We examined the effects of ultraviolet radiation (UVR) (280–400 nm) on epilithic algal metabolism, biomass, and taxonomy from early June to mid August 1999 (66 d) in Lake 224, an oligotrophic lake in the Experimental Lakes Area, northwestern Ontario, Canada. Epilithon colonized bare clay tiles placed under UVR-transparent (UVR ) and UVR-shielding (UVR−) acrylic filters, at depths of 0.5, 0.8, and 1.4 m. Filamentous chlorophytes and cyanophytes were the dominant algal taxa at 0.5 and 1.4 m depths, respectively, on day 28. Biovolumes did not differ between UVR treatments for any major algal class. We measured epilithic metabolism on days 28, 45, and 66 after the tiles were deployed. Photosynthesis was not affected by UVR on any date. On day 28, dark respiration was 33 to 49% greater in UVR than in UVR− treatments at all depths. On day 45, respiration was 11% greater in UVR than in UVR− treatments only at 0.5 m, and by day 66, respiration did not differ between treatments at any depth. Thus, the effects of UVR on respiration diminished with depth and over time and were strongest when epilithic biofilms were least developed. UVR probably has little impact on naturally colonized, fully developed epilithon deeper than 0.5 m in Lake 224, i.e., UVR affects <6% of the littoral zone. Comparison of our findings with those of an earlier study in Lake 224 suggests that the intensity and specific nature of UVR effects depend on the taxonomic composition of the benthic algal assemblage.
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