Ecosystem-level regulation of boreal lake phytoplankton by ultraviolet radiationThis paper is part of the series “Forty Years of Aquatic Research at the Experimental Lakes Area”.

Abstract

Unique and interactive effects of ultraviolet radiation (UVR), temperature, and water column mixing on phytoplankton abundance and community composition were quantified using regression and multivariate analysis for lakes of differing transparencies and UVR exposure regimes located at the Experimental Lakes Area in Canada. Abundance of planktonic diatoms and chrysophytes (as fucoxanthin) and total algae (as chlorophylls) were negatively correlated with UVB exposure (R2 = 0.57 and R2 = 0.64, respectively) in slightly stained lakes. In contrast, concentrations of both filamentous Cyanobacteria and dinoflagellates were positively correlated with UVB levels in eutrophic and humic lakes (dissolved organic carbon >9 mg·L–1) (R2 = 0.23 and R2 = 0.27), whereas all algal groups were uncorrelated with UVB in oligotrophic ecosystems. Although univariate analyses suggested that surface water temperature explained more variation in algal abundance than UVB, principal components analyses revealed that the two factors often covaried and could not be statistically disentangled. Instead, it appears that strong UVB effects on lake algal communities occur commonly when optical and thermal properties interact to maximize exposure to high irradiances. Unexpectedly, UVR could either suppress or stimulate surface algal growth depending on the precise combination of lake parameters and algal community composition.