Risk analysis of dissolved organic matter-mediated ultraviolet B exposure in Canadian inland waters

With depleted ozone levels and the possibility that climate change might lower dissolved organic matter (DOM) concentrations, the risk of exposure of fresh waters in Canada to ultraviolet B (UV-B) was evaluated. First, the distribution of a UV-B-sensitive zooplankton genus, Daphnia, was examined as a function of DOM concentration and maximum depth (Z(max)) in 258 systems. Distribution was not restricted by UV-B, although very clear, shallow systems are underrepresented. Secondly, the depth at which 1% of surface radiance at 320 nm occurs (Z(320,1%)) was compared with Z(max) in over 1000 aquatic systems in 15 ecozones to determine the proportions of optically clear systems (Z(max) less than or equal to Z(320,1%)) and systems that may become clear (i.e., are "at risk") should DOM decrease by 50%. South of the treeline, <6% of systems were clear, with the exception of two ecozones with 10%-20%. The proportion of systems at risk was 0% in most regions, with 5%-9% in four regions. DOM levels appear adequate to prevent extirpation of sensitive taxa like Daphnia through direct exposure to UV-B in most regions south of the treeline. However, optically clear and at-risk ponds were much more common in the three Artic ecozones, especially the Northern Arctic and Arctic Cordillera.