The sea-ice realm is defined by a complex of permanent or multi-year ice that can reach thicknesses in excess of 5 m and seasonal, first-year ice that is generally <1 to 2 m in thickness (Melnikov 1997).  The structure of sea-ice varies with the depth and age of the pack ice.  At the ice-water interface, the sea-ice surface is uneven and porous, with brine channels that extend upwards nearly 1 m into the ice pack.  Brine channels are created as the water freezes and salts are excluded from the structure of the ice.  In first-year ice, brine channel density can be 50 to 300 channels per m² and average 0.4 cm diameter (Arndt et al. 2009).  The ice temperature and interstitial water salinity increases with increasing distance from the ice-water interface.  Temperatures in the bottom meter of ice are relatively stable, remaining between 0 and -2°C and salinities ranging from brackish to marine (4 to 40‰; Petrich and Eiken 2010).  In the middle depths of the ice pack, ice density increases, brine channels become smaller, and interstitial water salinity increases (>100 ppt).  Temperatures in the mid to upper ice pack are more variable and can range from -35°C to >5°C.  In older multi-year ice, the upper layers of ice are comprised of fresh water ice, with more fully developed structure and no brine channels.   With the exception of some microbial and bacterial species, the majority of the flora and fauna associated with the sea ice are found on or in the bottom ~20 cm of ice.

Sympagic organisms are those species that live in and on sea ice and include autochthonous species (those that spend their entire life history in the ice) and allochthonous species (those that migrate to the sea ice from the benthic or pelagic realm to spend a portion of their life cycle associated with the sea ice.  The sympagic community includes ice algae, ciliates, nematodes, rotatorians, acoel turbellarians, cyclopoid and harpactacoid copepods, amphipods, and polychaetes (Melnikov 1997).  These species in turn support larger pelagic and avian predators that are closely associated with the ice (e.g. Arctic and polar cod).  The species composition and distribution of sympagic fauna can exhibit large spatial and inter annual variations due to the origin and history of the ice, the water depth, the physical and biological characteristics of the underlying water.  Despite variation, the dominant components of the sea-ice community are similar throughout the Arctic, and include the ice algae, cyclopoid and harpacticoid copepods, sea-ice amphipods, and polar or Arctic cod.  The following section will focus on these species, with reference to the associated upper trophic levels. 

ce algae form the base of the sea-ice food web.  Although primary production by sea ice algae is generally low compared to phytoplankton, they comprise the primary source of fixed carbon to higher trophic levels in ice covered waters (Arndt and Swadling 2006).  Gradinger (2010) noted that in portions of the Arctic Ocean, sea ice primary production accounts for 50% of the total annual production.  In addition, the sea-ice blooms in the spring coincide with the ice melt, representing an important early source of nutrition for zooplankton (Bluhm et al. 2011).  Ice algae primary production is controlled by available light and nutrients.  During the Polar night, production is limited by light availability.  However, increases in light during April and May initiates algal blooms.  As with phytoplankton production, nutrient limitation becomes a controlling force during these blooms.

The sea algal ice communities are diverse and variable throughout the Arctic, with hundreds of reported sympagic species (Sakshaug et al. 2009).  While centric diatoms are a dominant form in the phytoplankton community, ice algae are dominated by pennate diatoms (Melnikov 1997; Sakshaug et al. 2009).  Of 21 species observed in sea ice near Barrow, Alaska, only one centric diatom (Thalassiosirasp) was found in the sea ice.  Common diatoms found in sea ice include Nitzchia spp, Navicula spp, Pinnularia, Pleurosigma, Gomphonema, and Surirella.  Sea ice algae often exists as single cells within brine channels, with pennate diatoms being well suited to living in the limited space of brine channels.  Along the bottom of the ice, sea ice algae may also form dense mats or long strand communities (Melosira arctica).  Melnikov (1997) found long strands of the diatom Melosira arctica under multiyear ice that supports faunal communities.

