Aquatic systems supply humans with vast amounts of food, primarily in the form of finfish, shellfish and seaweed. More than 30% of the world's animal protein for human consumption comes from the sea, and in many countries, particularly the developing countries, this percentage is significantly higher. As a result, it is important to know how increased levels of exposure to solar UV-B radiation (280-315 nm) might affect the productivity of aquatic systems. In addition, the oceans play a key role with respect to global warming. Marine phytoplankton are a major sink for atmospheric carbon-dioxide, and they have a decisive role in the development of future trends of carbon dioxide concentrations in the atmosphere. The relative importance of the net uptake of carbon dioxide by the biological pump in the ocean and by the terrestrial biosphere is a topic of much current research. Phytoplankton form the foundation on which the very survival of aquatic food webs depends. Marine phytoplankton are not uniformly distributed throughout the oceans of the world. The highest concentrations are found at high latitudes while, with the exception of upwelling areas on the continental shelves, the tropics and subtropics have 10 to 100 times lower concentrations. In addition to nutrients, temperature, salinity and light availability, the high levels of exposure to solar UV-B radiation that normally occur within the tropics and subtropics may play a role in phytoplankton distributions. A major loss in primary biomass productivity may have significant consequences for the intricate food web in aquatic ecosystems and affect food productivity. It has been estimated that a 16% ozone depletion could result in a 5% loss in phytoplankton, which equals a loss of about 7 million tons of fish per year. Biological effects of small changes in UV-B exposure may be difficult to determine because the biological uncertainties and variations are large, and the baseline productivity for pre-ozone-loss eras is not well established. Phytoplankton productivity is limited to the euphotic zone, the upper layer of the water column in which there is sufficient sunlight to support net productivity. The position of the organisms in the euphotic zone is influenced by the action of wind and waves. In addition, many phytoplankton are capable of active movements that enhance their productivity and, therefore, their survival. Like humans, phytoplankton cannot perceive, and thereby avoid, UV-B radiation. Exposure to solar UV-B radiation has been shown to affect both orientation mechanisms and motility in phytoplankton, resulting in reduced survival rates for these organisms. Researchers have directly measured the increase in, and penetration of, UV-B radiation in Antarctic waters, and have provided conclusive evidence of direct ozone-related effects within natural phytoplankton communities. Making use of the space and time variability of the UV-B front associated with the Antarctic ozone hole, researchers assessed phytoplankton productivity within the hole compared to that outside the hole. The results show a direct reduction in phytoplankton production due to ozone-related increases in UV-B. One study has indicated a 6-12% reduction in the marginal ice zone. In recent years, there has been increased interest in UV-B effects on macroalgae and seagrasses. In contrast to the phytoplankton, most macrophytes are attached to their growing site, thereby restricting them to specific growth areas and the resultant exposure to UV-B radiation. Recent studies have demonstrated that photosynthesis is inhibited in many red, brown, and green benthic algae. Solar UV-B radiation has been found to cause damage to early developmental stages of fish, shrimp, crab, amphibians and other animals. The most severe effects are decreased reproductive capacity and impaired larval development. Even at current levels, solar UV-B radiation is a limiting factor, and small increases in UV-B exposure could result in significant reduction in the size of the population of consumer organisms. At high latitudes (over 40 degrees N) the late-spring increases in UV-B exposure may affect some species because the UV-B enhancement occurs at critical phases of their development. Even small increases or temporary fluctuations in UV-B may affect relatively sensitive species. Recent studies have addressed the potential impact of chlorofluorocarbon substitutes and their degradation products. Some hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs), notably HFC134a, HCFC123, and HCFC124, are degraded, generating trifluoroacetic acid (TFA) as their main product. TFA is mildly toxic to most marine and freshwater phytoplankton. It is still speculative if TFA is concentrated in the food web. Even if produced well into the next century, TFA is unlikely to reach toxic levels for oceanic phytoplankton; however, it could reach toxic levels in restricted aquatic systems. Although there is overwhelming evidence that increased UV-B exposure is harmful to aquatic ecosystems, the potential damage can only be roughly estimated at the present time.
