Differential responses of littoral communities to ultraviolet radiation inan alpine lake

Differential sensitivities of benthic and planktonic communities to UV radi ation may involve differences in habitat conditions (e.g., availability of physical refuge), taxonomic composition, UV-A (320-400 nm) and DNA-damaging UV-B (280-320 nm) irradiances, and potential indirect effects via food-web processes. These hypotheses were tested using 18 enclosures (corrals) with in an alpine lake. The factorial design consisted of three UV treatments ( UV, - UV-B, - UV) and two macroinvertebrate densities (ambient, 3x), High performance liquid chromatography was used to quantify changes in periphyto n and phytoplankton abundance and composition in response to UV radiation a nd macroinvertebrates over a period of 1 mo. Algal and invertebrate responses to UV radiation were habitat- and taxon-sp ecific. Epilithic standing crop was significantly suppressed by UV radiatio n, primarily due to UVB radiation inhibiting diatoms by 40%. In contrast, s tanding crop of epipelic (sediment-dwelling) organisms was significantly en hanced by UV-A radiation, which increased the abundance of cyanobacteria by 50%. UV radiation also significantly altered the taxonomic composition of both epilithon and epipelon. In comparison, picocyanobacterial phytoplankto n were unaffected by UV radiation. Zoobenthos (Gammarus lacustris, Chironom idae) and zooplankton (Hesperodiaptomus arcticus, Rotifera) did not signifi cantly alter periphyton or phytoplankton biomass or taxonomic composition. Although total zoobenthos and zooplankton biomass were unaffected by UV rad iation, UV-B significantly suppressed the final density of rotifers but not that of heavily pigmented calanoid copepods. These results show that UV radiation affects shallow-water communities in c old and unproductive systems mainly through direct effects, rather than by indirect effects mediated by food-web processes. Access to physical refuges was evidently a key factor determining habitat-specific responses to UV ra diation. UV radiation did not adversely affect motile epipelon and zoobenth os that could seek refuge in sediments, but it did suppress attached epilit hic taxa. In habitats devoid of physical refuge, UV tolerance was associate d with photoprotective pigmentation (i.e., H. arcticus), and possibly a cap acity for DNA repair (i.e., epilithic filamentous cyanobacteria and plankto nic picocyanobacteria). Our findings suggest that UV exposure can affect ab iotic regulation of littoral food webs in extreme environments, such as alp ine, polar, and anthropogenically acidified ponds and shallow lakes.