FY 2008

Patterns of spatial and temporal variation in the marine ecosystem of the southeastern Bering Sea, with special reference to the Pribilof Domain

Hunt, Jr., G.L., P.J. Stabeno, S. Strom, and J.M. Napp

Deep-Sea Res. II, 55(16–17), 1919–1944, doi: 10.1016/j.dsr2.2008.04.032 (2008)

Results from 2004 field observations, integrated with those from prior studies, allow definition of a unique “Pribilof Domain” in the southeastern Bering Sea. This domain results from its geographic location and bathymetry that supply water from the outer shelf and slope that is replete with dissolved and planktonic material. We highlight temporal variability in this domain and place this in the broader context of mechanisms identified as potentially important in regulating the flow of energy and material in this shelf ecosystem.

In the Pribilof Domain, unique circulation features and mixing are responsible for energy and material dynamics. Rectified tidal currents entrain water from the Outer Shelf Domain, thereby transporting nutrients and plankton to the vicinity of the Pribilof Islands. Near the islands, winds and strong tides interact with bathymetric features to mix nutrients into surface waters, resulting in localized regions of high new production well after the cessation of new production in the highly stratified waters of the Middle Shelf Domain. Large, oceanic species of zooplankton are episodically advected into the Pribilof Domain where they provide an important source of prey for predators.

In 1999, a cold year, sea ice was present in the southeastern Bering Sea until late May, summer stratification over the middle shelf was weaker than in 2004, “large” crustacean zooplankton were abundant, and microzooplankton productivity was greater than in 2004. In contrast, 2004 was a warm year in which sea ice retreated in early March, summer stratification was strong, summer primary production was lower than in 1999, and small shelf species dominated the mesozooplankton, the biomass of which was comparable to some of the lowest values in 50 years. In 2004, microzooplankton consumed about half of the primary production in the Middle Shelf Domain.

Evidence is mounting that the eastern Bering Sea is undergoing a major change in the structure and function of its ecosystem. The lack of large zooplankton in Middle Shelf Domain waters in summer, possibly linked to the lack of an early spring, ice-associated phytoplankton bloom, can be expected to affect the distribution, and possibly growth, of planktivorous groundfish, such as walleye pollock (Theragra chalcogramma). Bottom temperatures also affect the distribution of fish, with warm temperatures allowing many groundfish species to extend their distributions northward and eastward. We review mechanisms influencing productivity and its fate, and the impacts of a changing ecosystem on apex predators tied to the Pribilof Islands for reproduction.