No effects of ambient UV radiation detected in periphyton and grazers

The vulnerability of periphyton and grazers to current and future levels of ultraviolet radiation (UV) has important ramifications for energy flow in shallow waters. We reduced ambient UV levels in a Tennessee stream with UVopaque Plexiglas suspended over the stream and compared periphyton under these Plexiglas filters to periphyton under UV-transparent polyvinylidene film. The experiments were performed three times, from autumn 1993 to summer 1994. The last experiment included a treatment with reduced snail densities, so potential interactions between grazing and UV could be identified. Periphyton biomass and photosynthesis were not significantly affected by reduced UV in any of the three experiments, indicating ambient UV effects were minor, at most. Grazing by snails (Elimia clavaeformis) tightly controlled periphyton biomass and primary production in all three experiments. Snail densities were not significantly affected by ambient UV, eliminating the possibility of a solar cascade of indirect effects. Snails may be less vulnerable to UV than are soft-bodied grazers such as chironomids, and periphyton in southern U.S. streams may be adapted to naturally high levels of UV. The results of this study clearly imply that previous reports of UV impacts on periphyton and grazers cannot be generalized. Predicted increases in middle ultraviolet radiation (UVB) resulting from stratospheric ozone depletion at both high and middle latitudes have focused much attention on the potential ecological effects of UV (Madronich 1992; Kerr and McElroy 1993). In clear waters lacking appreciable levels of UV-absorbing dissolved organic matter, current UV fluxes penetrate to significant depths in the photic zone. Deleterious effects of these fluxes in open-water ecosystems include suppressed bacterioplankton activity (Herndl et al. 1993), inhibition of phytoplankton photosynthesis (Helbling et al. 1992), and increased zooplankton mortality (Williamson et al. 1994). Many of the organisms affected or potentially affected by UV cannot avoid it because they are either phototrophic or need to be in close proximity to phototrophs (e.g. grazers). Unless these organisms have protective mechanisms to mitigate potential ultraviolet effects, their distribution and abundance in aquatic ecosystems should be influenced by UV. Periphyton and grazing invertebrates in shallow streams and clear lakes are exposed to relatively high levels of both photosynthetically active radiation (PAR) and UV. Unlike phytoplankton and zooplankton, periphyton and benthic grazers exist at fixed depth, unable to reduce their UV exposure through passive or active movements into deeper waters. Recent experiments in artificial stream channels in British Columbia suggest that periphyton and invertebrate grazers in shallow waters are vulnerable to current levels of UV (Bothwell et al. 1993, 1994). In these experiments, development of periphyton communities was inhibited by the direct effects of near ultraviolet (UVA), but these effects were reversed in later developmental stages by the positive, indirect effects of UVB. Positive UVB effects were mediated by the dominant grazers, chironomid larvae. Chironomid larvae in the channels were particularly sensitive to UVB and, in its absence, became abundant enough to control algal development. Increased periphyton biomass therefore occurred in the presence of UV because of a “solar cascade” of indirect effects (Williamson 1995). The variability of UV effects on periphyton and grazers in streams and lakes is unknown. Fluxes of UV to the water’s surface are strongly affected by latitude, altitude, cloud cover, sun angle, stratospheric conditions, and shade cast by terrestrial objects (e.g. riparian vegetation). Penetration of UV reaching the water’s surface to the benthos depends on albedo at the water surface, water turbidity, depth, and concentrations of UV-absorbing dissolved organic matter. Direct effects of UV on periphyton are undoubtedly influenced by community type and architecture as well: self-shading in thick periphyton matrices should lessen UV exposure to underlying cells. The cascade of indirect effects will depend on the strength of grazing effects and the susceptibility of grazers to UV, UV sensitivity is likely to vary considerably among grazer taxa. Physiological or morphological adaptations to UV by periphyton and grazers also could affect potential impacts. In this study, we investigated UV effects on periphyton and grazers in White Oak Creek, a small, clear-water stream in Tennessee. This stream contains high densities (up to 1,000 rn-‘) of the pleurocerid snail Elimia clavaeformis, which grazes periphyton to low levels of biomass and is food-limited (Hill 1992; Hill et al. 1992). The tight interaction between periphyton and snails maximizes the potential propagation of indirect effects (Hill et al. 1995). Manipulations of in situ UV were performed in upper White Oak Creek, a second-order stream in the Oak Ridge National Environmental Research Park (35”55’N, 84’19’W). Water chemistry in White Oak Creek is typical of undisturbed streams draining limestone and dolomite geologic formations in eastern Tennessee: alkalinity is moderately high (-3 mEq liter’), pH is -8, and nutrient concentrations are moderately low (dissolved phosphate ~6 ,cLg P liter I and dissolved nitrate <90 pg N liter’). Water temperature ranges between 17” and 10°C in summer to late autumn. The section of stream that was used for the in situ manipulations was a riffle-run section with maximum depth of 30 cm. The experimental section lacked streamside trees, allowing full sunlight to illuminate the streambed for at least 6 h on clear days. Plexiglas MC plates were used to screen UV from portions of the streambed. Plexiglas MC transmits 90% of PAR but absorbs UVB and most UVA below 375 nm (50% transmittance at 375 nm). Four plates (40 X 40 X 0.6 cm) were suspended 5-10 cm over the water surface in an upstreamdownstream direction. Each plate was paired with a 40 X 40-cm piece of polyvinylidene film (Dow Saran Wrap), sus-