GRAZING BY MICROZOOPLANKTON AND MESOZOOPLANKTON IN THE HIGH-LATITUDE NORTH-ATLANTIC OCEAN - SPRING VERSUS SUMMER DYNAMICS

Grazing on chlorophyll by microzooplankton (<200 mu m) and copepods was measured in the mixed layer of the high-latitude North Atlantic Ocean during May and August 1991. No significant grazing by microzooplankton occurred in May during a spring bloom dominated by colonial Phaeocystis pouchettii and Nitzschia spp. As the bloom declined, the size distribution of chlorophyll shifted from dominance by the >20 mu m chlorophyll fraction to dominance by the <20 mu m chlorophyll fraction. The impact of grazing by microzooplankton increased as the bloom declined, with microzooplankton consuming 100% of potential daily chlorophyll production following the bloom. In August, when the phytoplankton was dominated by the <20 mu m chlorophyll fraction, microzooplankton consumed 37-53% (mean = 41%+/-11% s.d.) of potential daily chlorophyll production. Averaged over all experiments, microzooplankton grazing accounted for 81% bf daily chlorophyll production. The grazing impact of Calanus finmarchicus-stages C4 and C5, which dominated mesozooplankton biomass in the upper euphotic zone. in both spring and late summer, was concentrated on chlorophyll >20 mu m in both seasons; C. finmarchicus did not consume significant amounts of chlorophyll <20 mu m in either season. Compared to the microzooplankton, copepods did not consume a significant fraction of total chlorophyll in either season, accounting for only similar to 1% of daily chlorophyll production.