A comparative study of size-fractionated mesozooplankton biomass and grazing in the North East Atlantic

Measurements of biomass and grazing for three size classes of mesozooplankton were carried out at two drifting stations: at 59°N, 20°W during a tracer release experiment, and at 37°N, 19°W at a conventional drifting station. Mesozooplankton biomass at 59°N did not vary between day and night (6.7 and 6.4 mg C m^-3 for night and day samples, respectively). At 37°N, carbon biomass was much lower and in addition a diel cycle was observed (2.2 and 1.4 mg C m^-3 for night and day samples, respectively). No one size fraction was the major contributor to biomass at either station. At 59°N, grazing was dominated by the small size fraction for both day and night samples, and at 37°N grazing was dominated by this size fraction during the day only. At 59°N, mean ingestion (57.1 and 91.1 mg C m^-2 day^-1 for day and night samples, respectively) was much higher than at 37°N (5.3 and 1.7 mg C m^-2 day^-1 for day and night samples, respectively). Consumption of the standing stock of phytoplankton (based on total chlorophyll a concentrations) was higher at the southern station where 8.9% and 2.6% day^-1 of the standing stock was removed by night and day, respectively. Reverse diel consumption was observed at the northern station, where 1.5% (night) and 2.1% day^-1 (day) of the standing stock was removed. Comparisons are made between the two time series, and the results are set in the context of a decade of investigations into the role of mesozooplankton in this region of the North Atlantic.