Patterns in phytoplankton taxonomic composition across temperate lakes of differing nutrient status

We describe the relationships between summer average total phosphorus (TP) and the biomass of six major phytoplankton taxonomic groups from 91 north temperate lakes. Both regression and a locally weighted smoothing (LOWESS) analyses show that all groups increase with TP but over different nutrient ranges. At nutrient extremes, i.e. very low and high levels of TP, the few taxonomic groups that dominate total biomass are generally dissimilar. In oligotrophic and eutrophic lakes, most group biomass curves show corresponding increases or decreases, but different rates of change. The curves converge at intermediate TP levels (∼10–30 µg liter−1) where there is increased equitability among algal groups. In highly eutrophic lakes, the slope of the total biomass curve decreases, as do all the relative abundances of all groups except blue-greens and diatoms. We conclude that the curvilinearity of the TP-total biomass curve is not attributable to a single taxonomic group, because all groups show some nonlinearity in relation to TP. We suggest that morphological diversity, differential herbivory, and, in particular, mixing regime may explain some of the observed patterns.