Benthic communities are strongly influenced by the substrate-type and sediment grain size.  Benthic substrates in the Arctic are dominated by fine-grained sands, silts, and clays on the shelves and fine clays and silts in the oceanic basins (Bluhm et al. 2010).  Sediment in the expansive shelves of the Barents, Kara, and Laptev Seas are typically dominated by fine grained sediments (Stein et al. 2004, Semiletov et al. 2011; Cochrane et al. 2009), with areas of sandy substrate occurring in the nearshore areas.  Similarly the narrowing shelves of the Bering and Chukchi seas are fine in nature, with some sandy areas occurring in the Bering Strait.  In the central reaches of Russian, Greenlandic, and Baffin Island fjords, sediments are dominated by very fine, unconsolidated clays and silts (Aitken and Fournier 1993, Stein et al. 2004).  Sands and gravelly substrate is more common in the nearshore zone and in erosional areas.  Hard substrate is localized and includes subtidal boulder fields as well as nearshore rocky outcrops.  Notable examples are the boulder fields in the Beaufort Sea, portions of the Barents Sea, and the rocky shoreline in northern Greenland and Svalbard.

Intertidal benthic communities in the Arctic are generally thought to be less diverse than those found in lower latitudes due to ice scour and UV exposure.  The disturbance from ice comes from direct contact of foot or anchor ice and scouring during breakup.  Such disturbance results in a continually changing benthic community dominated by fast-settling, opportunistic meiofauna (Barnes 1999).  Estimates suggest that Arctic intertidal macrofaunal communities typically have no more than 100 species, with some areas nearly devoid of intertidal macrofauna (animals larger than 0.5 mm).  In contrast intertidal boreal communities in Alaska and Norway may have over 200 to over 300 species (Weslawski et al. 2011).  Most intertidal species are circumpolar; however, their regional distribution can be highly variable.  For example, areas of the Beaufort and Siberian coasts are devoid of intertidal macro-organisms as beaches are predominately gravelly with little stability and fast ice in the winter months (Weslawski et al. 2011).  Conversely, estuarine lagoons along the Beaufort are relative benthic hotspots, with well-developed benthic infaunal communities that are protected from fast ice and coastal erosion (Dunton et al. 2012).

Rocky intertidal and shallow subtidal communities are an important component in certain portions of the Arctic, particularly the Atlantic sector (Figure 2-9).  Benthic communities in these areas are more akin to those of north Atlantic rocky intertidal and shallow subtidal habitats.  For example, in the steep rocky substrate in Svalbard (Kuklinski and Barnes 2008) the most common species include the macroalgaeFucus spp., sessile (i.e., non-mobile) barnacles (Balanus balanoides), and motile gastropods (Littorina saxatilis) and amphipods (Gammarus setosus and G. oceanicus).  Associated subtidal communities are dominated by the barnacle Balanus balanus, brittle stars (Ophiopholis sp.), motile amphipods (Calliopidae sp.), isopods (Munna sp. and Janira maculosa), sipunculid polychaete worms, and snails (Alvaniasp.).

Figure 2-9. Locations with Shoreline Dominated by Hard Substrate (Red outline, from Lantuit et al. 2012)

In the western Arctic, hard-substrate communities are uncommon in intertidal and subtidal waters.  However, there are isolated communities associated with patches of boulders and cobble.  Dunton et al. (1982) characterized an Arctic kelp community associated with subtidal (<10m) boulder patches in the Alaskan Beaufort Sea.  Exposed boulders and cobble provided an attachment point for Laminarian kelp (Saccharina latissima, L. solidungula, Alaria esculenta).  The community associated with the kelp beds were characterized as being similar to those found in northern Atlantic waters.  Large sponges and cnidarians were common, as were the chitons and mussels; barnacles (B. balanus), snails, and sipunculid polychaete worms were also common.  The crab Hyas coarctatus was the dominant crustacean, along with mysids, amphipods, and hermit crabs (Pagurus trigonocheirus).

