Regulation of zooplankton biomass and production in a temperate, coastal ecosystem: I. Copepods

Seasonal distributions of biomass, egg production, and production rates of the pelagic copepod community were compared to the variation in abundance and size and taxonomic composition of the microplankton in the southern Kattegat, Denmark. Copepod production was episodic and occurred in bursts associated with three net phytoplankton blooms. The seasonal distribution of copepod biomass, on the other hand, was unimodal with peak concentrations in June-July. The vernal increase in copepod biomass coincided with the spring production burst, but copepod biomass was otherwise unrelated to the seasonal variation in copepod production. Specifically, the continuous decline in copepod biomass between early August and late October overlapped the most significant production event during the year (August-September) and must, therefore, have been caused by elevated mortality during this period. Weight-specific egg production rates in several species of copepods were similar and varied in concert. Egg production rates varied significantly with the concentrations of chlorophyll (> 11 mum) and total microplankton biomass but were unrelated or only weakly related to other components of the microplankton (dinoflagellates, nanoflagellates, ciliates, and copepod nauplii). Significant egg production occurred only when concentrations of diatoms and other large phytoplankters were high and ciliates were too dilute to contribute significantly to copepod diet during these production bursts. However, microzooplankton concentrations were sufficient to account for observed egg production rates when these were low during major parts of the year. Although microzooplankton may ensure a nutritionally complete diet throughout the year, integrated annual copepod production appears to depend primarily on episodic net phytoplankton blooms.