SUMMERTIME HERBIVORY OF COASTAL MESOZOOPLANKTON AND METAZOAN MICROPLANKTON IN THE NORTHERN BALTIC

Size-specific grazing by the coastal zooplankton community in the nort hern Baltic was studied during the summer. Experiments on grazing by m esozooplankton and metazoan microplankton, fractionated by filtration through 140 and 40 mu m mesh, were carried out in the laboratory with two C-14-labelled algae cultures, Brachiomonas submarina (size 10 to 1 5 mu m) and Pavlova lutheri (size 4 to 6 mu m). Zooplankton grazing on 2 size groups (>10 mu m and 2 to 10 mu m) of naturally occurring phyt oplankton was also estimated. Mesozooplankton was composed of copepods Acartia spp, and Eurytemora affinis, the cladoceran Bosmina longispin a maritima, rotifers Synchaeta spp, and meroplankton; metazoan micropl ankton consisted mostly of copepod nauplii and rotifers. Mesozooplankt on had greater biomass and higher community clearance rate (CCR) when feeding on B. submarina than had metazoan microplankton. For mesozoopl ankton, the CCR on B. submarina was dependent on successional changes of total biomass of copepods and cladocerans, whereas the CCR on P. lu theri was variable, probably because of less efficient grazing by cope pods. The CCR of metazoan microplankton on the test algae was variable and unpredictable, possibly because of the varying biomass specific c learance rates (BSCR) of rotifers and different naupliar stages of cop epods. The mean CCR on P. lutheri did not differ between mesozooplankt on and metazoan microplankton, because of the higher mean BSCR of the latter. The daily grazing of mesozooplankton on naturally occurring ph ytoplankton was estimated to average 9% of the daily primary productio n when rotifers, meroplankton or copepods were predominant, but 42% wh en B. longispina maritima was predominant. The integrated grazing of t he mesozooplankton and metazoan microzooplankton was estimated to acco unt for 13 and 6%, respectively, of primary production during summer.