Grazing by mesozooplankton and metazoan microplankton on nanophytoplankton in a mesocosm experiment in the northern Baltic

Abstract

Grazing by two size classes of metazooplankton, mesozooplankton(Meso; >140 um in size) and metazoan microplankton (Micro; 100-140 urn in size), was studied in a mesocosm experiment carried out off the SW coast of Finland, northern Baltic Sea, in late summer. During the 3 week study, the mesocosm was manipulated periodically by the addition of nutrient (ammonium and phosphate) and fish predators (stickleback fry). During the experimental period, the mesocosm was sampled five times to measure metazooplankton grazing, using 5 um pre-filtered and l4C-labelled natural nanoplankton as food. In spite of the presence of fish, Meso biomass increased throughout the experi- mental period. The biomass of Meso was composed mostly of different copepodite stages of Eury- temora affinis and the dadoceran Bosmma longispina maritima, and that of Micro by biomass of copepod nauplii N1II-NVI. Owing to its larger biomass, Meso could exert a greater grazing pressure on the nanophytoplankton than could Micro. The biomass specific clearance rate (BSCR) was gener- ally the same for both groups, occasionally higher in Micro. The BSCR increased during the first half of the study period, after which the clearance rates were depressed, coinciding with a sudden decrease in water temperature. The daily ingestion rate on nanophytoplankton (% of biomass as carbon) varied between 3 and 96% for Meso, and between 4 and 130% for Micro. When integrated over the study period, grazing on nanophytoplankton was estimated to provide about 60% of the carbon require- ments of metazooplankton, respectively, thus, protists were probably important food. The overall gross growth efficiency for the whole metazooplankton community was estimated to be 32%. Meso and Micro grazing was estimated to account for 8 and 2% of primary production, indicating that they were not able to control phytoplankton primary production enriched by nutrient additions.