Month

Figure 4. Seasonal variations in ambient UV (squares, 320 nm, from Biospherical Instruments Inc. RT95 radiative transfer model) and surface water temperatures (diamonds, monthly averages from our Lake Giles data 1989-1993) lead to strong variation in seasonal UV : T ratios (heavy line, triangles). These ratios may be critical to aquatic organisms due to the temperature dependence of molecular repair of UV damage. These seasonal UV : T patterns are for the surface of a lake. Underwater UV and temperature exposures of organisms will depend on interactions between CDOM, temperature, and UV at the ecosystem level (Figures 1,6), while the response of organisms to a given exposure level will depend on the interactive effects of temperature and UV at the molecular level (Figure 5). [Modified from [68], with permission.]

U V transparency is close to summer solstice when incident solar radiation is also high. This has important implications for the timing of UV impacts on aquatic organisms that may show strong seasonal peaks in reproduction [61,72]. Such time constraints imposed by seasonal variations in UV and temperature may have severe consequences for reproductive success and subsequent survival of some fish [73,74],

Both seasonal and spatial variations in UV : T ratios are likely to alter UV impacts (Figure 5). At the most fundamental level, organisms may have differences in their temperature optima. In addition, both antioxidants and molecular repair of UV damage are enzyme-driven and thus one would expect them to be temperature dependent. UV damage itself on the other hand is likely to be largely temperature independent. These relationships are not straightforward however. There is evidence that while in some cases UV tolerance does indeed increase at higher temperatures [67,75] in other cases UV tolerance shows either little change with temperature [76], or even decreases at higher temperatures [77,78]. The complexity of these processes is clearly illustrated by the fact that the specific activity of two antioxidant enzymes in some Daphnia (catalase and glutathione transferase) increases at higher temperatures, yet survival may be greater at lower temperatures over this same temperature range [78]. A partial explanation may be related, as mentioned above, to variations in the temperature optima of Daphnia and other aquatic organisms [79,80], and the importance of temperature-dependent photorepair of UV damaged DNA relative to photoprotection in a given species [68].

Dna Photorepair

Figure 5. Conceptual model showing how temperature influences the effects of different wavelengths of solar radiation on some components of living organisms (+) but not others (0). The nature of the " + " effects (positive or negative) will depend on the temperature optimum of the enzyme system involved. The width of the arrows approximates the strength of the effects.

Figure 5. Conceptual model showing how temperature influences the effects of different wavelengths of solar radiation on some components of living organisms (+) but not others (0). The nature of the " + " effects (positive or negative) will depend on the temperature optimum of the enzyme system involved. The width of the arrows approximates the strength of the effects.

The impact of temperature-UV interactions on aquatic organisms may also be modulated by changes in development rates as well as by other environmental factors. For example, lower temperature at higher elevations and higher latitudes may slow development rates and prolong the period of exposure of the juvenile stages of UV-sensitive vertebrates to damaging UV [81,82]. Other environmental factors such as pH may also interact with UV. For example, in some cases pH may act synergistically with UV exposure to reduce survival in amphibians [83,84], while in other cases it may not [85]. The presence of pesticides and other specifically phototoxic compounds may also influence UV-temperature interactions and consequent responses of zooplankton and other aquatic organisms [86-88].

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