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

Abstract

As part of mesocosm experiments to study effects of nutrients and zooplankton grazing on phytoplankton communities from the northern Adriatic Sea, we monitored metazoan zooplankton fluctuations in enclosures and performed grazing experiments in May and June of 1993. An effort was made to increase zooplankton abundances up to 10 times above normal levels in May, whereas in June, we attempted to initially exclude zooplankters > 100 μm in size. Neither manipulation mattered since, in both cases, a cohort of copepodites rapidly developed from eggs and nauplii added during initial filling of enclosures. Declines in abundances of nauplii, with concurrent increases in copepodites, during both experiments likely resulted from a combination of nauplii molting into copepodites, and possibly cannibalism of nauplii by copepodites and adults. In May copepodites were almost exclusively Acartia clausi, whereas 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 grazing had minimal impact, in that phytoplankton growth continued unabated until nutrients were exhausted.