The Arctic Circle, a parallel of latitude, has little value in understanding the distribution and limits of the marine Arctic flora and fauna. Its only significance lies in its relationship to the seasonal behaviour of light, which is of only limited importance and has nothing to do with temperature—which is extremely important—or, in the case of marine fauna, with salinity. The marine Arctic is defined as an area in which the upper layer (650–825 feet) is derived directly from the upper layer of the Arctic Ocean (Central Basin); the subarctic is the region in which Arctic and non-Arctic (Atlantic or Pacific) waters are found in close association or as mixed water. The subarctic marine fauna is much richer than the Arctic fauna, with which this article deals. The Arctic marine fauna is illustrated in terms of the whole ecosystem in the figure.
The fact that mammals are warm-blooded (homoiothermic) was clearly a great advantage when the climate cooled during the Pleistocene glaciations, and even now they dominate the macrofauna. Among the whales, the beluga, or white, whale and the narwhal are Arctic water species. The bowhead, in much depleted numbers, is found in the Beaufort Sea and in Baffin Bay and occasionally in Hudson Bay. Other whales, of worldwide distribution, appear in Arctic water occasionally (blue whale, little finback or lesser rorqual, finback, sperm whale, and killer whale). The killer whale is a fairly frequent visitor. The phocids, or hair seals, are represented principally by the ringed and the bearded seals, typical Arctic species, and by the migrant harp and hooded seals. The harp seal exists in three separate populations, breeding respectively in the Newfoundland region, the White Sea, and the waters south of Jan Mayen on sea ice in March and April. The fur seals, which are not strictly Arctic, appear in the North Pacific, breeding in Alaskan and Russian waters. A special ecological place is occupied by the polar bear, which is at home in the sea, on the sea ice, and on land but which is essentially an aquatic animal.
Fishes are not abundant in the Arctic zone, perhaps owing to the early competition with the homoiotherms. There are probably not more than about 25 species within the zone. The arctic char, an anadromous (river-ascending) migrant, is abundant and circumpolar, and the two small gadids, the polar cod and the arctic cod, are abundant throughout the region, their numbers being as yet only tentatively estimated.
Marine birds are abundant in summer, all of them migrants except, apparently, for a small proportion of the black guillemot population that winters in the Arctic, using the open water, such as the polynyas, for feeding areas. The seabirds in the true Arctic zone are represented by the auk family (murres, guillemots, auklets, and little auk), the sea duck (eider, scoter, old squaw), the gulls and terns (especially the glaucous and glaucous-winged gulls, many of the herring gull group of species, Sabine's gull, and the common and arctic terns), the jaegers (parasitic, pomarine, and long-tailed), and the waders (sandpipers, etc.). One of the petrel group, the fulmar, breeds on certain Arctic cliffs. The arctic tern, which breeds in the Arctic in the summer, makes a remarkable migration to subantarctic waters, where it winters.
There is a special ecosystem associated with the sea ice that is based on algae (mainly diatoms) living within the ice itself in considerable concentrations, especially in the lowest few inches. This plant growth supports a food web ranging from worms and copepods to amphipod crustaceans, polar cod, birds, and seals. The algae develop earlier in the season than do the planktonic algae (phytoplankton).