Changes in phytoplankton and microzooplankton populations during grazing experiments at a Mediterranean coastal site

Abstract

Microzooplankton grazing on phytoplankton populations was investigated by means of eight dilution experiments in different seasons and trophic conditions at station Mare Chiara, a long-term study site in the Gulf of Naples, Tyrrhenian Sea. To check for changes in prey and predator populations, size-fractionated chlorophyll and HPLC analyses and microzooplankton counts were performed before and after the incubations. On average, 68% of daily phytoplankton production was consumed by the microzooplankton in spring and summer, and in winter, more than 100% of phytoplankton daily production was consumed. Nutrients were added in four of our experiments. Diatoms (as recorded by fucoxanthin concentrations) increased significantly in the nutrient-enriched experiments, and unexpectedly, also heterotrophic dinoflagellate growth was enhanced by nutrient additions. Remarkable changes in grazer abundance were observed in several experiments; a substantial difference was found in microzooplankton grazing pressure (g/k × 100) as calculated by regression of apparent chlorophyll a growth against nominal dilution factor or against average predator concentrations, 72 ± 19% and 60 ± 23% of phytoplankton daily production, respectively. However, our observations suggest that if growth and mortality of grazers in the dilution series are not considered, microzooplankton grazing impact may be either underestimated, at low phytoplankton growth rates, or overestimated in more productive conditions.