Predation on marine picoplankton populations examined with an 'add-in' approach

NW Mediterranean surface water was spiked with picoplanktonprey (heterotrophic bacteria or cyanobacteria) or predators(bacterivorous microflagellates or ciliates) to investigatedifferential grazing pressure on picoplankton populations. Addinga particular prey type did not yield different growth patternsfor heterotrophic bacteria and cyanobacteria. but gave eithersimilar, positive, effects on both picoplankton types or similarnegative effects. Natural populations of both predator typesincreased with additions of cyanobacteria, but not heterotrophicbacteria. Ciliate additions gave marked decreases in cyanobacteria.While individual groups of grazers may preferentially consumecyanobacteria, selective grazing is probably not responsiblefor the maintenance of apparently stable populations of differentgroups of picoplankters during the summer.