The benthic communities of the coastal nearshore zone (<50 m) can be variable depending upon the salinity regime and substrate.  Infaunal biomass is variable across the Arctic, ranging from 41 gC/m² in the Beaufort Sea to over 250 gC/m² in the Baffin Island, Lancaster Sound area (Thompson 1982).  The nearshore zone in the Beaufort Sea has relatively low biomass and highly dynamic communities due to the gravelly nature of the shallow subtidal substrate.  In gravelly or cobble substrate meiofauna and barnacles dominate.  In sand and sandy silts, the sediment fauna is dominated by small polychaetes (Scoloplos armiger, Spio filicornis, and Chaetozone setosa) and oligochaetes.  Feder and Schamel (1976) found that species diversity, abundance and biomass increased with distance from shore, which was attributed to ice and wave action affecting stability.  Nearshore coastal areas of Lancaster Sound, the high Canadian Arctic, and Greenland, infaunal communities have higher biomass, with bivalve dominated communities including Astarte borealis, A. motagui, Serripes groenlandicus, Mya truncata, Cistenides granulata, and Macoma calcarea (Thompson et al. 1986).  Similar species complexes that also included the bivalvesPortlandia arctica, and Nuculana sp. are found along the Russian Arctic (Gebruk 2004).

Estuarine flats and lagoons can have abundant and well developed benthic communities.  Lagoon areas are often protected from fast ice and wave action, are enriched by terrestrial sources of organic matter, and have warmer water temperatures.  However they may also have highly variable salinities that can limit species diversity.  In lagoons along the Beaufort Sea and in the Kara Sea, infaunal diversity was noticeably lower in estuarine lagoons than the neighboring marine waters.  However, abundance and biomass were equivalent or greater.  As in temperate waters, the lagoons act as a rich habitat for phytoplankton, harpacticoid copepods, calanoid copepods, polychaetes (Nephtys sp., Prionospio sp., Spio sp., Terebellides sp., and Travisia sp.), Pandalus shrimp, mysids, clams (Astarte sp., Yoldia sp.,Macoma sp., Portlandia sp.), a variety of amphipods (Anonyx sp., Gammarus spp.), and isopods, anadromous fishes (Arctic and Least cisco, Arctic char, salmonids) and birds (Dunton et al. 2012; Gebruk 2004).  Many of the species found in the lagoon communities are similar to those found in boreal and temperate waters.

As indicated above, the majority of the benthic habitat in the Arctic basins is fine grained sand, silt, and clay.  In general, soft bottom infaunal and epifaunal communities in the Arctic are similar to those of other oceanic basin, being dominated by polychaetes, amphipods, mollusks, and echinoderms.  Abundance, biomass, and species diversity in Arctic shelf communities is considered to be similar to the lower range for boreal and temperate basins.  Community abundance, biomass, and diversity are variable throughout the Arctic, and are controlled by factors such as substrate type and availability of organic carbon appear to be the primary drivers for abundance and biomass (Thompson 1982; Grebmeier and Cooper 2012; Piepenburg et al. 2011).   In the Barents Sea, Cochrane et al. (2009) found that ice cover was inversely proportional to organic carbon and abundance.  A number of studies have noted the importance of pelagic-benthic coupling in determining the benthic assemblage (Grebmeier and Cooper 2012; Carmack et al. 2006; Carmack and Wassmann 2006).  Pelagic sources of organic carbon in the Arctic includes both water-column and sea-ice production.  In the high Arctic, the ice algae are the primary source of carbon for the benthic communities.  However, productivity in these regions is limited due to the reduced daylight and ice cover and this appears to limit abundance and biomass in the associated benthic communities.  Species assemblages have also been shown to vary across the shelf from east to west, with the southern Greenland, Norwegian, and Barents Seas affected by the species in the North Atlantic.  Similarly, the Siberian-Chukchi-Beaufort system is affected by Pacific species associated with the Bering Sea and Strait (Grebmeier 2012).