Ice algae also form communities in melt-ponds of the pack-ice surface.  The melt-pond communities occur in the summer and autumn in brackish to marine waters that are created by melting snow and ice and marine water that penetrate upwards through channels in the ice.  Melt pond communities are dominated by unicellular green algae and flagellates that are commonly found in Arctic freshwater basins at altitudes from sea level to 3000 m (Melnikov 1997).  The communities shift to diatom-dominated communities as salinity in the melt ponds increases (von Quillfelt et al. 2009).

The sympagic mesofaunal community is dominated by harpacticoid and cyclopoid copepods (Kramer 2010).  These smaller copepods are generally considered to be epibenthic in nature, with feeding habits and morphology that makes them well suited to living in and under the sea-ice.  Harpacticoid and cyclopoid copepods can occur in populations as high as tens of thousands per m² in pack ice and are several orders of magnitude greater in abundance than the surrounding water column (Kramer 2010).  The highest population densities generally coincide with the highest algal densities and the more moderate temperatures and salinities, within the bottom 20 cm of the ice pack (Gradinger et al. 1999; Bluhm et al. 2010).  As with ice algae, sympagic copepods biomass and abundance is highly variable depending upon the ice age and location.  The copepod genera Harpacticus, Halectinosoma, Tisebe and Cyclopina appear to have a circumpolar distribution (Arndt and Swadling 2006).  However, the relative proportion of the dominant taxa varies with the type of ice, the region, and with season.  For example, the copepod H. superfluxus and other Harpacticus species appear the be nearly absent from the interstices of perennial ice in the Arctic ocean and the northern Barents and Greenland Seas (Melnikov 1997) and are scarce underneath old ice.  In contrast, in the seasonal fast ice of Frobisher Bay Canada, H. superfluxuspopulations can reach >380 individuals per sq. m.  Another harpacticoid species, Halectinosoma sp. is typically found in multi-year ice in particular near Svalbard and northern Greenland.  Both species can be found in areas where young ice and multi-year ice mix (Kern and Carey 1983). 

In Frobisher Bay, Grainger (1991) found two dominant copepods, Tisbe furcata and Cyclospina schneideri move to the ice in the winter months at a time when the algal production in the ice exceeds that of the sea bottom or water column.  The timing of the ice-ward migration for the two species differed, with Tisbe migrating gradually from February to April at which time, a new generation was produced.  The migration of Cyclopina consisted of only young copepods moving to the ice in early winter and remaining there until April, emerging as mature adults. Despite phytoplankton blooms in the water column in early June, the two ice copepods descend to the bottom to take advantage of the organic material dropping from the melting ice and the water column (Grainger 1991).

The feeding habits of harpacticoid copepods is considered to be herbivorous with little selective feeding (Grainger and Hsiao 1990; Kramer 2011), though many species will supplement their diatom-based diet with detritus during periods of low productivity (Arndt and Swadling 2006).  Tisbe spp. has also been found with fish larvae and copepod eggs and copepodites (Grainger et al. 1985).  Cyclopoids appear to be more omnivorous, with gut contents and lipid biomarkers showing a diet of diatoms, copepod eggs, and detritus.

Cyclopoid copepods are known to use sea ice for reproduction and development.  A large portion of the Cyclopina population is comprised of ovigerous females and up to three cohorts (Arndt and Swadling, Kern and Carey 1983) with a generation cycle of 31 days.  While the full life history of harpacticoid copepods is less well known, Arndt and Swadling (2006) infer that the ice is used for reproduction and growth, given that the eggs, nauplii and copepodite stages, as well as gravid females are found in the ice.  Furthermore, harpacticoid copepods can have >10 broods per year and a generation cycle of 20 days (Tisbe furcata).

