Effects of solar ultraviolet radiation on stream benthic communities: An intersite comparison

The effects of solar ultraviolet radiation (UVR), both mid-ultraviolet (UVB; 280-320 nm) and near-ultraviolet (UVA; 320-400 nm), on benthic algal and invertebrate communities were compared in three reaches of a British Columbia coastal stream that differed in the degree of shading by riparian canopy (a full canopy, a partial canopy, and no canopy). At each of the three sites benthic communities were exposed to three different radiation treatments: photosynthetically active radiation alone (PAR; 400-700 nm), PAR+UVA, and PAR+UVA+UVB. Relative to the site with no canopy, UVR was 88% and 66% lower, and PAR was 83% and 49% lower at sites with full and partial canopy, respectively. Late summer increases in UVR to the streambed caused by declines in water level and dissolved organic carbon (DOC) were also lower at sites with high canopy. Sites with less canopy shading had greater algal accrual, decreased biomass of total invertebrates, mayflies, and stoneflies, and reduced invertebrate community diversity compared to the heavily shaded reach of the stream. UVR produced taxon-specific community responses that varied across sites and increased with increasing UVR as summer progressed. At the full canopy site UVR had no impact, and the final (day 91) biomass and diversity of invertebrates was highest, and algal biomass lowest. Higher UVA radiation under reduced canopies inhibited algal accrual but had little effect on algal-community composition. The biomass of several invertebrate taxa (e.g., Dicosmoecus spp., Limnephilidae) and community diversity were reduced by both UVA and UVB. Less sensitive taxa (e.g., Paraleptophlebia spp., Paraleptophlebidae) were inhibited only by the highest UVB levels in late summer when water transparency to UVR was greatest. Inhibition of grazers by UVR appeared to indirectly increase algal accrual, particularly at the partial canopy site. Our results indicate that riparian shading may moderate UVR effects on benthic communities, mainly through impacts on invertebrates with indirect effects on algae. By reducing UVR exposure of streambeds, riparian canopies may be important for ameliorating UVR effects on shallow lotic systems, especially during late-summer, low-flow periods when DOC concentrations are reduced.