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 wa s measured in the mixed layer of the high-latitude North Atlantic Ocea n during May and August 1991. No significant grazing by microzooplankt on occurred in May during a spring bloom dominated by colonial Phaeocy stis pouchettii and Nitzschia spp. As the bloom declined, the size dis tribution of chlorophyll shifted from dominance by the >20 mu m chloro phyll 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 p roduction following the bloom. In August, when the phytoplankton was d ominated by the <20 mu m chlorophyll fraction, microzooplankton consum ed 37-53% (mean = 41%+/-11% s.d.) of potential daily chlorophyll produ ction. Averaged over all experiments, microzooplankton grazing account ed for 81% bf daily chlorophyll production. The grazing impact of Cala nus finmarchicus-stages C4 and C5, which dominated mesozooplankton bio mass in the upper euphotic zone. in both spring and late summer, was c oncentrated on chlorophyll >20 mu m in both seasons; C. finmarchicus d id not consume significant amounts of chlorophyll <20 mu m in either s eason. Compared to the microzooplankton, copepods did not consume a si gnificant fraction of total chlorophyll in either season, accounting f or only similar to 1% of daily chlorophyll production.