INFLUENCE OF ULTRAVIOLET-RADIATION ON THE COLONIZATION DYNAMICS OF A ROCKY-MOUNTAIN STREAM BENTHIC COMMUNITY

Recent measurements have shown that levels of ultraviolet radiation (U VR) reaching the earth's surface at temperate latitudes have increased significantly over the past decade; however, little is known regardin g the ecological effects of UVR on temperate aquatic ecosystems. To ex amine the effects of WR 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 repeat ed measures, randomized complete block with 3 replicates per treatment (no UVB; no UVB+UVA; unfiltered control (full sunlight); filtered con trol). Algal biomass (chlorophyll a) and invertebrate assemblage compo sition on artificial substrates were sampled 4 times over a 30-d perio d. On day 30, chlorophyll a levels (mu/cm(2)) were 50% lower on substr ates exposed to full sunlight (UVB [280-320 nm]+UVA [320-400 nm]+PAR [ 400-500 nm]) than on substrates that were exposed to PAR (photosynthet ically active radiation) only. Similarly, total invertebrate abundance (p < 0.01), and abundance of ephemeropterans (p < 0.05) and trichopte rans (p < 0.001) were significantly lower on substrates exposed to ful l sunlight than on UVR-excluded treatments. For example, the abundance of heptageniid mayflies on day 30 was 10x higher on UVR-excluded subs trates than on UVR-exposed substrates. These changes in benthic commun ity structure were not obvious until the end of the experiment when wa ter depth and UV-254 absorbance, a measure of the aromatic fraction of dissolved organic carbon (DOG), were lower than at the beginning of t he study. Thus, the negative effects of UV radiation on Rocky Mountain stream organisms (algae and invertebrates) may vary temporally and ma y be more pronounced during low-water, summer conditions when DOC leve ls are typically low and UVR levels reaching the earth's surface are r elatively high.