Eco-evolutionary differences in light utilization traits and distributions of freshwater phytoplankton

We compiled light utilization traits for 56 species of freshwater phytoplankton to analyze group differences, trait trade-offs, and allometric scaling relationships. We also used these traits to explain differences in major group distributions along the light availability gradient in 527 lakes in the continental United States. Major taxonomic groups differed significantly in their light utilization traits. Cyanobacteria had the highest initial slope of the growth-irradiance curve (alpha) and low irradiance at the onset of photoinhibition, indicating adaptation to low light environments. Green algae had the highest maximal growth rates and low alpha, indicating adaptation to higher light environments. Groups capable of mixotrophy had traits indicative of 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; alpha and cell size were conserved at the highest taxonomic level (domain), indicating a fundamental trait divergence between prokaryotic and eukaryotic phytoplankton. In line with these 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 genera mean values of the initial slopes of the growth-irradiance curves were significantly negatively correlated with the slopes of the relationships between the genus's relative abundance and light availability characterized by Secchi depth in 527 lakes. This indicates that light utilization traits can be used to explain phytoplankton distributions in nature.