The sympagic macroinvertebrate community is dominated by ice amphipods, in particular the pan-Arctic species Gammarus wilkitzkii, Apherusa glacialis, Onismus nanseni and O. glacialis (Hop et al. 2000; Melnikov 1997; Arndt and Swadling 2006).  These autochthonous amphipods reside primarily in sea ice, occupying the three dimensional structure under the ice, as well as somewhat limited use of the brine channels and structure created by the algal mats and strands.  Amphipod abundance is highly variable across the Arctic.  While all five of the dominant species are found in areas impacted with the perennial Arctic ice, Onismus spp. is more commonly associated with young, seasonal ice (Arndt and Swadling 2006).  The amphipod G. wilkitzkii is more abundant in multiyear ice, but will move from drifting multi-year ice to first year ice.

Allochthonous amphipods that are pelagic or benthic in origin are also found under the pack ice, generally taking advantage of the high spring production associated with the sympagic communities (Melnikov 1997).  Planktonic amphipods may include Parathemisto spp. which can be common at the ice-water interface and may occur in swarms of several hundred individuals per m² (Dalpadao et al. 2001).  However, most allochthonous species are benthic in origin, occurring in the land fast ice, or seasonal ice forming over shallow coastal areas.  Species may include Anonyx nugax, Anonyx sarsi, and G. setosuswhich may occur in abundance of tens per m² (Arndt and Swadling 2006)

Apherusa glacialis is an herbivorous amphipod and an important grazer of ice algae (Arndt et al. 2005).  This species concentrates along ice edges and beneath more translucent new ice, where the onset of primary production takes place (Hop et al. 2000).  The diet may shift towards detritus when algae become less abundant in winter.  In areas with high amphipod abundance, the ice algae biomass may decrease at a rate of 30% to 60% of the standing stock per day (Arndt and Swadling 2006).  Phytodetritus is a major food item for Onismus spp.; however, O nanseni is a species repeatedly collected by baited traps, scavenging on carcasses and live prey including sympagic harpacticoid and cyclopoid copepods.  Gammarus wilkitzkii is primarily a predator, catching copepods, chaetognaths and other live prey, as well as amorphous organic debris, diatoms and microflagellates.  Other Lysianassid amphipods that move from pelagic or benthic communities to the pack ice are typically generalists feeding on ice algae, detritus, copepods, and fish eggs. (Arndt and Swadling 2006)  These species can occur in large swarms and in turn become an important food source for Arctic and Polar cod. 

Sympagic amphipods have perennial life cycles and are thought to spawn once a year. Apherusa glacialis can reach 2 years in age but has a high fecundity (>500 eggs develop for 6 months in a female’s brood pouch).  Offspring are released between March and August, presumably to take advantage of the spring growth of ice algae (Arndt and Swadling 2006).  The life span for Onisimus spp. is 2.5 to 3.5 years (Hop et al. 2000).  The reproductive cycles for the two species are offset, with O. glacialis spawning a few months earlier than O. nanseni.  Both species have one brood per year and produce approximately 100 eggs over the female’s life span. G. wilkitzkii is the longest lived ice amphipod, having a 5 to 6 year life span.  This species matures after two years and has one brood per year, releasing 90 to 250 eggs per year.   Eggs are deposited into the females marsupium during winter and release them in April and May when primary production peaks in ice-filled waters.  However, they can be flexible as the amphipods (such as G. wilkitzkii) are generalists and can release young April to September.

The pelagic calanoid copepods common in the Arctic are commonly found under the pack ice, particularly during the spring bloom (Melnikov 1997; Arndt and Swadling 2006).  While they do not appear to colonize the ice, the eggs and nauplii may be advected to the ice sheet.  Calanus glacialis, C. hyperboreus, Pseudocalanus acuspes, Metridia longa, and Oithona similis perform diel vertical migrations to the ice surface at dusk (Fortier et al. 2001) to feed at the ice-water boundary.  Biomass estimates for pelagic copepods are highly variable and are typically highest during the spring bloom, particularly during the ice melt (Arndt and Swadling 2006).  There are some indications that during this time of both ice algae and phytoplankton blooms, calanoid copepod build up a significant portion of their lipid reserves.  The ice-water interface can be an important nursery ground for Calanoid eggs and nauplii, providing food and shelter.  Other omnivorous copepods, such as M. longa come to the sea-ice to feed on Calanus eggs and the small ice crustacean, as well as sympagic diatoms.

