UV-B irradiance gradient affects photosynthesis and pigments but not food quality of periphyton

• 1 This laboratory study examined the effect of a gradient of UV‐B radiation (280–320 nm) on photosynthesis and food quality of periphyton, the trophic base of many freshwater benthic communities. Four irradiances of UV‐B (0, 0.6, 1.2, and 2.3 W m^‐2) were delivered by UV‐B lamps (313 nm peak irradiance) over a 13‐day period in the first experiment and over a 4‐h period in the second experiment. These irradiances were roughly equivalent to 0, 1, 2, and 4 times the ambient biologically effective (DNA) midsummer, midday UV‐B irradiance in Tennessee.
• 2 Rates of photosynthesis and photosynthetic pigments were significantly reduced by irradiances greater than ambient during the 13‐day experiment, suggesting that food supply rates to grazers would be depressed by increases in current UV‐B levels. Effects on community structure were minor, but mean diatom cell size decreased at higher UV‐B irradiances.
• 3 Irradiated periphyton was fed in surplus to juvenile snails (Physella gyrina) in the first experiment as a bioassay for food quality. Snail growth was the same on all four diets, suggesting that UV‐B did not affect food quality. Nitrogen and phosphorus content of the periphyton were not affected by UV‐B, either.
• 4 Photosynthesis by low‐biomass periphyton in the second experiment was significantly depressed by irradiances above ambient after only 4 h. Photosynthesis by the high biomass periphyton was not significantly affected by UV‐B, suggesting that self‐shading reduced UV‐B effects.