The microflagellate‐picoplankton food linkage in the water column of Lake Biwa

Seasonal variations of abundance and biomass of heterotrophic microflagellates (HMF), bacteria, and unicellular chroococcoid cyanobacteria (UCB) at a pelagic site (water depth, ∼72 m) in the north basin of Lake Biwa were measured during May to November 1984. The cell densities of HMF, bacteria, and UCB were 10 2 –10 3 , 10 6 , and 10 2 –10 5 cells ml −1 , and the biomass estimates were <1–7, 10–70, and <1–20 µ g C liter −1 . Active phagocytosis of UCB by HMF was indicated from microscopic observation of HMF food vacuoles; when UCB density was high, up to 52% of HMF cells contained one or more UCB cells. Incubation of size‐fractionated epilimnetic waters indicated that bacterial net production rates (5–25 µ g C liter −1 d −1 ) were usually almost equal to rates of consumption of bacteria by HMF populations. Estimates of gross growth efficiencies of HMF fed on natural picoplankton were 11–53% in terms of carbon. HMF may be a significant link transferring picoplankton energy to macrozooplankters which cannot collect picoplankton. Lack of information on the fate of HMF makes it difficult, however, to directly address the “sink vs. link” argument about the microbial loop in Lake Biwa.