DISSOLVED ORGANIC-CARBON RELEASED BY ZOOPLANKTON GRAZING ACTIVITY - AHIGH-QUALITY SUBSTRATE POOL FOR BACTERIA

Experiments were designed to investigate whether processes related to zooplankton feeding have a positive effect on bacterial growth. Bacter ial abundance and [H-3]thymidine incorporation rates were followed in grazer-free batch cultures originally containing either Scenedesmus qu adricauda or Rhodomonas lacustris as food sources, and Daphnia cuculla ta or Eudiaptomus graciloides as grazers. Compared with controls lacki ng either animals or algae, a significantly higher bacterial abundance and productivity occurred in cultures which contained both phyto- and zooplankton. The same experimental methodology was tested during the decline of a diatom spring bloom in a eutrophic, temperate lake. A sig nificant increase in bacterial biomass was observed due to the grazing activity of in situ mesozooplankters during the clear-water phase. Ou r results demonstrated that the dissolved carbon pathway from mesozoop lankton to bacteria averaged 57% (26-78%) of the algal carbon filtered from suspension.