Effects of solar ultraviolet radiation on epilithic metabolism, and nutrient and community composition in a clear-water boreal lake

Stratospheric ozone depletion, climate change, and acidification will incre ase the exposure of aquatic eco systems to ultraviolet radiation (UVR; 280- 400 nm). The objective of this study was to determine the ecological effect s of ambient UVR exposure on epilithon (algal communities attached to rocky substrata) relative to an artificially reduced UVR environment. UVR exposu re was altered in the littoral zone of a boreal lake by selectively filteri ng segments of the solar spectrum with large acrylic sheets. This 130-day s tudy took place at the Experimental Lakes Area, northwestern Ontario, in 19 98. Epilithon were monitored for changes in productivity, biomass, pigment, and nutrient and taxonomic composition. The absence of UVR increased epili thic photosynthetic rates 37-46% above epilithon exposed to ambient UVR, in creased cellular carbon and nitrogen content, and consequently lowered C:P. UVR effects on algal metabolism were dependent on seasonal trends. Epilith ic respiration rates and chlorophyll a concentrations were not significantl y different among treatments. A reduction in UV induced taxonomic shifts in epilithon, with diatoms increasingly favoured under low UV conditions. Spe cies richness and diversity were not affected. Overall, the results of this study, considered together with previous research, suggest that the epilit hic community is sensitive to changes in the UVR environment of aquatic eco systems.