GRAZING, GROWTH AND MORTALITY OF MICROZOOPLANKTON DURING THE 1989 NORTH-ATLANTIC SPRING BLOOM AT 47-DEGREES-N, 18-DEGREES-W

Grazing and growth rates of nano- and microzooplankton were measured as part of the 1989 North Atlantic Bloom Experiment, an interdisciplinary research program of the Joint Global Ocean Flux Study (JGOFS). Samples for shipboard experimental incubations were collected from the mixed layer of a drogued water mass (46-degrees-20'N, 17-degrees-50'W) over a 2 week period in May. Grazing and growth rates, measured using the size fractionation and dilution techniques, were calculated from changes in chlorophylls, accessory pigments, and cell abundances. The phytoplankton community was dominated by phytoflagellates, primarily prymnesiophytes, which passed 10 mum mesh. Chlorophyll a (Chl a) increased at an average rate of 0.9 doublings day-1 when incubated at 60% I0. Grazing by nano- and microzooplankton removed 37-100% of estimated primary production in samples from 10 m, and 100% of that at 30 m. An attempt was made to budget estimated rates of community grazing to major groups of nano- and microzooplankton, using measured biomass and specific ingestion or growth rates from laboratory studies. Aplastidic microflagellates were apparently the most important herbivores. In addition to ciliates and heterotrophic dinoflagellates, various developmental stages of copepods were abundant in the <200 mum fraction. Predation within the microzooplankton community appeared to be substantial. Given the evidence of tight coupling between production and consumption within the upper water column, little material appeared to be available for direct export from the mixed layer to depth during this phase of the spring bloom.