Plankton communities consist of complex microbial consortia which change over time with. These fluctuations can only be partially explained by limiting resources. Biotic factors such as herbivores and pathogens also contribute to the control of algal blooms. Here we address the effect of algicidal bacteria on a natural plankton community in an indoor enclosure experiment. The algicidal bacteria, introduced into plankton taken directly from the North Sea during a diatom bloom, caused the rapid decline of the bloom forming Chaetoceros socialis within only one day. The haptophyte Phaeocystis, in contrast, is resistant against the lytic bacteria and could benefit from the removal of the competitor indicated by an onset of a bloom in the treated enclosures. This cascading effect caused by the bacterial pathogen accelerated the succession of Phaeocystis which bloomed with a mere delay of several weeks in the in situ waters at Helgoland Roads, North Sea. The algicidal bacteria can thus modulate the community within the limits of the framework of the abiotic and biotic limitations of the local environment. Implications of our findings for plankton ecosystem functioning are discussed. [ABSTRACT FROM AUTHOR]