Production, consumption and nutrient cycling in a laboratory mesocosm

A mesocosm experiment was conducted in which estuarine plankton population dynamics and rates of production, consumption, and nitrogen uptake and release were compared for 13 d in 400 l vats with and without additions of the diatom Thalassiosira weissflogii and the copepod Acartia tonsa. In the vats with copepods, both phytoplankton biomass and production were reduced by grazing whereas in the vats without copepods, nutrient limitation reduced both phytoplankton production and standing stock. Bacteria dominated both estimated heterotrophic carbon biomass and production early in the experiment. However toward the end of the experiment, the biomass of zooplankton was often greater than the combined carbon pools of phytoplankton, bacteria and heterotrophic flagellates. Highest release rates of ammonium and dissolved free amino acid nitrogen occurred in the vats with copepods. Highest bacterial growth rates also occurred in the presence of copepods suggesting that grazing activity and copepod excretion stimulated the growth of bacteria. An inverse relation between the biomass of copepods and flagellates suggests that the copepods consumed significant quantities of heterotrophic nanoflagellates. Estimated growth rates of flagellates were highest in the vats with copepods. Our results demonstrate that the presence of copepods can result in more efficient nutrient recycling, higher primary production/chlorophyll and greater production by bacteria and heterotrophic nanoflagellates.