Light color and nutrients interact to determine freshwater algal community diversity and composition

Lakes experience a wide range of variation in resource availability; variation of essential resources such as light and nutrients can cause changes in algae community structure. Changes in light color and nutrient availability could lead to shifts in algal community diversity and composition, including shifting communities to be dominated by cyanobacteria. Due to eutrophication and brownification, lakes are experiencing increases in nutrient concentrations and shifts from blue towards red in the color of light available to algae for photosynthesis. We investigated whether differences in light color and nutrients affect algal community composition and diversity, and whether differences in light color alter the impact that nutrient levels have on algal communities. We used experimental microcosms with a fully factorial experimental design, crossing four light colors with two nutrient levels. We assessed community composition by enumerating algal taxa via light microscopy. We found that light color and the interaction between light color and nutrient availability led to large differences in community diversity, with blue light leading to the greatest diversity and broad light the lowest. Light color, nutrients, and the interaction between the two were all significant drivers of differences in community composition. Overall, we found that light color and nutrient availability interact to affect algal community diversity, composition, and cyanobacteria density. Consequences of light color are infrequently studied in aquatic ecology, but our results show that light color may need to be considered more broadly. Furthermore, our results suggest that concurrent eutrophication and brownification may yield environmental conditions favorable to cyanobacteria, including taxa that can form harmful algal blooms.