Effects of N : P : Si ratios and zooplankton grazing on phytoplankton communities in the northern Adriatic Sea. III. Zooplankton populations and grazing

As part of mesocosm experiments to study effects of nutrients and zooplankt on grazing on phytoplankton communities from the northern Adriatic Sea, we monitored metazoan zooplankton fluctuations in enclosures and performed gra zing experiments in May and June of 1993. An effort was made to increase zo oplankton abundances up to 10 times above normal levels in May, whereas in June, we attempted to initially exclude zooplankters >100 mu m in size. Nei ther manipulation mattered since, in both cases, a cohort of copepodites ra pidly developed from eggs and nauplii added during initial Wing of enclosur es. Declines in abundances of nauplii, with concurrent increases in copepod ites, during both experiments likely resulted from a combination of nauplii molting into copepodites, and possibly cannibalism of nauplii by copepodit es and adults, in May copepodites were almost exclusively Acartia clausi, w hereas in June, both A. clausi and particularly Oithona similis copepodites increased in enclosures after 6 to 7 d. Grazing rates by A. clausi adults and copepodites were low in both months (0.01 to 0.35 ml copepod(-1) h(-1) in May, and <0.21 ml copepod(-1) h(-1) in June). Zooplankton community graz ing had minimal impact, in that phytoplankton growth continued unabated unt il nutrients were exhausted.