Microzooplankton: Abundance, biomass and contribution to chlorophyll in the Eastern Bering Sea in summer

Summer microzooplankton abundance and biomass were determined for three years, 2008, 2009, and 2010, during a four-year cold period in the eastern Bering Sea. Average microzooplankton densities ranged from 4×103 to 25×103 cells l−1 in the mixed layer. Microzooplankton biomass was 21-25 µg C l−1 in the mixed layer on the middle shelf (South and North Middle Domains), which has relatively low chlorophyll during summer stratification. However, microzooplankton biomass was about 1/2 that in the less stratified waters near the shelf break, greenbelt, and in the bloom waters in the Pribilof Island Domain. Although phytoplankton biomass was higher in deep chlorophyll maxima (DCM) than in surface waters on the shelf, microzooplankton biomass was generally not elevated in the DCM. High ratios (>1) of microzooplankton biomass to phytoplankton biomass were observed at chlorophyll a concentrations <1 µg l−1. At times, average microzooplankton biomass was higher than the calculated phytoplankton biomass in the mixed layer in coastal (Inner Domain) and middle shelf (South and North Middle Domain) waters. A confounding factor in comparing microzooplankton and phytoplankton biomass was the contribution of plastid-retaining, mixotrophic, ciliates to chlorophyll a. On average, mixotrophic ciliates comprised 66% of the ciliate biomass, and in the North Middle Domain, on some cruises were estimated to have contributed over 50% of the chlorophyll a in the mixed layer. The 2008-2010 data suggest that extent of stratification, presence of localized blooms, and domain differences all have major influences on coupling of microzooplankton to phytoplankton stocks in summer in the eastern Bering Sea.