Most laboratory and short-term field experiments show that exposure to intense UVR has detrimental effects on cellular processes [1], individual survival [2], and population growth rates [3,4]. However, demonstration of substantial UVR impacts on entire lake ecosystems has proven elusive to date, possibly because of variable species sensitivity to UVR [2,4], habitat specificity of biotic responses [3], incomplete reciprocity of UVR doses [5], countervailing effects of individual wavelengths [5], or complex trophic interactions [6-8]. Potential underestimation of UVR impacts may have arisen also because initial investigations focused on minor increases in UVR exposure arising from ozone depletion [e.g., 9], rather than variability in terrestrial dissolved organic matter (DOM) fluxes, the principle control of UVR penetration into lakes [10,11]. However, with the demonstration that major environmental disturbances such as droughts [12,13], lake acidification [14], long-term climate change [15] and even cosmic events [16,17] greatly alter inputs, metabolism and degradation of photo-protective DOM, it seems likely that unique, ecosystem-level impacts of UVR will be identified [e.g., 18].

One approach to assessing the potential impacts of UVR on aquatic ecosystems is to quantify the magnitude and consequences of past variations in UVR. In this regard, paleoecological analyses can provide valuable insights into the history of UVR exposure and its potential impacts on aquatic ecosystems [19]. Modern lake surveys show that planktonic and benthic community composition varies predictably as a function of the key determinants of UVR exposure (lake depth, water clarity, DOM content) as well as the environmental drivers that regulate DOM fluxes and optical properties (e.g., climate, pH) [reviewed in 20]. Further, multivariate analysis of sub-fossil (1-5 yr old) assemblages of diatoms in lake sediments have been used to develop statistical models (termed transfer functions) of the relationships between present-day species composition and environmental conditions that allow quantitative reconstruction of past lake conditions, including DOM content and optical regime [e.g., 21-23]. Consequently, application of these models to sedimentary sequences can be used to quantify historical patterns of UVR variability over 100s to 1000s of years, potentially at annual resolution [15,23]. Because past episodes of UVR exposure appear to be greater than those arising from many modern processes [15,24,25], these analyses may also offer insights into the unique impacts of UVR on ecosystem processes [e.g., 26,27], including those occurring immediately after lake formation when DOM inputs are lowest [2,28].

The goal of this chapter is to review how paleoecological techniques can be used to reconstruct past UVR environments and impacts on lakes. In the first section of the chapter, we briefly summarize UVR impacts on key aquatic biota, as well as the main controls of UVR exposure. Next we critically review the main microfossil and biogeochemical methods used to reconstruct past UVR environments. Third, we use a series of case studies to illustrate the insights obtained from historical reconstructions of irradiance environments and ecosystem responses. Finally, we identify new research avenues that we feel may bring important insights into the role UVR in regulating the structure and function of aquatic ecosystems and that may illustrate the wide range of past irradiance environments. In this review, we use DOC to indicate dissolved organic matter measured as organic carbon concentration, chromophoric DOM (CDOM) to indicate similar measurements based on optical properties of water, and DOM for cases where carbon or optics are not directly measured, or when describing general sources and fluxes of dissolved organic compounds.

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