0 1996, by the American Society of Limnology and Oceanography, Inc. Effects of ultraviolet radiation on periphyton in an alpine lake

Many alpine lakes have low concentrations of ultraviolet (UV) radiation-absorbing dissolved organic matter, yet receive higher UV radiation flux than low-elevation lakes. We tested whether ambient UV radiation affected periphyton development in a small alpine lake in Banff National Park, Canada. After 30 d, total periphytic biomass and chlorophyll accrual on artificial substrates were enhanced- 100 % (t-test, P < 0.05) by removal of UV radiation (~400 nm). The inhibitory effect of UV radiation was species-specific, significantly suppressing (Bonferroni-adjusted t-test, P < 0.05) Achnanthes minutissima Kiitzing but not other colonists. Although taxa apparently differed in their sensitivity to UV radiation, periphyton commu-nities remained dominated by early successional taxa, especially A. minutissima (75 % of total biovolume). In contrast, natural epilithic communities were dominated by cyanobacteria (Anabaena subcylindrica Borge, Calothrix sp.). These findings suggest hat ambient UV radiation at alpine sites can suppress periphyton development by inhibiting littoral diatom production during the short ice-free season (July-September). Future climatic change may affect the ultraviolet (UV) radiation environment in aquatic ecosystems. For ex-ample, a warmer and drier climate in central North Amer-ica has reduced inputs and increased in-lake removal of dissolved organic matter (DOM), the primary attenuator of UV radiation in lakes (Schindler et al. 1990, 1992; Jones 1992). Low concentrations of DOM allow UV ra-diation to penetrate to greater depths (Scully and Lean 1994; Kirk 1994) and may combine with absolute in-creases in UV-B (280-320 nm) radiation flux (Blumthaler and Ambach 1990) to adversely affect aquatic organisms (e.g. Williamson et al. 1994). Alpine lakes may offer insights into the long-term ef-fects of elevated UV radiation on aquatic organisms. UV-B and total irradiance increase by-20 % and- 10 % pe