Trophic roles of heterotrophic nanoflagellates and ciliates among planktonic organisms in a hypereutrophic pond

We followed seasonal changes in abundance of bacteria, heterotrophic nanoflagellates (HNF), ciliates and crustaceans, and consumption of bacteria by the protozoans, to investigate trophic interactions among these organisms in a hypereutrophic pond from March to October 1997. Densities of HNF and ciliates were high and attained a maximum of 1.4 x 10 super(5) and 3500 cells ml super(-1), respectively. However, the high densities decreased as chlorophyll concentration increased. Since the predominant phytoplankton species was Microcystis aeruginosa (Cyanophyceae), toxin produced by the alga possibly affected growth of protozoans. Not only HNF but also ciliates were important consumers of bacteria, and consumption of bacteria by ciliates varied at the same level as that of HNF from August to October. Bacterial turnover rate (% d super(-1)) due to consumption by the protozoa ranged between 5.6 and 112 (mean 25), and there were significant relationships between densities of bacteria and specific ingestion rates (bacteria protozoan cell super(-1) h super(-1)) of the protozoans. These results suggest that the food linkage between bacteria and the protozoans is substantial in the pond. We could not find any significant trophic relationships between HNF and ciliates, and between the protozoans and crustaceans.