ZOOPLANKTON ACTIVITY FUELING THE MICROBIAL LOOP - DIFFERENTIAL GROWTH-RESPONSE OF BACTERIA FROM OLIGOTROPHIC AND EUTROPHIC WATERS

The impact of copepods on the pool of organic nutrients (monomeric carbohydrates and dissolved free amino acids) and on bacterial growth during phytoplankton bloom conditions was assessed in laboratory incubation experiments. Different batch cultures were performed with water and bacterial assemblages from oligotrophic and eutrophic sampling sites amended with either phytoplankton, zooplankton, or both. A significant increase in monomeric carbohydrate concentrations could be observed only in incubations supplemented with both types of plankton organisms. The increase in bacterial numbers was also most pronounced in that treatment. Compared to eutrophic bacterioplankton, the bacterial assemblage from oligotrophic waters exhibited elevated growth responses to the presence of eucaryotic plankton with lower generation times in all treatments. Zooplankton activity may play a key role in providing organic substrate for bacterial growth.