Background/Question/Methods   Functional traits are being increasingly used to explain community organization along environmental gradients. Here we compiled light utilization traits for more than 70 species of freshwater phytoplankton to analyze functional group differences, trait trade-offs and allometric scaling relationships. We then used these traits to determine whether they can explain differences in major group distributions along the light availability gradient in 537 lakes in the continental USA.

Results/Conclusions   Major taxonomic groups differed significantly in their light utilization traits. Cyanobacteria had the highest initial slope of the growth-irradiance curve and low onset of photoinhibition, suggesting adaptation to low light environments. Green algae had high maximum growth rates, lower slopes of the growth-irradiance curves, indicating adaptation to higher light. Groups capable of mixotrophy had suites of traits suggesting poor light competitive abilities and high light requirements. Key light utilization traits scaled allometrically with cell size and exhibited trade-offs leading to contrasting ecological strategies. The initial slopes and cell size were highly conserved at the highest taxonomic level (domain) suggesting fundamental trait divergence between prokaryotic and eukaryotic phytoplankton, explained by the constraints arising from differences in cellular organization. In line with the trait differences, major groups showed different responses to light availability in natural conditions. The relative abundances of low light adapted groups declined with increasing light availability and vice versa. The mean group values of the initial slopes of the growth-irradiance curves were significantly negatively correlated with the slopes of the relationships between the group's relative abundance and light availability in 537 lakes. This suggests that light utilization traits can be used to explain phytoplankton distributions in nature.