To date, paleoecological approaches to reconstruction of past UVR regimes and their impacts on lakes have demonstrated that changes in biological exposure to UVR caused by climate change are at least 10-fold greater than those associated with moderate (30%) ozone depletion [e.g., 15,24]. In all cases, climatic variability acts to change export of UVR-absorbing DOM from land, although the direction of lake response depends on whether warming reduces hydrologic export [cf., 125] or results in development of new C sources (e.g., soils; [47]). These patterns suggest that all glacial lakes have experienced severe UVR stress early in their evolution, and that this may be a key factor limiting early lake productivity. Similarly, preliminary comparison of historical irradiance regimes in both polar regions suggests that past UVR exposure is intrinsically variable due to both short-term processes and, potentially, millennium-long solar or atmospheric variability (Figures 5, 6). Finally, because human alteration of global C cycling and atmospheric processes is so pervasive [108], and because ecosystem and environmental monitoring efforts continue to decline, we foresee an expanded role for such retrospective analyses to identify past environmental change and to quantify the relative importance of multiple stressors on ecosystem structure and function [26].

Several avenues of future research may benefit from historical reconstructions of past UVR. For example, little is known of how long-term changes in irradiance impact trophic interactions in natural ecosystems [e.g., 6], yet most lake sediments archive a wide range of invertebrate and plant fossils that allow quantitative reconstruction of past grazer communities, predator-prey interactions, and even estimates of fish abundance [95]. Similarly, hydrated lime is frequently added to acidified lakes to mitigate impacts of low pH, yet this approach has long been known to greatly reduce water-column DOM concentration through adsorption onto carbonate particles [130]. Reconstruction of potential UVR impacts arising from the use of lime has not yet been attempted, but may provide critical information to explain unexpected delays in lake recovery from acidification. Similarly, quantitative analyses of the rate of change of fossil assemblages in response to natural and anthropogenic forcing is required to better formulate management plans for ecosystem protection. For example, preliminary analysis of declines in lake production following mid-Holocene climate change (Figure 3) suggests that past rates of ecosystem response (> 1% yr-1) are similar to those forecast for future global warming [e.g., 125] and may provide unexpectedly good analogues for its impacts [27]. Thus, when combined with ecosystem models, long-term monitoring, ecosystem experiments and short-term manipulations, paleoecology provides modern ecolo-gists and environmental scientists with key insights into the scale, causes and consequences of temporal variability in UVR.

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