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

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 (<5 μm, 5–20 μm, >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 (<0.5 d−1), a consequence of high microzooplankton biomass coupled with low grazing rates. Low ratios of grazing: growth (average 0.49) further indicated weak trophic coupling between phyto- and microzooplankton during summer 2004. Some regions near the Pribilof Islands contrasted with this scenario, exhibiting high phyto- and microzooplankton biomass and, in some cases, elevated growth and grazing rates. This lower trophic level enhancement was a response to natural nutrient addition events caused by flow–bathymetry interactions near the islands. These predictable sources of summer production are likely important to zooplankton and to the birds and mammals that breed on the Pribilofs. Comparison with a data set collected in the same region in 1999 (a cold, late ice year) showed that summer production at lower trophic levels can be strongly modified by environmental conditions that vary intra- and interannually. Warmer temperatures, weaker winds, and stronger stratification in 2004 likely led to nutrient limitation of phytoplankton growth, reduced microzooplankton grazing, and weak trophic coupling. Such responses at lower trophic levels could negatively affect zooplankton, birds and mammals dependent on summer production in the southeast Bering ecosystem.