Paleoecological reconstructions of past irradiance regimes provide critical insights into the scales, causes and consequences of temporal variability in UVR. Such retrospective analyses using microfossil remains (diatoms), photo-protective pigments (scytonemin) or bulk sedimentary characteristics (% organic matter) can allow reconstruction of past concentrations of UVR-absorbing dissolved organic carbon compounds (DOC), spectral irradiance regimes (UVR : PAR ratio), lake transparency (depth of UVR penetration), as well as algal and invertebrate responses to altered UVR flux. Application of fossil approaches to lakes at treeline show both that climatically-induced changes in the export of terrestrial DOC to lakes are up to 100-fold more effective than modern ozone depletion at altering biological exposure to UVR, and that naturally occurring droughts can increase UVR exposure in lakes by up to 10-fold. When applied to Arctic and Antarctic lakes, preliminary fossil analyses suggest that historical variability in UVR exposure has been high, but that lakes may have received twice as much UVR prior to ~ 3000-4000 years ago as they do at present. Finally, when used in combination with long-term environmental monitoring, historical reconstructions have proven valuable at identifying the importance of UVR relative to other stressors in regulating lake production. Thus, although further research is required to validate fossil interpretations, paleoecological analyses can provide critical information on the role of UVR in regulating lake structure and function.

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