Trophic linkage among heterotrophic nanoflagellates, ciliates and metazoan zooplankton in a hypereutrophic pond

Abstract

We examined seasonal abundance, production and loss to predation of heterotrophic nanoflagellates (HNF) and ciliates in order to clarify food linkages among HNF, ciliates and metazoan zooplankton in a hypereutrophic pond. Production and loss rates of the protists were estimated using size fractionation (<5 μm, <30 μm and total) experiments. There were coupled oscillations in seasonal abundance between HNF and ciliates, whereas we found none between the protists and metazoan zooplankton. Seasonal trends in production of HNF (-12.4 to 86.4 ng C ml-1 d-1) and predation rates on them by ciliates (-25.6 to 52.2 ng C ml-1 d-1) were similar. There were significant correlations (p < 0.05) between HNF production and predation on HNF by ciliates. These results indicate that ciliates are major consumers of HNF, but we have insufficient evidence of predation on HNF by metazoan predators to do more than suggest that food linkage between HNF and metazoan predators is not substantial. The dominant ciliate species were Cinetochilum margaritaceum, Urotricha spp., Halteria grandinella and Cyclidium glaucoma. Growth rates of Urotricha and Cyclidium populations were enhanced by removal of metazoan predators when the ciliates were dominant. We detected substantial predation pressure on Urotricha spp. during its period of dominance and found a significant correlation (p < 0.02) between production and predation of C. glaucoma. Thus, we suggest that there are active food linkages between these 2 ciliates and metazoan predators. For C. margaritaceum and H. grandinella, we did not have enough evidence to suggest predation by metazoan predators. Thus, we showed over a 1 yr period that HNF production was almost balanced by loss to predation by ciliates, and that loss processes other than predation might be important for some ciliate taxa.