Jacqueline M. Grebmeier

Carbon and nitrogen cycling within the Bering/Chukchi Seas: Source regions for organic matter effecting AOU demands of the Arctic Ocean

JDepartment of Marine Science, University of South Florida, St. Petersburg, Florida 33701, U.S.A. Institute of Marine Science, University of Alaska, F airbanks, Alaska 99701, U.S.A. JDepartment of Oceanography, University of Washington, Seattle, Washington 98195, U ~ A. 4Geohydrodynamic and Environmental Research Laboratory, University of Liege, B--4000 Liege, Belgium 5Marine Science Institute, University of Texas, Port Aransas, Texas 78373, U~ .A. 61nstitute of Ecology and Genetics, University of Aarhus, DK--8000 Aarhas C, Denmark 7Department of Applied Science, Brookhaven National Laboratory, Upton, New York 11973, U.S~4. aLaboratory for Oceans, Goddard Space Flight Center, Greenbelt, Maryland 20771, U.S.A. 9Graduate Program in Ecology, University of Tennessee, Knoxville, Tennessee 37996, U.S.A.

Influence of the St. Lawrence Island Polynya upon the Bering Sea benthos

The influence of a polynya, a persistent ice-free region, on water column production and subsequent transport to the shallow continental shelf benthos of the Bering Sea was evaluated by studying spatial patterns of organic material deposition, benthic biomass, community sediment metabolism, benthic population structure, and other potential indicators of enhanced organic carbon transport to benthic communities underlying the St. Lawrence Island Polynya. Despite suggestions that polynyas may be important localized centers of primary production in polar waters, we found that the St. Lawrence Island Polynya does not obviously enhance the biomass of benthic communities directly below the polynya. However, southward flowing, baroclinic currents generated as a result of brine injection at the polynya edge do appear to have an influence on the biomass and ecological structure of Bering Sea benthic communities south of St. Lawrence Island. These currents appear to affect mean sediment oxygen consumption, surface organic carbon/nitrogen ratios, total organic content, and bottom water ammonia by sweeping phytodetrital matter south and to the west of the island. A particle-reactive, short-lived, natural radioisotope, 7Be, used as an indicator of rapid (days to weeks) deposition of particulate material from the water column, was detected only in surface sediments to the southwest of the island, indicating enhancement of particle deposition to the southwest of the island. Finally, the 18O content of tunicate cellulose was highest in the polynya region, consistent with increased filter feeding in the late winter when the polynya is present, and presumably promoting primary production in the open water. The Anadyr Current, consisting of nutrient-rich, deeper Bering Sea water that is upwelled onto the shelf in the Gulf of Anadyr, flows west to east in the region south of St. Lawrence Island throughout the year and is the major forcing function for high production in the region. The interaction of Anadyr Water with the winter/spring ephemeral polynya and associated baroclinic currents combine to positively influence benthic communities.

Late winter water column and sea ice conditions in the northern Bering Sea

[1]xa0Seasonal sea ice concentration and thickness were evaluated on a weekly basis during two years with contrasting ice coverage, 1998–1999 and 2000–2001, using data provided by the U.S. National Ice Center. Ice in the Bering Sea during 1998–1999 was extensive and thick, but by contrast, in 2000–2001, winter sea ice formed late with thin ice, and ice melt proceeded earlier during spring. The presence and timing of a winter polynya (an area of relatively open water or thin ice surrounded by heavier ice) located south of St. Lawrence Island also varied between these two winters. Shipboard measurements south of St. Lawrence Island during two late winter/early spring cruises in 1999 and 2001 showed that some brine injected water was present, resulting in localized areas of bottom water with salinities approaching, or exceeding, 33 psu. The mean salinity of bottom water was significantly higher in 2001 (32.6 psu) than in 1999 (32.3 psu). These varying degrees of brine injection associated with large differences in ice conditions (heavy in 1998–1999, light in 2000–2001) influenced bottom water salinity and density, but there were indications that variation in water mass structure also can explain differences in salinity observed between the two winters. The mean δ18OH2O values of bottom seawater were significantly higher in 2001 (−0.78 ± 0.16‰ SD) than in 1999 (−0.98 ± 0.20‰ SD) consistent with a more saline, nutrient-rich water being present in 2001. Consistent with these indications, inorganic nutrients (nitrate, phosphate, and silicate) in bottom water were significantly higher in 2001 than in 1999. Although it is seemingly paradoxical that salinities were higher in 2001 (light ice) than in 1999 (heavy ice) when brine injection might have been more prevalent, the sampling period was approximately 1 month earlier in 2001, when ice formation may have been more active. In addition, δ18OH2O values indicate a higher proportion of Anadyr Water, with higher salinity and nutrients in 2001. Despite these differences in both ice conditions and water mass nutrient chemistry, water column chlorophyll was uniformly low in both years. This indicates that changes in Bering Sea ice regimes during the late winter months, such as may occur under various climate change scenarios, will not necessarily lead to any higher productivity or an earlier onset of seasonal biological production. Despite relatively high inorganic nutrient concentrations in both years, a well-mixed water column was observed with low water column chlorophyll-a (chl-a) concentrations. These observations suggest that there was little primary production during either cruise in spite of the relatively open water and high water column nutrient concentrations during 2001. Strong southerly winds during winter likely impeded ice formation, vertically mixed the water column, and prevented early spring open water primary production.