Results/Conclusions The major phytoplankton groups differed significantly in their responses to an environmental temperature gradient. Relative abundance of diatoms decreased with increasing temperatures, while that of greens and cyanobacteria increased. Cyanobacteria experienced the greatest positive response of the three groups. This pattern mirrors seasonal succession patterns within individual temperate lakes, and was explained by differences in response to temperature between groups. Diatoms were found to have the lowest optimum temperatures & cyanobacteria the highest, with greens being intermediate. Anthropogenic climate change is predicted to lead to temperature changes large enough to alter current spatial and temporal patterns. Cyanobacteria and greens are likely to be favored over the diatoms, which play an important role in carbon sequestration.