Long-term phytoplankton changes in an artificially divided, top-down manipulated humic lake

Lake Grosse Fuchskuhle (Brandenburg, Germany) is a naturally acidic bog lak e that was artificially divided into four basins by large plastic curtains for biomanipulation experiments in 1990. Different numbers of perch were ad ded to each compartment beginning in the spring of 1993. The species compos ition and abundance of phytoplankton, pH, nutrient concentrations, dissolve d organic carbon (DOC) and chlorophyll a content were analyzed at regular i ntervals during 1991 and 1998. The division of the lake resulted in diverge nt developments in the physical and chemical environment of the compartment s. This study compared the phytoplankton assemblages of the Northeast- (NE) and Southwest- (SW) basins which differed strongly in chemistry during the investigation period. Divergent developments in phytoplankton species comp osition in both basins can be explained by changes in physical and chemical conditions (bottom-up effects). Increased pH values and DOC concentrations probably favoured mass developments of the dinoflagellate Gymnodinium uber rimum since 1993, while increased nutrients (dissolved inorganic carbon, to tal nitrogen and especially total phosphorus) as well as further changes in pH and DOC led to the dominance of the raphidophyte Gonyostomum semen in 1 998. This bloom was characterized by extreme biomasses of up to 143 mg l(-1 ) wet weight, corresponding with high chlorophyll a concentrations of up to 413 mug l(-1) at the same time. In contrast, no significant relationship b etween experimental manipulations by piscivorous fish stocking (top-down ef fects) and phytoplankton biomass were observed.