Effects of UV-B dose and irradiance: comparison among grazers

Short-term ultraviolet-B (UV-B) radiation was administered in the laboratory to mayfly nymphs (Diphetor hageni) midge larvae (Corynoneura taris) and snails (Elimia claveformis and Physella gyrina) to determine relative susceptibility to W-B stress. Invertebrate response was a function of the total amount of UV-B received (dose, kJ/m(2)), and the fluence rate at which UV-B was administered (irradiance, W/m(2)). Reciprocity did not hold for organisms tested in this study; effect of a given dose was dependent on irradiance. Invertebrates received a gradient of cumulative doses between 0 and 30 kJ/m(2) UV-B while simultaneously receiving UV-A and photosynthetically active radiation. UV-B wavelengths were weighted with the DNA action spectrum to obtain biologically effective doses between 0 and 0.214 kJ/m(2) UV-B-DNA. This dose gradient was repeated at 3 different UV-B irradiances (1.6, 2.0 and 2.7 W/m(2)). Total dose, irradiance, and organism size all contributed to W-B mortality. Mayfly nymphs and midge larvae had similar UV-B thresholds, and the lethal dose at which 50% of the organisms died (LD50) decreased with increased irradiance. For instance, LD(50)s for the midges were 17.2, 15.2 and 10 kJ/m(2) at 1.6, 2.0, and 2.7 W/m(2), respectively. Both snails were relatively resistant to W-B. The prosobranch snail Elimia showed no mortality at any of the UV-B doses administered in this study. A size-class experiment using the pulmonate snail Physella suggested that organism size also affects its ability to cope with UV-B stress: small snails (1.2 +/- 0.1 mm,(x) over bar +/- 1 SD) were 5 times more likely to die from W-B exposure than medium (5.1 +/- 0.3 mm) or large (7.8 +/- 0.6 mm) snails. Our experiments demonstrated that benthic invertebrates are differentially susceptible to acute UV-B stress, which could lead to alterations in invertebrate assemblages where UV-B levels are high.