Seasonal changes in abundance of heterotrophic nanoflagellates and their consumption of bacteria in Lake Biwa with special reference to trophic interactions with Daphnia galeata

Abstract

We followed seasonal changes in the abundance of bacteria, heterotrophic nanoflagellates (HNF), rotifers and crustaceans, and the consumption of bacteria by HNF in mesotrophic Lake Biwa from March to November 1993. From March to July, the changing pattern of HNF densities was similar to that of bacteria, and HNF specific ingestion rate (bacteria flagellate -1 h -1 ) increased. These results suggest that the food linkage between bacteria and HNF was substantial during that period. In August, both the density and specific ingestion rate of HNF dramatically decreased concomitantly with dominance of Daphnia galeata. Thus, there were two clear seasons of bacterial consumption: one dominated by HNF and the other by D. galeata. A significant negative correlation between densities of HNF and D. galeata (r = -0.773, p<0.05) was found by Spearman rank correlation analysis over the study period, suggesting additional grazing by this zooplankter on HNF. Densities of rotifers and other crustaceans did not significantly correlate with those of HNF.