Effects of copepods on natural microplankton communities: do they exert top-down control?

Abstract

Top-down effects in the pelagic realm are quite well known in freshwater ecosystems. However, our knowledge of these effects in the ocean remains scant. It is known that copepods prefer to prey on ciliates and heterotrophic dinoflagellates, and their high or low abundances can change the structure of microplankton communities. Field studies in subtropical waters have shown parallel increases of mesozooplankton and phytoplankton without a lag, suggesting a top-down effect of mesozooplankton preying upon microzooplankton and releasing primary producers from predation. In the present work, we added copepods at increasing densities to natural plankton in 24 h experiments. A decrease in aloricated ciliates abundance of nearly 50% and increases in the abundances of picoeukaryotes, Synechococcus, Prochlorococcus, diatoms, and chlorophyll a were observed. No effect of nutrient additions was observed in parallel grazing experiments. Thus, a top-down effect of copepods upon microzooplankton explains the observed changes in the abundance of the different phytoplankton groups. Copepods promote important changes down the food web, structuring the community by predation upon microzooplankton. There are biogeochemical consequences of zooplankton variability over short time scales in the ocean.