeper waters of the shelf  and slope at depths ranging from 200 to >1000 m while large shoals of YOY Arctic cod were found nearer to the surface (20 to 100 m) in nearshore waters extending into waters over the outer shelf and slope (Figure 2-8). The near bottom aggregations of Arctic cod at depths of 200 to 400 m appear to span the Canadian Beaufort shelf into the fall and winter months (Geoffrey et al. 2013; Benoit et al. 2008). Acoustic surveys during the winter months have found massive aggregations of adult cod at depths of 140 to 230 m under the pack ice in the Canadian Beaufort waters (Benoit et al. 2008) and at depths of 300 to 1,300 m near the North Pole (Geoffrey 2013). In winter months, adult cod appear to remain in schools at the deep inverse thermocline (160-230 m, -1 to 0°C) throughout the Polar night to avoid seal predation; whereas smaller cod (<25 g) periodically migrate into the isothermal cold intermediate layer (90-150 m) to feed on Calanoid copepods and then return to the deeper layer (Benoit et al. 2010).

An exception to the Arctic cod dominated pelagic food web is in the Barents Sea and White Sea.  The Barents Sea is located between the Arctic and boreal oceanic systems and is influenced by the variations in the Atlantic current and the Polar front.  In the southern portions of the Barents Sea, where the Atlantic water is more predominant, capelin (Mallotus villosus) is the primary link between pelagic crustaceans (e.g. copepods and amphipods) and higher trophic levels (Blanchard et al. 2002; Hamre 1994; Mehl and Yaragina 1992; Titov et al. 2006).  The importance of capelin to the Barents Sea ecosystem was demonstrated when capelin stocks decreased markedly in the 1980s, resulting in decreases in stocks of the commercially important Atlantic cod (Gadus morhua) and ringed seals.  Subsequent studies have demonstrated linkages between the location of the polar front, the population of capelin and subsequent changes in the population of Atlantic cod (Titov et al. 2006).  Capelin are also an important forage fish in the northern boreal waters of Greenland, the Sea of Okhotsk and the Bering Strait.  Unlike Arctic cod, Atlantic cod are more demersal in nature, generally feeding near the bottom.  The diet of Atlantic cod is remarkably varied, with Mehl and Yaragina (1992) reporting with over 180 prey species.  While capable of feeding on a variety of invertebrate and vertebrate prey, capelins appear to be the primary energy source.

Figure 2-8. Arctic cod (Boreogadus saida) vertical distribution during the ice-free season along the Mackenzie Slope 2012 (from Geoffrey et al. 2013).

Pacific herring (Clupea harengus pallasi) are another nearshore forage fish that represents an important link between zooplankton and higher level consumers, particularly anadromous fishes of the Bering-Chukchi-Laptev and Lofoten-Barents Sea systems (Mehl and Yaragina 1992; Hamre 1994).  Herring feed primarily in the water column on copepods, euphausiids, and mysids.  Herring spawn en masse and their eggs and larvae are a vital food source for nearshore migrating anadromous fish in the Arctic such as salmonids, cisco, and char. In the Arctic they are generally found in the nearshore waters and avoid the colder, Arctic water (Craig 1984).  While numerically less important than the Arctic cod, capelin and herring are important forage species for upper trophic level consumers.

Anadromous fish include those species with a life history that includes both freshwater and marine habitats.  In the Arctic, utilization of marine waters is typically limited to the brackish waters found in nearshore corridor immediately following breakup.  Anadromous fishes in the Arctic include Arctic char (Salvelinus alpinus), least and Arctic cisco (Coregonus sardinella and C. autumnalis), broad and humpback whitefish (C. pidschian and C. nasus), Inconnu (Stenodus leucichthys), and several species of salmonids (Craig 1984; Mecklenburg et al. 2011).  These species spawn in fresh water and typically do not enter coastal waters until months, or often years, after hatch.  Thus, the most sensitive life stages for anadromous fish are spent in the Arctic rivers and lakes.  Use of marine waters is limited to feeding and migration.  Coregonids are the most common anadromous fish in the Laptev-Kara-East Siberian waters, with Arctic cisco more commonly found in the marine waters than Least cisco (Sherman and Hempel 2008; Craig 1984).  Arctic char range widely from their stream of origin and might be found in more open water during high flow years (Jarvela and Thorsteinson 1999; Johnson 1980).  When occupying the nearshore brackish water, anadromous fish feed nearly exclusively on epibenthic fauna (e.g. polychaetes, mysids, and amphipods; Dunton et al. 2012).  In turn, anadromous fish become an important food source for seals as well as subsistence fishers.

