UVR effects in aquatic ecosystems are distinguished by strong vertical gradients (see Chapter 3), so that most effects occur in a "photoactive" zone near the surface. Vertical mixing mediates UVR effects by determining how much of a water body (ocean, lake, etc.) exchanges through this photoactive zone. Processes influencing this exchange are mainly those that determine (1) the depth of the upper mixed layer (UML) and (2) the rate of transport within the UML. In this chapter, we will catalog and briefly discuss primary physical processes influencing each of these UML characteristics. For more detail on these processes, the reader is referred to several recent general reviews on mixing in the upper ocean [1,2] and in lakes [3,4]. We then consider how changes in mixed layer depth and rate of mixing can influence the chemical and biological effects of UVR exposure.

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