Grazing of phytoplankton by microzooplankton in the Barents Sea during early summer

Phytoplankton growth rates and grazing losses to microzooplankton were determined in surface waters of the central Barents Sea during a cruise in June/July 1999. Five stations were occupied which had been studied repeatedly over the past 15–20 years. Dilution experiments using chlorophyll a (chl a) as a tracer were used to estimate daily rates in three size fractions; image-analyzed fluorescence microscopy provided quantitative estimates of standing stocks of auto- and heterotrophic nano- and microplankton. Phytoplankton contributed the largest share of protistan biomass, followed by bacteria and microzooplankton. On average, nanophytoplankton (<20 μm) contributed half of the microphytoplankton (<200 μm) biomass. All stocks were low relative to peak spring bloom concentrations reported in previous years. Different taxonomic groups of microzooplankton were relatively more important under the ice, in the marginal ice zone (MIZ), and in open water. Phytoplankton growth and microzooplankton grazing rates were 0.1 to 0.5 day−1, and were closely coupled. Neither growth nor grazing rates alone was closely related to phytoplankton biomass, but the net difference between growth and grazing explained about 2/3 of the variance in chl a standing stocks. Grazing losses ranged from 64% to 97% of daily chl a production, and were greater for smaller size fractions. Growth and grazing coefficients of all size classes exhibited Q10ʹs of 2–3. These results support the growing body of evidence that small-celled phytoplankton and zooplankton are ubiquitous and important in cold waters as well as temperate and tropical ecosystems.