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

Abstract

Seasonal 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. Copyright 2004 by the American Geophysical Union.