Beyond the shelf, species diversity, abundance and biomass decreases markedly with depth.  Unlike other oceanic basins, there was no increase in diversity and biomass at the mid-depths, rather all three benthic measures decrease steadily with depth (Table 2-2; Bluhm et al. 2011).  Many of the species that comprise the deep water communities are eurybenthic and are found on the outer continental shelves.  Similarly, many of the dominant deep water species are similar to those of the boreal and temperate deep water communities.  When considering deep water benthos, it is important to bear in mind that data at depth in the Arctic is limited.  Sampling methods commonly deployed in other ocean basins cannot be deployed in the Arctic.

Table 2-1.  Abundance and Biomass for Arctic Deep-Sea Benthos (Bluhm et al. 2011).

The general taxa groups that dominate the benthic macro- and megafauna in the Arctic are similar to those of other oceanic basins, namely polychaetes, amphipods, isopods, mollusks, and echinoderms.  Other epibenthic crustaceans such as crab and shrimp are also found in the Arctic but are not as common throughout the region.  While polychaetes are the most abundant of the major taxa throughout the shelf, Arctic clams are perhaps a more important component in the Arctic food webs, relative to other regions, with a number of larger and important predators (e.g. walrus, bearded seal).  The following section discusses each of these taxa groups.

Bivalve mollusks are an important component of Arctic shelf communities, serving as a primary prey item for walrus and bearded seals, among other higher vertebrates.  Benthic communities of the inner shelf are often defined by the dominant bivalve species.  Bivalves (clams and mussels) can comprise up to 15% of the infauna in slope and plain sediments, but often dominate the biomass.  In substantial portions of the East Siberian Sea-Chukchi-Beaufort and Barents-Kara-Laptev shelves, bivalves were the dominant taxa (Grebmeier and Cooper 2012; Denisenko 2007; Filatova and Zenevich 1957).

The Greenland cockle, Serripes groenlandicus, is a common circumpolar bivalve found up to 100 m depth on a variety of substrates.  A fairly large (up to 112 m) and long-lived clam (up to 39 years) it is a main component of the diet for walrus and Bearded seals.  Macoma calcarea are also a common component in the shallower portions of the Siberian-Chukchi- Beaufort and Barents-Kara-Laptev seas (<50 m depth), as well as the fjords of northern Canada and Baffin Island.  Areas with organic enrichment had higher Macoma abundance (Grebmeier and Cooper 2012; Filatova 1957).  Other dominant clams in the shelf include Astarte sp., Portlandia, Mya truncata, Telina sp. and Yoldialla solidula. 

Deep-sea species are generally smaller and less abundant.  Common deep sea species include the taxa Axinopsidae, Nucula, and Nuculanacia while other species are unique to the deep sea (e.g. Bathymodiolus spp. and Abra spp.; Gage and Tyler 1991). Clams are generally suspension or deposit feeders that are well adapted to processing particulate organic matter (POM) in the deep sea.  The digestive tract in many deep-sea species is elongated with intra and extracellular digestive enzymes that allows for efficient digestion of recalcitrant forms of carbon.  In addition, deep-sea clams have modified palps that sort particles before sending them to the mouth (Allen 1979).  Some deep-sea species are carnivorous, with modified siphons that allow for predation on copepods and ostracods. 