A number of salmonid species are found in river systems throughout the Arctic; however, their use of marine waters is limited.  Atlantic salmon are the most common form found in the Lapotov-Barents Sea and Hudson Bay river systems (Sherman and Hempel 2008).  Pink and chum salmon (O. gorbuscha) are the most abundant species of Pacific salmon documented in the Arctic, with populations in Russian (Yana and Lena Rivers), Canadian (Mackenzie River), Alaskan, and Norwegian waters (Sherman and Hempel 2008; Mecklenburg et al. 2002).  With the exception of some juvenile use of the brackish nearshore waters, adults are the most common life stage found in marine waters.

Demersal fishes are those that are found in close association with the bottom.  Sculpins (Cottidae) and eelpouts (Zoarcidae) are the most speciose fish taxa in the Arctic, comprising over 50% of the species in polar waters (Mecklenberg et al. 2011).  Both taxa are well adapted to living in the dominant substrates found in the Arctic (sand, silt, and mud), as well as rocky bottoms, with most species spending the majority of their life cycle in close association with the bottoms.  Adults often deposit eggs directly on benthic substrates or on bottom oriented vegetation.  Larval and early juvenile forms may remain on the bottom near the adults or move into the water column or into vegetation before descending to the bottom as young fish.  Common and pan-Arctic sculpin species include the Arctic Staghorn sculpin (Gymnocanthus tricuspis), the large Short-horned sculpin (Myoxocephalus scorpius: 90 cm length), and Spatulate sculpin (Icelus spatula; Mecklenberg et al. 2011).  Two genera account for most Arctic eelpouts, Gymnelus and Lycodes.  Sculpin and eelpout diets vary, but many sculpin and eelpouts feed on benthic infauna and epifauna, including polychaetes, benthic amphipods, small mollusks, and epibenthic crustaceans, with larger species feeding on fish, including cods, flounders, and smelts (Dunton et al. 2012).  Anti-freeze proteins have been found in several species of Myoxocephalus sculpins, showing their adaptation to Arctic waters (Fletcher et al. 1982).

Flatfishes or flounders live on the bottom, usually in shallow marine waters, and burrow into the surface sediment to rest and wait for prey. They eat worms, mollusks, echinoderms, crustaceans, other benthic invertebrates, and fishes. Arctic flounder (Pleuronectes glacialis) are a pan-Arctic species that prefer coastal and nearshore waters (Mecklenberg et al. 2011).

The deep-sea fishes are perhaps the poorest known group in the Arctic.  Recently there have been several targeted efforts to sample the deep basins in the Arctic (Reist and Majewski 2013; Dolgov et al. 2009; Jorgensen et al. 2005). In the deep Canadian Beaufort, Arctic cod were the dominant species in the water column and associated with the demersal community (Reist and Majewski 2013). Other species found in midwater trawls included species found in other deep ocean basins, including Myctophids (lanternfish) and Gonostomids (bristlemouths).  The myctophids, Benthosoma glaciale and Protomyctophium arcticum, spawn above the Polar front (Dolgov et al. 2009) and were found in deep water trawls (Riest and Majewski 2013).  Other species have been recently collected from the Kara Sea, includingMyctophum punctulum (Dolgov et al. 2009; Weinrroither et al. 2010).  The wide-spread gonostomid, Cyclothone microdon, has been observed in trawls in Baffin Bay (Riest and Majewski 2013; Mecklenburg et al. 2011).  Based on observations of myctophids and bristlemouths throughout the world, their diet is dominated by pelagic crustacean, migrating to the surface from depth to feed.  It is not known if the diel vertical migrations occur in the Arctic.

Deepwater demersal fish taxa are generally similar to those found in other oceanic basins and include Zoarcids (eelpouts) and Macrourids (grenadiers).   The globally distributed macrourids, Coryphaenoides rupestris and Macrourus berglax have been found in the Baffin Bay (Jorgensen et al. 2005).  The Glacial eelpout (Lycodes glacialis) is one of the most dominant demersal fishes in the Arctic basins.  This large eelpout (~70 cm) moves along the bottom stirring up the bottom sediments to feed on small benthic crustaceans and mollusks; L. glacialis also feeds on other fishes and cephalopods (Mecklenberg and Mecklenburg 2011).  This is a truly deep water species, spending its entire life cycle at depths >1,000m.  The Greenland halibut (Reinhardtius hippoglossoides) is a right-eyed flounder typically associated with deep waters of the Arctic (200 – 1600 m), as well as deep waters of the Atlantic and Pacific Oceans.  The Greenland halibut is epibenthic, and feeds on epibenthic crustaceans, demersal fish, and other invertebrates.  Deepwater rays found in the southern Arctic waters in Alaska and Baffin Bay include Rajella spp. and Amblyraja radiata (Dolgov et al. 2009; Mecklenburg et al. 2002).

