Phytoplankton temperature-response traits determine biogeography and seasonal succession patterns

Background/Question/Methods   Phytoplankton exhibit spatial & temporal patterns in their distribution. Various abiotic and biotic stressors have been linked to their geographic distribution and seasonal succession patterns, including temperature, light intensity, pH, stoichiometry and grazing pressure. Understanding the drivers behind these patterns is necessary to accurately predict future changes in lake community structure & harmful algal bloom frequency. We investigated whether 1) the major phytoplanktonic groups (which include diatoms, green algae & cyanobacteria) respond differently to an environmental temperature gradient, and 2) laboratory determined temperature-response traits can explain this spatial distribution and commonly observed succession patterns in temperate lakes. The responses of phytoplankton groups to environmental temperatures was determined using data from the EPA 'National Eutrophication Survey', which measured environmental parameters and community composition in 543 lakes across the US from 1973-1974. Temperature-response traits (optimum temperature, range, maximum/minimum temperature for growth) were estimated for ~100 species of phytoplankton from a literature survey.

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.