Polychaetes dominate the infaunal community on the slope, rise and abyssal plain.  MacDonald et al. (2010) found dominant families in the Arctic including Cirratulidae and Paraonidae (Aricidea sp).  Other species considered to have pan-oceanic distributions based are Capitella capitata, Lumbrineris minuta, Maldanid, Oweniid, and Chaetozone complexes.  There are a number of different feeding strategies used by polychaetes, including filter feeders, surface deposit feeders, subsurface-burrowing feeders, and carnivores.  On the shelf and in fjords, the species assemblages change with feeding strategies.  Renaud et al. (2007) found that the inner fjords and nearshore areas were typically dominated by few species and species that were highly motile or surface deposit feeders (e.g. Lumbrineris sp., Chaetozonesp.). In the mid- to outer fjords, the polychaete community was more diverse with an increase in subsurface deposit feeders.  There is a strong indication for pelagic-benthic coupling in the Arctic with the distribution of benthic fauna strongly associated with patterns in the associated water column, particularly near polynyas where regionally high production in the water column is reflected in the benthic community (Piepenburg et al. 2005).

Amphipods are common members of the benthic community, including both infaunal and epibenthic species.  Amphipods are broadly distributed throughout the world’s shelf and deep sea basins.  The most widely distributed species across the Arctic are Ampelisca eschrichti, Anonyx nugax, Arrhis phyllonyx, Pontoporeia femorata, Gammarus setosus, and Byblis gaimardi (Piepenburg et al. 2011; Dunton et al. 2012).  Petrova and Dzhurinskiy (2012) found that Ampeliscidae were more common in the inner shelf areas, while Pontoporeiidae were more common in Bering Strait.  In the Gulf of Finland, a similar trend was observed with Pontoporeiidae more abundant at the mouth of the Gulf and Ampelisca more common near the head of the Gulf.  Amphipods are primarily deposit feeders or active scavengers, either free-burrowing or living in tubes.  Infaunal amphipods are commonly used in toxicological evaluations of whole sediment, with established methods for testing whole sediments or spiked water samples.

Lysianassid amphipods are 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, extendable guts for food storage, and a highly motile foraging behavior.  Premke (2003) has reported on food-fall scavenging activities of the Lysianassid amphipod Eurythenes gryllus  in deep Arctic waters of the Norwegian Deep.  However, in shallower waters, Lysianassid amphipods may have a more diverse diet.  Lysianassid amphipods  dominate invertebrate macrofauna in certain environments.  On tidal flats, Onismus litoralis constitutes up to 95% of the macrofaunal density (Weslawski et al. 2000). 

Many species of Lysianassid amphipods are barotolerant. E. gryllus may be easily retrieved and decompressed from deep water with no apparent deleterious effects, as long as temperature is kept below 4 °C (Sainte-Marie 1992).  Their geographic distribution and depth range is also considered to be extensive, being found throughout the world to depths up to 2,500 m above the bottom (Krapp et al. 2008, Sainte-Marie 1992, Premke 2003).  Anonyx nugax is also a common benthic amphipod in the Arctic found associated with the sea ice and with abyssal communties as deep as 1700 m.

In general, decapod crustaceans are less common in the Arctic.  There are however, several notable exceptions, particularly in the boreal-Arctic waters of the Bering Sea-Chukchi and the Barents-Greenland Seas.  Crab and shrimp are the primary decapods in the Arctic. These include the shrimp, Pandalus borealis and the deep water prawns Pandalopsis dispar.  In the eastern Arctic, the densest concentrations are found in the central region of the Barents Sea, Hopen Deep, Thor Iversen Bank and near the western Murman coast at depths from 200 to 350 meters.  The caridean shrimp, Nematocarcinus ensifer is found from the North Sea into the Arctic.  Epibenthic shrimp are primarily detritivores; however the diet is augmented with slow-moving prey such as gastropods, ostracods, and hydrozoan polyps (Cartes 1993).  In general, larval and juvenile demersal shrimp appear to occur deeper than adults, migrating to the shallower end of their distribution as they grow older.  Demersal shrimp are an important food source for demersal fish, in particular Alepocepalus and Macrouridae.  There is a substantial fishery for P. borealis in the Russian Arctic.

