Freshwater Biology (1998) 40, 357–365 APPLIED ISSUES Diel emigration and colonization responses of blackfly larvae (Diptera: Simuliidae) to ultraviolet radiation WILLIAM F. DONAHUE AND DAVID W. SCHINDLER Department of Biological Sciences, University of Alberta, Edmonton, Alberta, T6G 2E9, Canada SUMMARY 1. Total counts of blackfly larvae densities over 30- and 57-h periods in experimental channels during May of 1996 and 1997 indicate that ultraviolet radiation (UV; 290– 400 nm) may be important in stimulating emigration. 2. Under experimentally controlled solar UV exposure, larval densities at dawn in UV- shielded channels were 161% and 168% higher than in the UV-exposed channels. Larval densities in UV-exposed channels then decreased by 68.2% and 81.1% between dawn and early afternoon of the two days; density decreases in UV-shielded channels were slight, and not statistically significant, during the same periods. 3. Larvae within UV-exposed channels occupied shaded microhabitats during hours of intense solar radiation, suggesting that simuliid larvae can detect and respond to UV radiation over very short periods of time. 4. A cyclical pattern of UV-induced emigration during hours of increasing solar flux (06.30–13.30) and net immigration in the hours of decreasing solar flux and at night emerged. Thus stream invertebrates may be very sensitive to environmental changes, resulting in either increased UV flux or decreased shading of streams. Diel cycles in invertebrate densities should be taken into account in research designs and sampling protocols in order to identify and interpret correctly results of both periodic surveys and experiments. Introduction In general, abiotic factors such as light, substrate type, temperature, water depth and velocity have been found to affect rates of drift, densities, development and species composition of simuliid larvae in streams (Pearson & Franklin, 1968; Lewis & Bennett, 1975). This paper describes the potential role of solar ultraviolet radiation in the initiation of drift behaviour in simu- liid larvae. Depletion of stratospheric ozone has resulted in recent increases in ultraviolet B (UVB) fluxes (Crutzen, Correspondence: W. F. Donahue. E-mail: bdonahue@gpu.srv.ualberta.ca © 1998 Blackwell Science Ltd 357 1992), including at temperate latitudes (Blumthaler & Ambach, 1990; Kerr & McElroy, 1993). Perhaps more important in aquatic environments is the depletion of dissolved organic carbon (DOC), the primary attenu- ator of UV radiation in aquatic systems, as a result of anthropogenic acidification and regional climate change involving decreased precipitation, decreased runoff from the catchment, decreased streamflow and increased lake residence time of water (Schindler et al., 1992; Schindler et al., 1996a). These 70–80% decreases in DOC have resulted in increases, up to 900%, in UV- exposure in aquatic environments (Schindler et al., 1996b). This may be especially important to the com-