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 degrees N, 20 degrees W during a tracer release experiment, and at 37 degrees N, 19 degrees W at a conventional drifting station. Mesozooplankton biomass at 59 degrees N did nor vary between day and night (6.7 and 6.4 mg C m(-3) for night and day samples, respectively). At 37 degrees 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 degrees N, grazing was dominated by the small size fraction for both day and night samples, and at 37 degrees N grazing was dominated by this size fraction during the day only. At 59 degrees 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 degrees 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.