Trophic Interactions Between Heterotrophic Nanoflagellates and Bacterioplankton in Manipulated Seawater Enclosures

Biomasses of bacterioplankton and heterotrophic flagellates and bacterial net production rates were measured intensively during three periods of 16-25 d in estuarine experimental enclosures manipulated by additions of inorganic nutrients, fish, and mussels. Tightly coupled oscillations in biomasses of bacteria and flagellates were found, particularly during summer. The amplitude of the oscillations increased in response to artificial eutrophication with inorganic nutrients and decreased in response to predation control from added mussels. Losses of bacterioplankton by grazing were assessed indirectly with carbon balances for each enclosure and directly with procaryotic inhibitors and with3H-labeled bacteria. Estimates obtained by these three independent approaches showed clearance rates averaging 1.5-2.5 nl h-1 flagellate-1. Comparison of grazing rates to increases in flagellate biomass revealed minimum estimates of flagellate carbon yield averaging 33%. High minimum yields were found in some enclosures during summer. Carbon sources other than bacteria may have been used by heterotrophic flagellates in these situations.