Effect of copepods on estuarine microbial plankton in short-term microcosms

We examined the effects of copepod feeding on estuarine microbial plankton, including bacteria, cyanobacteria, heterotrophic and autotrophic nanoflagellates, microzooplankton and larger phytoplankton cells, to elucidate any short-term control or release phenomena. As an example of such potential effects, it was hypothesized that through omnivory Acartia tonsa would reduce both competitors (large phytoplankton) and predators (microzooplankton) of small autotrophic flagellates, allowing the latter to proliferate. Copepod addition and removal treatments were conducted in 241 containers in triplicate. As expected, the copepods significantly decreased both microzooplankton and large phytoplankton populations, and blooms of ciliates and diatoms (the dominant phytoplankton group) were apparent in the copepod removal treatment. Effects were qualitative (species composition changes) as well as quantitative. However, beyond this immediate impact, community level cascades in the microbial food web were not found, likely due to consumer recycling of resources and trophic level heterogeneity. Bacterioplankton responded positively to the addition of copepods, but negatively to the bloom of diatoms in the copepod removal treatment. These results suggest that the effects of copepods on the structure and function of estuarine microbial plankton communities may be more complex than simple linear effects would predict.