Impacts of UVR on lake biota

16.2.1 Natural controls of UVR in lakes

Biotic exposure to UVR varies as a function of solar production, atmospheric attenuation, orbital precession, seasonal factors (ice cover), meteorological conditions, lake chemistry (DOM content, pH, salinity), basin depth, the presence of physical refuges, and individual behavior. Variations in solar production of UVR are thought to be minor except on scales of planetary evolution (e.g., supernova, neutron star merger), and are usually ignored in ecological and most evolutionary studies [16,29]. While increased sunspot activity can deplete stratospheric ozone [30], instrumental records suggest such changes account for

~9% of inter-annual variation in UVR [31]. Similarly, volcanic eruptions or meteor impacts can increase or reduce UVR transmission through the atmosphere depending on the relative abundance of dust, aerosols and chemicals ejected from the Earth's crust (e.g., 03-depleting CI) [17]. In contrast, greater variation in stratospheric ozone and atmospheric UVR penetration has been recorded over short timescales (e.g., 26% in 3 days; [18]) due to transient changes in local ozone content, cloud cover, aerosols, particulates and pollutants [32]. However, with the exception of meso-scale depletions of ozone in polar regions [33,34] and variations in snow, ice and debris cover on frozen lakes [e.g., 35], most recent research suggests that the largest changes in exposure of aquatic biota arise from variation in landscape and in-lake processes related to DOM biogeochemistry [12,13,15,24].

Comparison of physical and chemical factors regulating UVR penetration into water show that variation in concentrations of chromophoric dissolved organic compounds is the main factor regulating biotic exposure to energetic irradiance in most lakes [10,11,36-38]. In general, changes in DOM influx provide the single largest source of variation [e.g., 13], although variation in source water content can be important in marine-influenced systems [39], Physical attenuation by algae or particulates may be important also in either highly unproductive [40], hypereutrophic or turbid systems [10,38], or in shallow lakes where the carbon-specific attenuation of DOM is low [41]. Because of the strong negative exponential relation between UVR transmission and wavelength, variations in mass-specific attenuation of DOM [41,42] and spectral attenuation characteristics ("S"; [43]) influence exposure of biota in polar, alpine, acidified or saline lakes where terrestrial inputs of DOM are low, DOM is highly degraded, or where phytoplankton are a quantitatively important source of DOM [41,44],

Finally, virtually all surveys of modern and sub-fossil algal communities show that lake depth is a critical factor influencing algal community composition [45]. While not explicitly linked to irradiance to date, we hypothesize that spatial and temporal variation in basin morphology is a key factor regulating biotic exposure to UVR on a biogeographic and historical basis, through the provision of deepwater refugia [15] or by controlling the development of thermal stratification ("UVR traps"; [18,46]). Further, as a consequence of the supremacy of in situ processes in regulating biotic exposure to UVR, we suggest that historical changes in UVR impacts are most likely to arise from variations in catchment and lake characteristics [cf., 28], rather than from factors which influence UVR transmission to the lake surface [15,24].

16.2.2 Impacts of UVR on aquatic biota

Most short-term experimental and observational research concludes that intense UVR, particularly UV-B (280-315 nm), damages cellular structures, decreases metabolic efficiency and reduces growth or survival of individuals [1,47]. Direct inhibition of algal photosynthesis and growth appears to be greatest when phytoplankton are trapped in surface waters [18,48; Chapter 11], restricted from exploiting refugia by hard substrates [3,6,49], or are exposed to photochemical toxins [50,51]. However, algal sensitivity to UYR varies greatly among taxa [e.g., 4], partly because of taxon-specific differences in cell structure [52], photo-protective pigment content [53], efficiency of cellular repair mechanisms [54] or behavioral exploitation of refugia [55,56]. Similarly, pelagic invertebrates are differentially impacted by UVR [57-59; Chapter 12], reflecting variations in species' photoprotective pigmentation [60], body size [58], repair mechanisms [5,61], or avoidance behavior [62]. Finally, recent evidence suggests that UVR may alter the reproductive success of both macrophytes [63] and predaceous fishes [7,8], thereby altering both trophic interactions and habitat structure within lake ecosystems.

Comparison of experimental investigations suggests that while the absorbance of high-energy irradiance must necessarily initiate cellular damage [64], complex indirect effects may be the predominant mechanism by which UVR effects are expressed. Unfortunately, no consistent pattern has been noted for integrated food-web or ecosystem response to changes in UVR exposure [3,6,9,57,65], probably because most experimental studies have been conducted on physical and temporal scales that are tractable for experimentation, but which may lack ecological realism. Such scale dependent problems may include insufficient experimental duration (days-weeks), unrealistic irradiance regimes (continuous light; spectral composition; instantaneous change in flux), absence of key food-web components (predators, competitors) and simplified physical structure (refugia, sediments, plants). While these approaches clearly identify the potential for UVR impacts to alter biotic interactions, research on broader physical and temporal scales is required to evaluate the importance of UVR relative to other processes in regulating ecosystem structure [66]. In this regard, we feel that paleoecological analyses may provide significant insights into the role of high-energy irradiance in structuring aquatic communities. Below we review the main methods for reconstruction of past UVR environments and use a series of case studies to illustrate UVR impacts on ecosystem organization.

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