Marine mammals are both permanent and seasonal members of the Arctic and include baleen and toothed whales, seals, walrus, and Polar bear.  The following section focuses on those mammals closely linked to the marine food webs.

Bowhead whales are circumpolar, residing in the high latitudes from late April to October.  In the spring months, bowheads migrate northward as the sea ice breaks up.  Five stocks have been identified, including the Spitsbergen, Baffin Bay-Davis Strait, Hudson Bay-Fox Basin, Sea of Okhotsk, and Bering-Chukchi-Beaufort stocks (Rugh et al. 2003).  The latter stock overwinters in the Bering Sea.  In the spring, the Bowheads move north and east, past Point Barrow into the western Beaufort Sea (Lowry et al. 2004).  The majority of whales move into the Canadian Beaufort Sea for the summer months; however, some Bowhead whales will either remain in the eastern Alaskan Beaufort or move into the Arctic Ocean.  While Bowhead whales appear to favor continental slope waters in the spring and summer, they appear to favor the inner shelf waters (<200 m depth) of the western Beaufort in September and October (Moore et al. 2000).  As the ice cover increases in late fall, the whales migrate into the Bering Sea for the winter months. 

Bowhead whales are considered to be second order consumers (Hoekstra et al. 2002), with a diet dominated by euphausiids and calanoid copepods (Bluhm and Gradinger 2008).  Stomach content analyses indicate that, while benthic crustaceans and fish occur, their consumption is either occasional or incidental (Frost and Lowry 1984; Lowry et al. 2004).  In the Bering-Chukchi-Beaufort system, there appears to be some seasonality in the diet which is dominated by euphausiids in spring and by copepods in the summer and autumn months.  This distribution of diet is likely to be a result of opportunity, rather than selection.  Dominant species in stomach contents included the euphausiids Thysanoessa raschii and the copepods C. hyperboreus and C. glacialis (Frost and Lowry 1984; Lowry et al. 2004).

The White whale, or Beluga, is a circumpolar species inhabiting cold waters of the Arctic and subarctic waters (Rice 1998; NAMMCO 2005a).  During the winter months, White whales retreat to subarctic regions with loose pack ice and winter polynyas (Barber et al. 2001).  During the summer months, White whales live in coastal waters, estuaries, shelf breaks and deep basins.  In the Alaskan Arctic, White whales move into the Beaufort Sea in May, moving east into the Canadian Beaufort or north to the Arctic Ocean for much of the summer (Loseto et al. 2006).  In the fall as the Arctic cod congregate in the nearshore waters, White whales cross along the Alaskan mid-shelf in late August and September (Suydam and Moore 2004).  The  Eastern Chukchi White whale typically remains in the more open waters (>200 m depth) throughout the year, whereas the Eastern Beaufort Sea White whales move closer to shore when in the eastern Beaufort Sea (Suydam et al. 2001).

White whales are considered to be third order consumers (Hoekstra et al. 2002), with a diet dominated by Arctic cod (B. saida and A. glacialis), and to a lesser extent whitefish (Coregonidae) in the Russian and Greenland Arctic.  A variety of other prey items have been observed in stomach contents, including capelin, herring, smelt, sculpins, cephalopods and benthic invertebrates (Bluhm and Gradinger 2008).  Nitrogen and carbon isotope signatures indicate that they also feed on copepods and euphausiids, particularly in the spring and fall (Hoekstra et al. 2002; Frost and Lowry 1984).  Known predators of White whales include Orca whales, Polar bears, and humans (NAMMCO 2005a).

Narwhal occur in the deep and offshore waters of the Canadian High Arctic, the Barents and Kara Seas, eastern Laptev Sea and the waters surrounding Greenland (Sherman and Hempel 2008).  Narwhal appear to have high site fidelity, remaining in close association with the ice in winter.  Winter feeding grounds appear to be more important than summer feeding areas, with remarkable aggregations of narwhals found in polynyas.  In winter, a large number of whales can share the limited open water areas; near Greenland, between 17,000 and 19,000 narwhals were found to occupy 2% of the surveyed area (approximately 73 whales per km² of open water (Laidre, personal communication).  Narwhals feed mostly in deep water and possibly at or near the bottom. Dives of up to nearly 1,500 m and 25 minutes are documented (Laidre et al. 2003), and there are some seasonal differences in the depth and intensity of diving (Laidre et al. 2002, Laidre et al. 2003).  Arctic cod (B. saida and A. glacialis) and the squidGonatus fabricii dominate the narwhal diet, with lesser amounts of Greenland halibut and other deep-sea fish (Laidre and Heide-Jørgensen 2005a; Bluhm and Gradinger 2008). Predators include Orca, Polar bears and humans (Hay and Mansfield 1989). 

