Bottom-up and top-down controls on the microzooplankton community in the Sargasso Sea

Microzooplankton, mainly heterotrophic unicellular eukaryotes (protists), play an important role in the cycling of nutrients and carbon in the sunlit (euphotic) zone of the world’s oceans. Few studies have investigated the microzooplankton communities in oligotrophic (low-nutrient) oceans, such as the Sargasso Sea. In this study, I investigate the seasonal and interannual dynamics of the heterotrophic protists, particularly the nanoflagellate, dinoflagellate, and ciliate communities, at the Bermuda Atlantic Time Series site and surrounding areas in the Sargasso Sea. In addition, I test the hypotheses that the community is controlled though bottom-up and top-down processes. To evaluate the bottom-up hypothesis, that the protists are controlled by prey availability, I test whether the protist abundance co-varies with the abundance of potential prey groups. Predation experiments with zooplankton were conducted and analyzed to test top-down control on the protists. I found distinguishable trends in biomass of the different protist groups between years and seasons. Nanoflagellates and dinoflagellates had higher biomass during the summer (28 ± 5 mgC/m2 and 44 ± 21 mgC/m2) than during the winter (17 ± 8 mgC/m2 and 30 ± 11 mgC/m2). Ciliates displayed the opposite trend with a higher average biomass in the winter (15 ± 9 mgC/m2) than in summer (5 ± 2 mgC/m2). In testing my bottom-up hypothesis, I found weak but significant positive grazer/prey relationships that indicate that nanoflagellates graze on picophytoplankton in winter and on the pico-cyanobacterium Prochlorococcus in summer. I found evidence that ciliates graze on Synechococcus in winter. I found weak but significant negative correlation between dinoflagellates and Prochlorococcus in summer. The predation experiments testing the top-down hypothesis did not show a clear top-down control, yet other studies in the region carried out during our investigation period support predation of the protists by the zooplankton. Overall, my results suggest a combination of bottom-up and top-down controls on these heterotrophic protists, however, further investigation is necessary to reveal the detailed trophic dynamics of these communities.