Abstract
NW Mediterranean surface water was spiked with picoplankton prey (heterotrophic bacteria or cyanobacteria) or predators (bacterivorous microflagellates or ciliates) to investigate differential grazing pressure on picoplankton populations. Adding a particular prey type did not yield different growth patterns for heterotrophic bacteria and cyanobacteria, but save either similar, positive, effects on both picoplankton types or similar negative effects. Natural populations of both predator types increased with additions of cyanobacteria, but not heterotrophic bacteria. Ciliate additions gave marked decreases in cyanobacteria. While individual groups of grazers may preferentially consume cyanobacteria, selective grazing is probably not responsible for the maintenance of apparently stable populations of different groups of picoplankters during the summer.