The oceans are currently absorbing over one million tons of CO 2 from the atmosphere each hour, and play an important role in mitigating global warming (Sabine et al. 2004). At the same time, enhanced dissolution of CO 2 from the air is also acidifying the oceans, a process known as ocean acidification (Doney et al. 2009). While standing biomass in the oceans accounts for only about 1% of that in terres-© Inter-Research 2012 • www.int-res.com

Interactions between climate change and UV radiation are having strong effects on aquatic ecosystems due to feedback between temperature, UV radiation, and greenhouse gas concentration. Higher air temperatures and incoming solar radiation are increasing the surface water temperatures of lakes and oceans, with many large lakes warming at twice the rate of regional air temperatures. Warmer oceans are changing habitats and the species composition of many marine ecosystems. For some, such as corals, the temperatures may become too high. Temperature differences between surface and deep waters are becoming greater. This increase in thermal stratification makes the surface layers shallower and leads to stronger barriers to upward mixing of nutrients necessary for photosynthesis. This also results in exposure to higher levels of UV radiation of surface-dwelling organisms. In polar and alpine regions decreases in the duration and amount of snow and ice cover on lakes and oceans are also increasing exposure to UV radiation. In contrast, in lakes and coastal oceans the concentration and colour of UV-absorbing dissolved organic matter (DOM) from terrestrial ecosystems is increasing with greater runoff from higher precipitation and more frequent extreme storms. DOM thus creates a refuge from UV radiation that can enable UV-sensitive species to become established. At the same time, decreased UV radiation in such surface waters reduces the capacity of solar UV radiation to inactivate viruses and other pathogens and parasites, and increases the difficulty and price of purifying drinking water for municipal supplies. Solar UV radiation breaks down the DOM, making it more available for microbial processing, resulting in the release of greenhouse gases into the atmosphere. In addition to screening solar irradiance, DOM, when sunlit in surface water, can lead to the formation of reactive oxygen species (ROS). Increases in carbon dioxide are in turn acidifying the oceans and inhibiting the ability of many marine organisms to form UV-absorbing exoskeletons. Many aquatic organisms use adaptive strategies to mitigate the effects of solar UV-B radiation (280-315 nm), including vertical migration, crust formation, synthesis of UV-absorbing substances, and enzymatic and non-enzymatic quenching of ROS. Whether or not genetic adaptation to changes in the abiotic factors plays a role in mitigating stress and damage has not been determined. This assessment addresses how our knowledge of the interactive effects of UV radiation and climate change factors on aquatic ecosystems has advanced in the past four years.

Natural phytoplankton populations from both Antarctic and tropical waters were exposed to solar radiation to determine the effects of ultraviolet radiation (UVR) on rates of photosynthesis. Radiation in the UV-A region (320 to 400 nm) was responsible for over 50 % of the total inhibition due to UVR, with less than 50 % due to UV-B (280 to 320 nm). Wavelengths c305 nm, which is the spectral region most enhanced under conditions of low ozone concentrations in the atmosphere, accounted for only 15 to 20% of the total inhibition due to UV-B radiation. Under high-light conditions on sunny days, photosynthetic rates were increased 200 to 300 % by screening off all UVR below 378 nm. When the average UVR (295 to 385 nm) during the incubation period was below a threshold value of 5 to 10 W m-', there was no significant depression of photosynthetic rates. Microscopic examinations of a phytoplankton population which was allowed to grow for 5 d under high solar irradiance indicated that UVR is more inhibitory to microplankton than to nanoplankton, and that it induced the formation of resting spores in diatom species of the genus Chaetoceros. Phytoplankton from tropical waters showed marked resistance to UVR as compared to Antarctic phytoplankton. Under the highest light conditions an increase in photosynthetic rates of 10 to 20 % was recorded in some experiments by screening off all UVR c 3 7 8 nm, but when data from all experiments with samples from surface waters in the tropics were analyzed, there was no significant difference between samples with or without natural solar UVR. Phytoplankton samples from below the pycnocline in tropical waters, however, were very sensitive to solar radiation, suggesting that the resistance shown by phytoplankton from surface waters reflects a photoadaptive process.

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