Ice begins to cover the northern Bering Sea in November and reaches its maximum by February, when it usually extends as far south as the edge of the continental shelf, and covers nearly 75% of the surface of the Bering Sea (Lisityn 1969). Coverage can vary greatly from year to year. In certain years Bristol Bay may be completely covered and in others ice is found only in the northern part of the Bay. Almost all ice in the Bering Sea is first-year ice. This ice tends to be flat on the top and underside and in general lacks the extensive keels and pressure ridges found on multi-year ice.
The Bering Sea ice has a number of large-scale features of importance to birds. The "front" is a zone of ice south of the consolidated pack that is composed of small floes, ice pans, and brash ice. This zone is prevented from forming large floes by the action of swells from the open water to the south. The front continually changes in width. When winds are from the south, it is compressed into a narrow band; when winds are from the north, it is a broad zone composed of bands of ice interspersed with open water.
Polynias (areas of open water) are found immediately south of the large islands in the northern Bering Sea. They are formed by the southward movement of ice caused by the prevailing winds. This movement causes ice to be pushed away from the south side of islands, leaving areas of open water. Large polynias are associated with St. Lawrence, St. Matthew, and Nunivak islands and with the south side of the Seward Peninsula (Shapiro and Burns 1975).
The most biologically active area of the Bering Sea in winter is the ice front. Studies of primary productivity in April show that production at the surface in the ice front is high (1.98 mg C/m³ per h). Surface waters directly under the pack ice have much lower production (0.29 mg C/m³ per h), and that in the water south of the ice is lower yet. At this time production within the ice is very high (more than 5 mg C/m³ per h) (McRoy and Goering 1974). Because this phytoplankton bloom is trapped in the ice, it is not available to grazers. Thus, before the spring melt the ice front is the only area where a large quantity of phytoplankton is available to higher levels of the marine food chain.