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 a s part of the 1989 North Atlantic Bloom Experiment, an interdisciplina ry research program of the Joint Global Ocean Flux Study (JGOFS). Samp les for shipboard experimental incubations were collected from the mix ed layer of a drogued water mass (46-degrees-20'N, 17-degrees-50'W) ov er a 2 week period in May. Grazing and growth rates, measured using th e size fractionation and dilution techniques, were calculated from cha nges in chlorophylls, accessory pigments, and cell abundances. The phy toplankton community was dominated by phytoflagellates, primarily prym nesiophytes, 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. G razing by nano- and microzooplankton removed 37-100% of estimated prim ary production in samples from 10 m, and 100% of that at 30 m. An atte mpt was made to budget estimated rates of community grazing to major g roups of nano- and microzooplankton, using measured biomass and specif ic ingestion or growth rates from laboratory studies. Aplastidic micro flagellates 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 w ithin the microzooplankton community appeared to be substantial. Given the evidence of tight coupling between production and consumption wit hin the upper water column, little material appeared to be available f or direct export from the mixed layer to depth during this phase of th e spring bloom.