Both B. saida and A. glacialis spend a portion of their life history in close association with the sea-ice, utilizing cavities and ledges on the underside of the pack ice, as well as the edges of melting ice floes (Lonne and Guilliksen 1983; Gradinger and Bluhm 2005).  The larval and juvenile life stages are commonly found individually or in small groups, with adults are seldom found in close association with the ice.  In multiyear and first year ice, Lonne and Guilliksen (1983) found both one and two-year old fish living in and under the pack ice.  No fish older than 2-year-olds were observed. 

The diet of Arctic cod in multiyear ice is comprised of a mixture of sympagic amphipods as well as pelagic copepods (Lonne and Gulliksen 1989; Melnikov 1997).  In first-year ice, Lonne and Gulliksen (1989) found the Arctic cod diet dominated by calanoid copepods, however, this was likely due to the distribution and abundance of food items.  Arctic cod have been observed feeding on sympagic amphipods in first year ice in other regions of the Arctic.  Younger cod tended to favor harpacticoid and cyclopoid copepods, shifting to the larger prey with age (Bradstreet 1979).  The amphipod, G. wilkitzkii, was seldom found in Arctic cod stomach contents.  This is likely due to the large size and spiny morphology of the G. wilkitzkii.

Ice amphipods and the more energy rich polar cod are subject to strong predation by top carnivores.  The sympagic macrofauna is a major link in the transfer of energy from sympagic primary producers to the ice-associated sea birds and marine mammals. 

Ice seals are perennial predators under the ice.  The seal diet is variable and is based on food availability.  Based on stomach contents and fecal analysis from seals in the high Canadian Arctic, Arctic cod comprised the majority of the adult ringed seal (P. hispeda) diet, with small proportions (approximately 8%) of amphipods and larger pelagic copepods (Bradstreet and Cross 1982).  Immature seals had a diet that was numerically dominated by ice amphipods, with approximately 3% cod.  However, the cod comprised 62% of the biomass in the stomach contents.  Both A. glacialis and B. saida were present in the stomach contents of young seals.  When Arctic cod are less common, a variety of crustacean species dominate the stomach contents, including ice-amphipods, mysids and other under-ice fauna (Melnikov 1997).

Other seals living amongst the ice include the leporine or bearded seal (E. barbatus), the harp seal (P. groenlandica), and hooded seal (C. cristata).  Erignatus barbatus is primarily a benthic feeder primarily on mollusks, crustaceans, and demersal fish including B. saida and A. glacialis (Melnikov 1997; Finley and Evans 1983).  Four taxa dominate the hooded and harp seal diets in Greenland: Arctic cod, capelin, the squid Gonatus fabricii and the pelagic amphipod, Parathemisto sp. (Haug et al. 2004).  Based on the stomach contents and satellite tracking data, the feeding habits of these two species is more pelagic in nature and not necessarily associated with the communities under the ice.

Sea birds are a primary consumer of sympagic fauna while their access is limited to areas with open water along ice edges.  Important avian predators include northern fulmars, black legged kittiwakes (Rissa tridactyla), guillemots (Uria lombia and Ceppus grille), murres, thick-billed murres, Little auks (Alle alle), and gulls.

Kittiwakes, Black guillemots (C. grille) and Brunnichs guillemots (U. lombia) and murres feed mainly on polar cod.  Other guillemots feed on fammasur in the Canadian Arctic and Themisto spp when the water is ice free.  In the Barents Sea, Little auks forage in multiyear ice mainly on A. glacialis (makes up 80% of their diet).  In the Canadian and Norwegian Arctic, the diet of Little auks is primarily C. glacialis (Falk-Petersen et al. 2007).