Protozoan control of bacterial abundances in freshwater

We have quantified the bacteria, heterotrophic nanoplankton (HNAN), and other microorganisms in 108 lakes, ponds, rivers, and bogs worldwide. These water bodies span the range of biological productivities in freshwater. Numbers of HNAN and bacteria are correlated over four orders of magnitude in each (3 × 10 2 to 4 × 10 6 ml −1 and 3 × 10 5 to 1 × 10 9 ml −1 , respectively) and both increase with the productivity of the water body. Most HNAN are small (2– 5 µ m), colorless, flagellated protists. They grow at about the same rate as bacteria ( µ = 0.01–0.02 h −1 ) and are capable of consuming the entire bacterial production. We suggest that bacterial abundances are regulated by substrate supply and HNAN grazing pressure. Ciliates and other grazing microzooplankton probably limit HNAN abundance, especially in the more productive water bodies. The structure and function of microbial food webs in freshwater environments may thus be similar to those suggested for marine systems.