MESOZOOPLANKTON STANDING STOCK DURING THE NORTH-ATLANTIC SPRING BLOOM STUDY IN 1989 AND ITS POTENTIAL GRAZING PRESSURE ON PHYTOPLANKTON - A COMPARISON BETWEEN LOW, MEDIUM AND HIGH-LATITUDES

Within the framework of the JGOFS Pilot Study in 1989 mesozooplankton (0.2-20 mm) was sampled by means of a Hydro-Bios multinet in five depth strata (0-25, 25-50, 50-100, 100-200, 200-500 m) during four Lagrangian drift experiments of 8-14 days' duration at 18, 33, 46 and 58-degrees-N, to follow the seasonal progress of the phytoplankton spring bloom development in the northeast Atlantic. Mesozooplankton standing stock, measured as dry weight and ash-free dry weight, increased by a factor of about 6 from 18 to 58-degrees-N. Day/night differences amounted to 10-20% of the average and were-with one exception at 18-degrees-N-not statistically significant. Using the data on weight-specific respiration rates measured by colleagues on the same cruise, the ingestion rates and potential community grazing of mesozooplankton on phytoplankton within the upper 100 m of the water column were calculated. During all four drift experiments, quasi-steady-state conditions were observed in phyto- and zooplankton standing stock, primary production and daily sedimentation at 100 m depth. The maximum potential grazing rate by mesozooplankton accounted for about half of the daily primary production. Since sedimentation of fresh phytoplankton was negligible, it is concluded that the grazing pressure exercised by mesozooplankton together with micro- and nanozooplankton was responsible for keeping the phytoplankton standing stock at a more or less constant level during the investigated spring bloom in the four areas. Particle flux was thus dominated by zooplankton faecal material.