Ice seals of the Arctic include Ringed seals, Ribbon Seals, Spotted Seal, and Bearded Arctic seals.   Ringed seals (Phoca hispida) are the most common and widely distributed ice seal in the Arctic (Reeves 1998).  Ringed seals are relatively small seals (1.5 m) that are generally found on permanent ice or large floes, maintaining breathing holes allowing it to use ice habitats when other seals cannot (NAMMCO 2005b).  Major foods eaten are Arctic cod, nektonic crustaceans (hyperiid amphipods and euphausiids), capelin, sculpin, and sea-ice and benthic crustaceans (Bluhm and Gradinger 2008).  The balance of the diet varies seasonally.  Ringed seals are a primary prey item for Polar bear, Arctic fox and Glaucous gulls (NAMMCO 2005b).

The Bearded seal (Erignathus barbatus) is a solitary seal with a circumpolar distribution.  It is most abundant where it can reach the sea bottom to feed.  The bearded seal is generally found in the pack ice where openings are common since it cannot maintain a breathing hole.  In the Beaufort and Chukchi system, E. barbatus consumed crab, shrimp, and clams (Lowry et al. 1980).  In the Kara and Barents Seas, and Sea of Okhotsk bearded seals fed primarily on Arctic cod (B. saida) as well as shrimp (Sclerocrangon boreas) and mollusks (Finley and Evans 1983).  In the areas of NW Greenland and the Canadian High Arctic, bearded seals had a varied diet including fish, crustaceans, gastropods, cephalopods and polychaetes (Finley and Evans 1983).

Other seals that are found in the Arctic include Ribbon seals (Phoca fasciata), Harp seals (P. groenlandica), and Spotted seals (P. largha).  Both the Ribbon and Spotted seals feed primarily on Arctic cod, capelin, as well as demersal fish and large benthic crustaceans.  Harp seals feed primarily on Arctic cod and shoaling fishes, such as herring and capelin (Bluhm and Gradinger 2008).

Three subspecies of walrus occur in the Arctic: the Pacific walrus (O. rosmarus divergins) in the Bering-Chukchi), the Laptev walrus (O. rosmarus laptevi) in the Laptev Sea, and the Atlantic walrus (O. rosmarus rosmarus) in the Barents-Kara, Greenland, and High Canadian Arctic waters (NAMMCO 2005c).  Walrus are extremely gregarious, often hauled out on land or ice floes, with several thousand individuals in a herd. 

Walrus feed in shallow waters (10-50 m), foraging through bottom sediments with their stiff beard bristles. Their primary diet appears to be dominated by bivalve clams, however, stomach contents analysis also indicates that other benthos are also important, including snails, echinoderms, and crabs (Outridge et al. 2003; Bluhm and Gradinger 2008).  Dominant clam species found in stomachs included Mya truncata, Serripes groenlandica, and Macoma sp.  Due to their size, tusks, and their gregarious behavior walrus predators are limited to Polar bear, Orca, and humans (NAMMCO 2005c).

Orcas occur in the Arctic during open water periods.  Orcas move northward from the Bering Sea or the North Atlantic.  Distribution likely varies, with movements tracking those of favored prey species or pulses in prey abundance of availability (such as seal pups or fish runs).  In the Arctic, Orcas rarely move close along or into the pack ice (Reeves et al. 2002).  The frequency and abundance of Orcas in the Arctic appears to increase during years with decreased ice coverage.  Orcas are a top predator and feed on a variety of large vertebrate prey including anadromous and pelagic fish, ringed seals, and whales.  Orcas are considered a significant threat to Bowhead whales (COSEWIC 2009).

Polar bears are linked to the marine habitat through diet and daily or seasonal migration.  Although they reside on the ice during the winter, polar bears are accomplished swimmers and inhabit the open waters along the ice edge; they migrate toward land once the ice melts.  Polar bears are linked to the marine pelagic food web, feeding primarily on ringed seals, although they will also feed opportunistically on other marine mammals (Thiemann et al. 2007). 

