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Physical forcing of ecosystem dynamics on the Bering Sea Shelf

P. J. Stabeno,1 G. L. Hunt, Jr.,2 J. M. Napp,3 and J. D. Schumacher 4

1NOAA, Pacific Marine Environmental Laboratory, Seattle, Washington, 98115
2University of California
3Alaska Fisheries Science Center
4Two Crow Environmental Consultants, Silver City, NM, 88061

Chapter 30 in The Sea, Vol. 14, A.R. Robinson and K. Brink (eds.) ISBN 0-674-01527-4.
Copyright ©2005 by the President and Fellows of Harvard College. Further electronic distribution is not allowed.

5. Processes in the Northern Shelf of the Eastern Bering Sea

While many of the observations of recent changes come from the southeastern shelf, similar ecosystem-level change has been observed in the northern Bering Sea. There are indications of changes in benthic biomass south of St. Lawrence Island, and in the Chirikov Basin between Lawrence Island and Bering Strait (Grebmeier and Cooper, 2002). Studies begun in the mid-1980s have shown declines in the biomass (Grebmeier, 1993; Sirenko and Koltun, 1992; Grebmeier and Dunton, 2000) and mean sizes of the dominant bivalves in the area (Grebmeier and Cooper, 2002). Sediment respiration rates, which indicate carbon loading to the sea floor, have also declined since the late 1980s. Seasonal patterns of sediment chlorophyll concentrations show that deposition of carbon in this area is related to the ice-edge spring bloom (Cooper et al., 2002), thus any change in the timing of ice retreat during the late winter/early spring will likely have a major impact on this ecosystem. Although commercial fishing may have played a role in changes to the southern Bering Sea ecosystem, there is little commercial fishing on the northern shelf where changes in benthic faunal populations and declines in dominant fauna have occurred, resulting in a cascade effect on higher trophic levels (Grebmeier and Dunton, 2000; Grebmeier and Cooper, 2002).

Ecosystem processes related to sea ice are dominant features of the northern shelf. In years with extreme occurrences of sea ice (e.g., extremely high in 1975/76 and extremely light in 1978/79), sea ice can be present at 62°N over the northern shelf from mid-November until mid-June, or only from late January to late April. While the presence of sea ice provides a substrate for marine mammals, it also limits at-sea observations. In other regions, under-ice algae provide a wintertime food source for zooplankton during periods of reduced water column production (Tourangeau and Runge, 1991). Most of our knowledge of this high latitude ecosystem comes from summertime observations. Primary production here is not consumed by pelagic secondary consumers (Coyle and Cooney, 1988; Springer and McRoy, 1993), but rather by a rich macro-benthic community. Large populations of benthic, rather than pelagic-feeding marine mammals and birds serve as apex predators in this food chain (Grebmeier and Cooper, 1995).

The primary physical process that results in high primary production over the northern shelf is the onshelf transport of nutrient-rich waters, which are then advected across the shelf. This current, known as the Anadyr Current (Shuert and Walsh, 1993), flows through Anadyr Strait and then northward through Bering Strait (Schumacher and Stabeno, 1998). As the bathymetry shoals (<40 m) in the northern Gulf of Anadyr, nutrients enter the euphotic zone and a production-deposition center is formed (Coachman, 1993). Another center is located over the Chirikov basin north of St. Lawrence Island. As the Anadyr Current flows between St. Lawrence Island and Siberia, the bathymetry deepens (50–60 m) and bottom-generated turbulence results in strong vertical mixing. This tends to mix phytoplankton below the critical depth and interrupts the bloom in the strait (Springer and McRoy, 1993). As the current speed is reduced over Chirikov Basin, a region of extremely high primary production occurs (12–16 gCm day; Springer and McRoy, 1993). The strength of the Anadyr Current is related to the magnitude of flow through Bering Strait. This net northward flow is driven by the difference between sea level in the North Pacific and Arctic Oceans (0.4–0.5 m). On short time scales, this northward flow is modified by wind-generated coastal changes in sea level (Aagaard et al., 1985; Overland and Roach, 1987). During weak winds of summer, northward transport is greatest (Coachman, 1993) and provides the strong flux of nutrients for primary production. The Anadyr Current also transports a high biomass of large oceanic copepods from the slope regions of the basin onto the northern shelf (Springer et al., 1989). In the Chirikov Basin, these support high numbers of planktivorous auklets (Springer and Roseneau, 1985; Springer et al., 1987) that forage in stratified Bering shelf water (Hunt et al., 1990) or in frontal regions on aggregations of these copepods (Hunt and Harrison, 1990). This combination of biophysical coupling supports a pelagic seabird community far from the origin of the zooplankton on which they are dependent.

Flow along the Alaskan coast of the northern Bering Sea is a combination of river runoff (mainly the Yukon River) and a continuation of the BCC of the southeastern Bering Sea (Schumacher and Stabeno, 1998). Production here has been identified as typical of shallow shelves elsewhere: once nutrients are exhausted during an initial bloom, production is low and only ~10% of that for Chirikov Basin and the Gulf of Anadyr (Springer and McRoy, 1993). The east-west gradient in water properties, nutrients, primary production and fauna (transported in the Anadyr Current) result in two different community structures, a rich benthic system to the west and a less productive eastern system (McRoy, 1993). Benthic macro faunal biomass ranges from ~30–60 gCm for the western portion and <10 gCm in the Alaskan coastal waters.

Over the northern shelf, regions exist that are usually ice-free throughout wintertime, and these regions are known as polynyas. These features often are found on down-wind facing coasts (e.g., the south coast of St. Lawrence Island). In the polynyas ice formation is caused by frigid winds blowing from the northeast. This and the resulting brine rejection are important features of the physical environment. As ice is formed, brine is released and sinks to the bottom. The current field generated by this process results in both offshore flow and sometimes a reversal of the mean eastward flow of the branch of the Anadyr Current that flows south of St. Lawrence Island (Schumacher et al., 1983). This wintertime set of physical processes apparently affects sediment patterns, e.g., surface C/N ratios, total organic carbon, sediment oxygen uptake, and benthic biomass (Grebmeier and Cooper, 1995). As a result of the enhanced production, the region southwest of St. Lawrence Island has long been a favorite feeding ground for gray whales.

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