Systematics and ecology of chlorophyte picoplankton in German inland waters along a nutrient gradient

The community structure and succession of autotrophic picoplankton in sever al oligotrophic to hyper trophic German freshwater ecosystems were studied with emphasis on the occurrence and characterization of chlorophyte picopla nkton. Depending on the trophic status and the time of the year, the relati on of green eukaryotic picoplankton to picocyanobacteria, the contribution of the picoplankton to the total phytoplankton biomass, and the succession and dominance of picoplankton groups changed considerably. A significant co rrelation between the picoplankton abundances, their biomass and their biom ass contribution could not be found. Although the chlorophyte picoplankton were similar with respect to their ul trastructure, phylogenetic analyses of the rbcL genes revealed that these o rganisms evolved independently within several green algal lineages. The mos t common picoplanktonic green algae in the lakes that were studied belong t o the genera Choricystis and Pseudodictyosphaerium. Considering the new mol ecular biological findings, the systematics of picoplanktonic green algae f rom freshwater and marine habitats are discussed.