The effects of ultraviolet radiation and nutrient additions on periphyton biomass and composition in a sub-alpine lake (Castle Lake, USA)

Rising levels of ultraviolet radiation (UVR) striking the Earth's surface h ave led to numerous studies assessing its inhibitory effects on phytoplankt on and periphyton in aquatic systems. Mineral nutrients such as nitrogen (N j and phosphorus (P) have been shown to increase aspects of algal metabolis m and compensate fur UVR inhibition. An in situ substratum enrichment techn ique and UV shielding was used to assess the effects of nutrient additions on pcriphyton exposed to different levels of UVR in Castle Lake. California during July-August, 1997. UV shielding had no effect on total periphyton b iomass, but caused shifts in species composition. The dominant periphyton s pecies, Anabaena circinalis R-AB., demonstrated sensitivity to ambient leve ls of UV radiation possibly due to UV inhibition of N-2-fixation. Total dia tom biovolume decreased when shielded from UVR. Phosphorus additions contin ually elicited an increase in periphyton biovolume at all levels of analysi s. These results suggest an interaction between nutrient status/availabilit y and UV sensitivity.