The eastern Bering Sea shelf is a marginal ice zone where polar and subarctic systems interact. During the summer and fall the shelf is usually ice free. In winter the juxtaposition of the Aleutian Low and Siberian High results in strong, frigid winter winds from the northeast. These winds cause extensive ice formation along leeward coasts, which is then advected southwestward. The leading edge of the ice is continuously melting, introducing cold freshwater to the water column. Maximum ice extent is typically in late March (Wyllie-Echeverria, 1995). The location of the southernmost position of the ice varies over hundreds of kilometers. In extreme years (e.g., 1976), ice covers the entire southeastern shelf (Fig. 1). The extent of the ice influences the temperature and horizontal extent of a cold pool that occurs in the lower layer of the middle shelf. The cold pool, which is 40-50 m thick, persists through the summer, warming only slightly (often <2°C).
The eastern Bering Sea shelf is one of the most productive regions of the world's oceans (Walsh and McRoy, 1986). Ice-edge phytoplankton blooms are common features in marginal ice zones during the spring (Smith, 1987; Niebauer, 1991) and produce a large fraction of the annual primary production over the shelf. This occurs as the ice melts back in the spring and a surface mixed layer is formed as a result of the increase in freshwater (Niebauer et al., 1995). The rapid increase in chlorophyll can be assisted by the seeding of the water column with ice algae (Schandelmeier and Alexander, 1981).
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