Effects of ambient and enhanced UV-B radiation on periphyton in a mountain stream

We examined the influence of UVR on an open and a shaded section of a mountain stream (2,850 m a.s.l.) by comparing periphyton and grazers on clay substrates under four light regimes (UVR blocked and UV-B enhanced treatments and two controls). Neither ambient levels of UVR nor enhanced UV-B appeared to have an adverse impact on periphyton in the shaded section. However, enhanced UV-B in the open section appeared to cause a decline in periphyton biomass and diversity. Diatoms seemed particularly susceptible as twenty-eight species were absent from the UV-B enhanced treatment but were present 2 m downstream in the control. Tube-dwelling diatoms were only present at UV-B enhanced treatments (shade and open). Hydrurus foetidus (cells embedded in mucopolysaccharide) was relatively rare at all sites except the UV-B enhanced treatment in the open where it was the dominant alga. Periphyton may use mucopolysaccharide to diminish harmful UV-B effects. Grazer densities in the UV-B enhanced sites were similar to controls and treatments blocking UVR. Our results suggest that potential UV-B increases due to thinning of the ozone layer could have direct adverse effects on the periphyton assemblage in open sections of high elevation streams.