Intense stratification leads to phytoplankton nutrient limitation and reduced microzooplankton grazing in the southeastern Bering Sea

Abstract

Abstract During July and August 2004, we used the seawater dilution technique to investigate relationships among environmental conditions, phytoplankton growth, and microzooplankton grazing in the southeast Bering Sea. During summer 2004 the southeast Bering was unusually warm and strongly stratified. Compared with previous observations in the subarctic Pacific and Bering Sea, mixed-layer phytoplankton growth rates were typical (average 0.35 d−1) while microzooplankton grazing rates were low (average 0.13 d−1). Phytoplankton growth rates were strongly nutrient-limited, increasing to an average of 0.69 d−1 in response to N+P addition. The largest phytoplankton cells (>20 μm) grew at the highest rates both with and without added nutrients. However, all phytoplankton size classes ( 20 μm) responded strongly to nutrient addition, and all showed equivalent growth increases in response to added ammonium versus nitrate. In contrast to growth, microzooplankton grazing rates did not vary with phytoplankton size class. Microzooplankton biomass ranged from 11 to 118 μg C L−1 and was dominated by ciliates and, especially, heterotrophic dinoflagellates. Estimated microzooplankton biomass-specific grazing on phytoplankton was generally low (