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decomposability of litter. Newly published studies have shown that enhanced UV-B radiation increases tannin, lignin and other secondary compounds, thereby reducing the microbial digestibility of the litter from mid-latitude dune grassland species and high latitude shrubs. New evidence has appeared that UV-B photodegrades the lignaceous component of surface litter and dissolved organic matter in the sea. Studies of the effects of enhanced UV-B and elevated CO2 have indicated no combined effects on CO2 respired from sub-arctic shrub litter. Recent climate changes are enhancing fire frequency and extent in the boreal forest. Increased boreal fires may be having important regional effects on the UV-induced release of CO from the forest to the atmosphere. Although the floors of mature boreal forests generally take up CO via microbial oxidation processes, the charred surface residues following fire produce high levels of CO on exposure to solar UV radiation. The production is sufficiently large that the burned forest becomes a net source of CO to the atmosphere. The photodegradation of contaminants in aquatic ecosystems involves both direct and sensitised photoreactions.19 Possible future reductions in UV irradiance due to ozone recovery and reduced UV penetration into aquatic environments would result in increased persistence of contaminants associated mainly with decreased direct photodegradation rates. Direct photodegradation results from the absorption of solar radiation by the contaminant itself. For many organic contaminants such as pesticides and “emerging chemicals”, e.g., pharmaceuticals, direct photodegradation is caused primarily by the shortest wavelength component of sunlight in the UV-B range 280–15 nm. Sensitised photodegradation is initiated through light absorption by another substance in the system with the contaminant, such as CDOM in aquatic environments,16,20 which produces short-lived reactive transients that react with the contaminants.21–27 Recent developments in remote sensing techniques and surface measurements are providing new insights into the spatial and temporal distribution of the transients e.g., hydrogen peroxide and superoxide,28 as well as additional information on the spatial and temporal distribution of CDOM in aquatic ecosystems.29– Enhanced terrestrial runoff due to more frequent heavy precipitation events could result in increased CDOM concentrations and reduced penetration of UV radiation into surface waters. Taking this climate-change effect together with potential super-recovery of stratospheric ozone,14 one can speculate that the overall effect might be a shift from direct to indirect UV-induced transformations of chemical and biological contaminants. The term “biogeochemical cycles” refers to chemical andor biological transformations of natural and man-made substances e.g., carbon, halogen compounds, and contaminants in terrestrial and aquatic ecosystems.