Influence of Ultraviolet Radiation on the Colonization Dynamics of a Rocky Mountain Stream Benthic Community

Recent measurements have shown that levels of ultraviolet radiation (UVR) reaching the earth's surface at temperate latitudes have increased significantly over the past decade; however, little is known regarding the ecological effects of UVR on temperate aquatic ecosystems. To examine the effects of UVR on a Rocky Mountain stream benthic community (algae and invertebrates), we used clear plastic filters to exclude UVR from the stream bottom. The experimental design consisted of a repeated measures, randomized complete block with 3 replicates per treatment (no UVB; no UVB+UVA; unfiltered control (full sunlight); filtered control). Algal biomass (chlorophyll a) and invertebrate assemblage composition on artificial substrates were sampled 4 times over a 30-d period. On day 30, chlorophyll a levels (μg/cm2) were 50% lower on substrates exposed to full sunlight (UVB [280-320 nm] + UVA [320-400 nm] + PAR [400-500 nm]) than on substrates that were exposed to PAR (photosynthetically active radiation) only. Similarly, total invertebrate abundance (p < 0.01), and abundance of ephemeropterans (p < 0.05) and trichopterans (p < 0.001) were significantly lower on substrates exposed to full sunlight than on UVR-excluded treatments. For example, the abundance of heptageniid mayflies on day 30 was 10× higher on UVR-excluded substrates than on UVR-exposed substrates. These changes in benthic community structure were not obvious until the end of the experiment when water depth and UV-254 absorbance, a measure of the aromatic fraction of dissolved organic carbon (DOC), were lower than at the beginning of the study. Thus, the negative effects of UV radiation on Rocky Mountain stream organisms (algae and invertebrates) may vary temporally and may be more pronounced during low-water, summer conditions when DOC levels are typically low and UVR levels reaching the earth's surface are relatively high.