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Abstract. Solar ultraviolet radiation (UVR) may have beneficial as well as detrimental effects on living systems. For example, UV-B radiation (280-320 nm) is generally damaging, while UV-A radiation (320-400 nm) may cause damage or stimulate beneficial photorepair of UV-B damage. The nature of both direct and indirect effects of UVR in nature depends on both the photon flux density and the spectral composition of the radiation incident on aquatic organisms across environmental UVR gradients in space (depth, transparency, elevation) and time (diel, seasonal, interannual). Here we use the common and widespread freshwater cladoceran Daphnia pulicaria as a model organism to demonstrate the potential importance of these wavelength-specific effects of UVR to the ecology of aquatic organisms. UVR-exposure experiments are used to manipulate both natural solar and artificial UVR sources to examine the beneficial as well as detrimental effects of different wavelengths of UVR. Changes in the spectral composition of solar radiation are also examined along several natural environmental gradients including diel gradients, depth gradients, and dissolved organic carbon (DOC) gradients. The implications of variation in the spectral composition of UVR for aquatic organisms are discussed.The first biological weighting function (BWF) for a freshwater cladoceran is presented here. It demonstrates that the shortest UV-B wavelengths in sunlight are potentially the most damaging per photon. However, due to the greater photon flux density of longer wavelength UVR in sunlight, the net potential damage to Daphnia in nature is greatest for the longer wavelength UV-B and shorter wavelength UV-A radiation in the 305-322 nm range. Overall the contribution of UV-B to the total mortality response of Daphnia exposed to full-spectrum solar radiation for 7 h on a sunny summer day is 64% while UV-A contributes 36%. The BWF for Daphnia is used with the transmission spectrum for Mylar D to demonstrate that Mylar D cuts out only about half of the damaging UVR in sunlight. Following exposure to damaging UV-B, Daphnia exhibits a dramatic increase in survival in the presence of longer wavelength UV-A and visible radiation due to the stimulation of photoenzymatic repair. We present data that demonstrate the importance of both atmospheric ozone and DOC in creating strong environmental gradients in the intensity (irradiance) and spectral composition of solar UVR in nature. The light-absorbing component of DOC, chromophoric dissolved organic matter (CDOM), is particularly important in creating depth refugia from damaging UV-B in freshwater ecosystems. CDOM may also cause intense variations in the ratio of potentially beneficial UV-A to detrimental UV-B radiation to which aquatic organisms are exposed. In addition to changes in atmospheric ozone, future changes in CDOM related to climate change or other environmental disturbances may substantially alter the underwater exposure of a variety of aquatic organisms to different wavelengths of solar UVR.

Climate warming and stratospheric ozone depletion increase temperature and ultraviolet (UV) in mid‐ to highlatitude ecosystems; however, little is known about the interactive effects of temperature and UV on organisms. We exposed Daphnia catawba, Leptodiaptomus minutus, and Asplanchna girodi to UV‐B at four different temperatures: 10, 15, 20, and 25°C. Elevated temperatures increased UV tolerance in D. catawba and L. minutus, species that depend heavily on photoenzymatic repair (PER), but decreased UV tolerance in A. girodi, a species that has less PER. Also, body size in Daphnia decreased with increasing UV dose. These results demonstrate that climate change can alter responses to UV through temperature‐mediated effects in aquatic ecosystems, and these effects can be species‐specific and dependent on PER ability.

1. This review addresses the effects of UV‐radiation on the morphology and biochemistry of phytoplankton and the potential effects on grazers. 2. UVA and UVB radiation inhibit the uptake of inorganic nutrients in phytoplankton. Reduced rates of ammonium and nitrate uptake in marine diatoms, and reduced uptake of phosphorus in freshwater flagellates are reported. The effects on cell stoichiometry are not settled. 3. UVA and UVB radiation promote increased cell volumes owing to a decoupling between the photosynthetic processes and cell division. Loss or inactivation of flagellae and loss of motility are also reported for a number of phytoplankton species. 4. UVA and UVB radiation may affect major biochemical constituents. Accumulation of intracellular, photosynthetic products (lipids or carbohydrates) is a common, although not unique, property of UV‐stressed algae. Fatty acid (FA) profiles seem susceptible to UV radiation. A relative increase of short‐chained, and a decrease in polyunsaturated FA (PUFA) are reported. The important membrane FA like eicosapentaenoic acid (EPA, 20 : 5ω3) and docosahexaenoic acid (DHA, 22 : 6ω3) seem particularly susceptible, owing to lipid peroxidation or reduced biosynthesis. 5. UV‐related responses are highly dependent on taxonomy, cell‐cycle stage, nutrient‐limitation and the UV : PAR (photosynthetic active radiation)‐ratio. 6. Nutrient deficiency, cell size, cell wall properties and FA can all have significant impacts on grazers. Thus the reported effects on cell morphology and biochemical constituents could have profound effects on grazers and energy transfer in aquatic foodwebs.

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