The crab, Chionoecetes spp., is native to waters in Alaska, the east coast of Canada and west of Greenland, and is an invasive species in the Barents Sea.  Main items in the Chionoecetes spp. diet in the southeastern Barents Sea are polychaetes, mollusks, crustaceans and echinoderms.  Chionoecetes spp.in the Barents Sea were recorded in waters from 39 to 387 m depth, predominantly on muddy or sandy and muddy grounds, at temperatures from –1.6° C to 5.9° C and salinity from 34.5 to 35.1 ‰ in the near-bottom layer.  In the Bering Strait and Chukchi Sea, Chionoecetes spp. has shown increased abundance (Bluhm et al. 2009; Iken et al. 2010). 

The Red king crab (Paralithodes camtschaticus) occurs in portions of the Barents Sea and was deliberately introduced to the Barents Sea at several locations during the 1960s and 1970s from the northern part of the Pacific (Orlov and Ivanovo 1978). It has continuously spread to new areas and is now distributed from the Kluge Island to east, the Goose Bank to north, and west to Lofoten and Kvænangen to west along the Norwegian coast. The expansion of the area inhabited by red-king crabs occurred during years when water temperature in Atlantic currents was higher than normal (Pinchukov and Karsakov, in press). Several studies have revealed that the crab besides being an important fishing resource, also significantly impact the bottom ecosystem in areas of high densities of crabs (Sundet and Berenboim 2008).

In the Russian waters of the Barents Sea, red-king crabs occur in areas from shallow waters to the depths below 335 m. In spring, April-May, they form spawning aggregations of individuals of both sexes, whereas in August-September, red-king crabs form separate aggregations where males aggregate in concentrations within the temperature range 4-6° С and females within 5-7° С.  Red-king crabs are benthic predators (Gerasimova and Kachanov 1997; Manushin 2008), but in areas with intensive fishing, they predominantly feed on fish offal (Pinchukov and Pavlov 2002; Anisimova and Manushin 2003). The main red-king crab predators in the Barents Sea are cod and skates (Matyshkin 2003).

Galatheid crabs (Munida spp) are widely distributed on bathyal bottoms in most deep ocean regions (Cartes et al. 2004).  Munida tenuimana is common in the north Atlantic along the middle and lower slope from depths of 300 to 1900 m (Cartes 1993).  The diet of the galatheid crabs includes polychaetes, crustaceans, and fish remains.  Galatheid crabs are also found in the hydrothermal vent communities, living within the large tube worms (Martin and Haney 2005).

As in other oceanic basins, Ophiuroid brittle stars are among the most common megafauna occurring in Arctic shelf habitats.  Dominant species include Ophiocten sericeum, Ophiura sarsi, Ophiura robusta,Ophiopleura borealis, and Ophiacantha bidentata (Piepenburg et al. 1997; Gebruk 2004; Thomson 1982 Anisimova 1989).  In the Laptev Sea, populations of O. sericeum and O. sarsi were abundant (as high as 36 ind/m²) but highly variable with nearby areas devoid of brittle stars (Piepenburg et al. 1997; Sirenko et al. 2010).  Similar trends were observed in shelf habitats in Greenland and Barents Sea and the outer shelf in the Siberian Sea (Bluhm et al.2009). 

Photo 2-4. Dense aggregations of Brittle Stars

Most ophiuroids are motile epifaunal grazers using their flexible arms to feed on detritus, suspended organic material, small epifauna, and infaunal organisms.  Ophiuroids have the ability to aggregate in areas of organic carbon deposition and have been associated in the Arctic with polynyas and marginal ice zones, where ice algae and associated detritus are deposited at higher rates (Photo 2-4).  Predators of ophiuroids include shrimp, crabs, and epibenthic feeding fish such as Zoarcidae and small Macrouridae.

At deeper sites, holothoroids become a dominant component of the demersal invertebrate community.  Holothoroids, or sea cucumbers, are ubiquitous on the abyssal plain and relatively abundant.  Sea cucumbers are detritivores, ingesting sediment and digesting the incorporated organic material.