Penetration of ultraviolet radiation (UVR) into natural waters depends on the concentration and optical qualities of dissolved organic matter (DOM), phyto-plankton, other suspended particles, and on the optical properties of pure water. Optical classification schemes developed for lakes or ocean regions, and relevant to UVR penetration, indicate which of these factors contribute to underwater attenuation of solar radiation. In most cases the best predictor of UV attenuation is optical absorption by chromophoric DOM (CDOM). In natural waters in which UVR penetration is greatest (Crater Lake, USA, and Lake Vanda, Antarctica), CDOM and phytoplankton are so scarce near the surface that UV attenuation by water molecules is significant. Recent evidence from these lakes supports a downward revision of older estimates for UV attenuation by pure water. Variations in atmospheric scattering, sun angle, and depth can cause diffuse attenuation measurements to deviate from the Beer-Lambert Law, but a simple correction is available. Bio-optical models to predict the penetration of UVR into natural waters from measurements will require better understanding of specific absorption of CDOM and phytoplankton, of the reactivity of CDOM, and of the linkage between the microbial community and autochthonous production of CDOM. Spatial and temporal patterns relating UV attenuation with dissolved organic carbon (DOC) concentration and optical quality in aquatic ecosystems appear to be driven by the rates of DOC flux and photobleaching, and by hydrologic properties (residence time and evaporation rate).

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