Abstract:
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 105 and 3500 cells ml-l, respectively.
However, the high densities decreased as chlorophyll concentration increased. Since the predominant
phytoplankton species was M~crocystisa erugjnosa (Cyanophyceae),t oxin 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-') due to consumption by the protozoa ranged between 5.6 and 112
(mean 25), and there were sigrllficant relationships between densities of bactena and specific ingestion
rates (bacteria protozoan cell-' h-') 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.
