Climatic control of ultraviolet radiation effects on lakes

Ultraviolet radiation (UVR) damages most biota, yet little evidence exists for its long-term effects on natural ecosystems. We used paleoecological techniques at three low-elevation lakes to show that algal abundance in lakes was depressed 10-fold by UVR during the first millennium of lake existence. Likewise, analysis of data from a lake near treeline showed that algal biomass declined 10–25-fold both early in the lake history and during the last ;4000 yr, when inputs of UVR-absorbing dissolved organic matter (DOM) declined despite constant nutrient levels since ;10,000 14C yr before the present. This rapid (21.25% yr21), sustained (.600 yr) suppression of algal abundance arose from directional climate change that reduced DOM inputs and occurred despite initial reservoirs of photoprotective DOM that are typical of most boreal lakes. Hence, we conclude that many lakes may be vulnerable to order-of-magnitude declines in algal abundance arising from future climate-DOMUVR interactions. Despite increasing evidence for the detrimental effects of ultraviolet radiation (UVR) on individual lake biota (Karentz et al. 1994), there have been few demonstrations of its longterm effects on natural ecosystems (Xenopoulos et al. 2000). Most laboratory and field experiments have suggested that exposure to intense UVR reduces the rates of individual or population growth (Karentz et al. 1994). However, quantification of the net effects of UVR on total primary production remains difficult, because the sensitivity to UVR varies among algae (Vinebrooke and Leavitt 1999a), herbivores (Bothwell et al. 1995), their predators (Williamson et al. 1997), and biota in different lake habitats (Vinebrooke and Leavitt 1999a), and because recent variance in UVR exposure is slight relative to past changes (Leavitt et al. 1997; Pienitz and Vincent 2000). Here we use the wide range of past UVR exposures seen during the Holocene era to show 1 Corresponding author ([email protected]).