GRAZING BY MESOZOOPLANKTON AND METAZOAN MICROPLANKTON ON NANOPHYTOPLANKTON IN A MESOCOSM EXPERIMENT IN THE NORTHERN BALTIC

Grazing by two size classes of metazooplankton, mesozooplankton(Meso; >140 mu m in size) and metazoan microplankton (Micro; 100-140 mu m in size), was studied in a mesocosm experiment carried out off the SW coa st of Finland, northern Baltic Sea, in late summer. During the 3 week study, the mesocosm was manipulated periodically by the addition of nu trient (ammonium and phosphate) and fish predators (stickleback fry). During the experimental period, the mesocosm was sampled five times to measure metazooplankton grazing, using 5 mu m pre-filtered and C-14-l abelled natural nanoplankton as food. In spite of the presence of fish , Meso biomass increased throughout the experimental period. The bioma ss of Meso was composed mostly of different copepodite stages of Euryt emora affinis and the cladoceran Bosmina longispina maritima, and that of Micro by biomass of copepod nauplii NIII-NVI. Owing to its larger biomass, Meso could exert a greater grazing pressure on the nanophytop lankton than could Micro. The biomass specific clearance rate (BSCR) w as generally the same for both groups, occasionally higher in Micro. T he BSCR increased during the first half of the study period, after whi ch the clearance rates were depressed, coinciding with a sudden decrea se in water temperature. The daily ingestion rate on nanophytoplankton (% of biomass as carbon) varied between 3 and 96% for Meso, and betwe en 4 and 130% for Micro. When integrated over the study period, grazin g on nanophytoplankton was estimated to provide about 60% of the carbo n requirements of metazooplankton, respectively; thus, protists were p robably important food. The overall gross growth efficiency for the wh ole metazooplankton community was estimated to be 32%. Meso and Micro grazing was estimated to account for 8 and 2% of primary production, i ndicating that they were not able to control phytoplankton primary pro duction enriched by nutrient additions.