Collaborative Research: Hydrographic Change and Effects on Bivalve Communities and Top Avian Consumers in the Bering Sea

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This project will examine whether oceanographic changes have resulted in long-term shifts in bivalve communities and thus the energetics and survival of the world population of Spectacled Eiders, a diving seaduck that feeds on benthic invertebrates near the St. Lawrence Island Polynya (SLIP) during winter. The project has four objectives: 1) retrospective analyses of the species and size structure of dominant bivalves collected over the past 25-50 years, along with similar analyses of samples to be collected over an annual cycle, 2) studies of recent changes in indicators of physical/biological hydrographic processes (sediment oxygen uptake rates, grain size, and carbon content), 3) investigation of alternative influences on bivalve production (ice cover, current transport, productivity, walrus/sea star predation), and 4) simulation modeling to relate bivalve prey characteristics to eider energy budgets. Previous studies indicate that the SLIP and Gulf of Anadyr (GOA) cold pool have an important influence on benthic community structure and biomass. Oceanographic studies south of St. Lawrence Island conducted in the May-August summer season since 1988 show that very high standing stocks of benthic infauna (up to 45 g C m-2) are maintained by nearshore primary production enhanced by hydrographic dynamics, and subsequent transport of waters rich in particulate organic carbon southwestward from the island. In March 1995, large flocks of up to 300,000 Spectacled Eiders were located for the first time south of St. Lawrence Island amid high concentrations of sea ice. Surveys in 1996 and 1997 confirmed that this area downstream of the SLIP is the major overwintering site for virtually the entire world populations of Spectacled Eiders. While it may seem surprising that these avian benthivores would dive in 40-60 m of sub-freezing water in ephemeral leads in the shifting pack ice as an overwintering strategy, prior studies showed that very large densities of small bivalves occur in this region (Nucula belloti and Nuculana radiata, 1-3 cm long). Each bivalve species has a preference for a specific sediment grain size, so changing sedimentation patterns and carbon loading may greatly impact bivalve population structure. Spectacled Eider populations have declined dramatically in the last 25 years, and historical data suggest that dominant bivalve communities south of St. Lawrence Island have also changed during this period. Possible alterations in the intensity of ice formation in the SLIP, and associated changes in transport conditions affecting the GOA cold pool, might have affected the species and size structure of bivalves. Simulation models suggest that these prey characteristics strongly influence eider energy budgets. Thus, eiders may be key indicators of how climate-mediated interactions between ice, hydrography, pelagic production, and benthos affect upper trophic levels. This project will characterize and analyze the strength of these interactions in an Arctic benthic system.This project will interact with an already funded, yearlong physical oceanographic program. The physical oceanographic program will deploy an array of moorings south of St. Lawrence Island in September 1998 to study transport conditions. Cooperation with this hydrographic program will greatly enhance interpretation of the biological and sediment measurements to be made in fall 1998, late winter 1999, and summer 1999. This collaboration also involves interaction with scientists investigating sea ice dynamics using SAR satellite imagery in a real-time manner. The end result of the collaborations will be evaluation of the interactions between physical forcing functions (hydrographic, ice) and trophic processes (benthic to top predators). This will allow simulation of eider energetics and population sensitivity to varying ice conditions and changes in underlying benthic communities under different climate scenarios.