EFFECTS OF INCREASED SOLAR ULTRAVIOLET-RADIATION ON AQUATIC ECOSYSTEMS

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 man y countries, particularly the developing countries, this percentage is significantly higher. As a result, it is important to know how increa sed levels of exposure to solar UV-B radiation (280-315 nm) might affe ct the productivity of aquatic systems. In addition, the oceans play a key role with respect to global warming. Marine phytoplankton are a m ajor sink for atmospheric carbon-dioxide, and they have a decisive rol e in the development of future trends of carbon dioxide concentrations in the atmosphere. The relative importance of the net uptake of carbo n dioxide by the biological pump in the ocean and by the terrestrial b iosphere is a topic of much current research. Phytoplankton form the f oundation on which the very survival of aquatic food webs depends. Mar ine phytoplankton are not uniformly distributed throughout the oceans of the world. The highest concentrations are found at high latitudes w hile, 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 availabili ty, the high levels of exposure to solar UV-B radiation that normally occur within the tropics and subtropics may play a role in phytoplankt on distributions. A major loss in primary biomass productivity may hav e significant consequences for the intricate food web in aquatic ecosy stems and affect food productivity. It has been estimated that a 16% o zone depletion could result in a 5% loss in phytoplankton, which equal s 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 becaus e the biological uncertainties and variations are large, and the basel ine productivity for pre-ozone-loss eras is not well established. Phyt oplankton productivity is limited to the euphotic zone, the upper laye r 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 phyto plankton are capable of active movements that enhance their productivi ty and, therefore, their survival. Like humans, phytoplankton cannot p erceive, and thereby avoid, UV-B radiation. Exposure to solar UV-B rad iation has been shown to affect both orientation mechanisms and motili ty in phytoplankton, resulting in reduced survival rates for these org anisms. Researchers have directly measured the increase in, and penetr ation of, UV-B radiation in Antarctic waters, and have provided conclu sive evidence of direct ozone-related effects within natural phytoplan kton communities. Making use of the space and time variability of the UV-B front associated with the Antarctic ozone hole, researchers asses sed phytoplankton productivity within the hole compared to that outsid e the hole. The results show a direct reduction in phytoplankton produ ction 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 th eir growing site, thereby restricting them to specific growth areas an d the resultant exposure to UV-B radiation. Recent studies have demons trated 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 ra diation is a limiting factor, and small increases in UV-B exposure cou ld result in significant reduction in the size of the population of co nsumer organisms. At high latitudes (over 40 degrees N) the late-sprin g 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 sen sitive species. Recent studies have addressed the potential impact of chlorofluorocarbon substitutes and their degradation products. Some hy drofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs), notably HFC134a, HCFC123, and HCFC124, are degraded, generating trifluoroaceti c acid (TFA) as their main product. TFA is mildly toxic to most marine and freshwater phytoplankton. It is still speculative if TFA is conce ntrated in the food web. Even if produced well into the next century, TFA is unlikely to reach toxic levels for oceanic phytoplankton; howev er, it could reach toxic levels in restricted aquatic systems. Althoug h there is overwhelming evidence that increased UV-B exposure is harmf ul to aquatic ecosystems, the potential damage can only be roughly est imated at the present time.