Limnol. Oceanogr., 44(2), 1999, 470–473

In contrast to earlier studies (Bothwell et al. 1993, 1994; DeNicola and Hoagland 1996; Kiffney et al. 1997b), Hill et al. (1997) recently concluded that there was no effect of ambient solar ultraviolet (UV) radiation on periphyton and grazers in a small Tennessee stream. These conclusions were based on the appearance that grazing pressure by pleurocerid snails (Elimia clavaeformis) controlled algal biomass and primary production in three in situ stream experiments designed to test the role of UV radiation in trophic interactions. We assert that Hill et al.’s experimental design, site selection, and assumptions about the organisms within the system minimized biotic response to any UV radiation treatment and thus, preclude them from making general claims about lack of sensitivity of stream systems to UV radiation. UV effects on periphyton communities were investigated by Hill et al. in White Oak Creek, a small, north-south– oriented, clear-water stream with high densities of Elimia. Plexiglas sheets (40 3 40 cm) impermeable to UV radiation (50% cut-off @ 375 nm) were suspended 5–10 cm above the water surface approximately 1 m apart in an upstreamdownstream orientation (Hill pers. comm.), and each of these sheets was paired with a directly adjacent sheet of UV-permeable plastic of the same size. In each of three experiments performed between June and November, ten 1-in. square ceramic tiles were placed under each sheet; chlorophyll a (Chl a), ash-free dry mass, and photosynthetic rates were measured from these. During these experiments, tiles were 10– 25 cm below the surface of the water and were preconditioned in full-spectrum sunlight for at least 1 yr prior to the experiment. During all three experiments, experimental substrates were exposed to sunlight for at least 6 h during the day (varying according to season). We suggest that a fundamental error in Hill et al.’s experimental design resulted from the small size of the Plexiglas sheets. As a result of varying solar angles, both elevational and rotational, during the day as well as over seasonal scales, the actual area under the screens that was shielded or exposed to solar UV changed position with time. Using solar elevation and azimuth angles, the site longitude and latitude, and the experimental dates, we demonstrate that, during the experiments performed by Hill et al., there would have been times when area thought to be shielded from UV would have been exposed to UV. A corollary of this is that the ‘‘UV-exposed’’ substrate, directly under the adjacent UVtransparent plastic, would have been shielded from UV during parts of the day when the shadow of the UV-opaque Plexiglas passed over it.