Irgarol is a highly effective biocide used in antifouling coatings to prevent the growth of periphyton. Environmental concentrations of Irgarol in marine and freshwater have often exceeded the effect concentrations of autotrophic organisms tested in the laboratory and give reason for concern that natural periphyton communities may be endangered. A 150 days freshwater mesocosm study in 8 indoor ponds was conducted at nominal concentrations between 0.04 and 5 microgL(-1) in order to investigate the effects of Irgarol on periphyton and plankton. The results demonstrated that periphyton communities were strongly affected after single applications of 1 and 5 microgL(-1) Irgarol. For these concentrations no recovery was observed in the course of the study. For chlorophytes, the EC(50) (nominal, 135 days) was 0.34 microgL(-1). Phytoplankton also decreased in abundance directly after Irgarol application but recovered after a few weeks, as Irgarol concentrations rapidly decreased in the water body and nutrient levels increased due to lack of competition with periphyton and macrophytes. Zooplankton was indirectly affected by Irgarol. Principle response curve analysis revealed a species shift from macrophyte associated zooplankton species to free-swimming species. For species of cyclopoid copepods and ostracods the EC(50) was, respectively, 0.09 and 0.11 microgL(-1). The study simulated a best-case scenario since the mesocosms were dosed only once. Under field conditions, however, permanent exposure of organisms to Irgarol is more likely due to permanent leaching from painted ship hulls. Therefore, the effects presented in this study most likely underestimate the effects under natural conditions.