Only recently has the potential for strong interactions between climate change and UV effects on aquatic ecosystems been recognized. Chromophoric dissolved organic matter (CDOM) largely mediates these interactions. CDOM influences water transparency and the attenuation of both visible and UVR in the water column. It also influences thermal gradients, mixing processes, and the timing and extent of ice cover. Here we review existing knowledge of these processes and present some conceptual diagrams to illustrate our current understanding of how the key processes interact. The general conclusion from numerous studies is that, for inland waters, climate change and consequent changes in CDOM are likely to have a greater effect on underwater UV environments than will stratospheric ozone depletion. Recent quantitative modeling efforts are also briefly reviewed. Sensitivity testing with these models indicates that mixing depth and water column attenuation of UV are at least as important as ozone depletion in regulating underwater UV exposures. Ozone depletion may be of greater importance for smaller organisms such as bacteria that are less protected from UV damage. Variations in ozone may also be of relatively greater importance in marine environments where CDOM concentrations are low and UV transparency high.

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