Arctic seabirds are dependent on marine resources from the Arctic 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.

Black-legged kittiwakes are one of the most numerous seabirds with a circumpolar distribution.  Arriving in southern portions of the Arctic in February and moving northerly through April.  Kittiwakes feed in ice floes as well as in open water, skimming the water surface or feeding from the surface.  Based on stomach contents analysis from kittiwakes in Lancaster Sound, the summer diet is dominated by Arctic cod (93% to 98%; Bradstreet 1976).  Dahl et al. (2003) indicate that kittiwakes in the vicinity of Svalbard feed primarily on capelin, Arctic cod, and hyperiid amphipods.  A similar diet was observed in Barents Sea.  Isotope analysis in the High Canadian Arctic indicated that amphipods may play an important role to fish over the course of the year (Hobson 1993).

Black guillemots are a common bird in open water and amongst the ice floes.  Black guillemots are generalists.  Stomach contents analysis in Lancaster Sound showed that amphipods, copepods, and Arctic cod were all important components in guillemot diets.  Amphipods and copepods appeared to be a more dominant component of the diet when the guillemots fed along the ice edge in later spring to early summer (Bradstreet 1980), with fish being an important component of the diet throughout the year (Hobson 1993).

Thick-billed murres or Brunnichs guillemots are members of the auk family (Alcids).  Thick-billed murres over-winter in boreal regions where there are open waters.  In summer, U. lomvia congregate and breed in the Chukchi Sea, the Siberian coast, eastern Canadian Arctic, Greenland and northern Norway.  Murres are agile diving birds that consume both fish and crustaceans, with Arctic cod comprising the majority of the diet in both coastal and offshore ice edges (Bradstreet 1980; Hobson 1993).  Summer diet was more variable feeding on pelagic amphipods when cod are unavailable.  Murre chicks’ diet is dominated by Arctic cod and sculpin.

Northern fulmars are long-lived (32 years) migratory birds, moving northwards to the Arctic between May and July.  Fulmars are pelagic birds, preferring shelf habitats, particularly shelf breaks or over the continental slope, though they are seldom further than 100 km from shore (Dewey 2009).  As with many other sea birds, the fulmar diet is dominated by Arctic cod, copepods and pelagic amphipods.  Fulmars also prey on the squid, Gonatus fabricii.  Seasonal analysis showed that amphipods and copepods were dominant in the diet of adult and nesting fulmars.  Arctic cod are the primary diet once chicks have hatched and during rearing.  After that time, amphipods (Hyperia sp., Gammarus sp, Themsto sp.) and copepods once again dominated the diet.

The Common eider is a large diving duck common in the Arctic, particularly in the High Canadian and Atlantic sectors.  Common eiders feed primarily on benthic prey including mollusks (Buccinum glacialis, Hiatella arctica), barnacles (Balanus balanus), decapods (e.g. Hyas araneus), and amphipods (Gamarellus homari; Dahl et al. 2003).  Eiders are an upper level consumer in kelp forest (Fredriksen 2003) and estuarine lagoons (Dunton et al. 2012).  Isotope analysis confirms that eiders feed at the lower trophic levels (Hobson 1993).  Unlike other Arctic species, lipids analysis and stomach contents analysis show that copepods are not an important prey item for eiders.

Dovekies are a small marine diving bird that is circumpolar in distribution (Day et al. 1988).  This small auk overwinters in boreal waters, such as the North Sea and Norwegian Sea.  In early spring they migrate northwards to feed on the sympagic copepods and amphipods (Dahl et al. 2003).  The Little auk feeds on the herbivorous sea-ice amphipod, Apherusa glacialis (Kramer 2010).  Dovekies also rely heavily on Calanus copepods, relying on the lipid rich C. glacialis and C. hyperborealis to successfully raise their chicks (Falk-Petersen 2007).   Breeding colonies can be quite large with 30 million birds observed in northwestern Greenland. 

Glaucous gulls are pan-Arctic and are a primary avian predator in the Arctic that feed on a wide variety of fish, mollusks, crustaceans, eggs, chicks, and adult seabirds, as well as carrion.  They will often prey on young and adult birds, as well as the catch from other birds.  Arctic fox are important predators of gulls and skua, as well as other nesting birds, preying on eggs and chicks.

Arctic jaegers are a top avian predator during the summer months, migrating annually to overwinter in the Antarctic.  Jaeger primarily on fish, though they will also feed on insects and berries.  While they can catch their own food, they will often steal fish from other birds.  They will also prey on the nests of waterfowl, eating the eggs and young, as well as small mammals (e.g. lemmings).