Effects on aquatic ecosystems

Regarding the effects of W-B radiation on aquatic ecosystems, recent scient ific and public interest has focused on marine primary producers and on the aquatic web, which has resulted in a multitude of studies indicating mostl y detrimental effects of UV-B radiation on aquatic organisms. The interest has expanded to include ecologically significant groups and major biomass p roducers using mesocosm studies, emphasizing species interactions. This pap er assesses the effects of UV-B radiation on dissolved organic matter, deco mposers, primary and secondary producers, and briefly summarizes recent stu dies in freshwater and marine systems. Dissolved organic carbon (DOC) and particulate organic carbon (POC) are deg radation products of living organisms. These substances are of importance i n the cycling of carbon in aquatic ecosystems. UV-B radiation has been foun d to break down high-molecular-weight substances and make them available to bacterial degradation. In addition, DOC is responsible for short-wavelengt h absorption in the water column. Especially in coastal areas and freshwate r ecosystems, penetration of solar radiation is limited by high concentrati ons of dissolved and particulate matter. On the other hand, climate warming and acidification result in faster degradation of these substances and thu s enhance the penetration of UV radiation into the water column. Several research groups have investigated light penetration into the water column. Past studies on UV penetration into the water column were based on temporally and spatially scattered measurements. The process of spectral at tenuation of radiant energy in natural waters is well understood and straig htforward to model. Less known is the spatial and temporal variability of i n-water optical properties influencing UV attenuation and there are few lon g-term observations. In Europe, this deficiency of measurements is being co rrected by a project involving the development of a monitoring system (ELDO NET) for solar radiation using three-channel dosimeters (UV-A, W-B, PAR) th at are being installed from Abisko (North Sweden, 68 degrees N, 19 degrees E) to Tenerife (Canary Islands, 27 degrees N, 17 degrees W). Some of the in struments have been installed in the water column (North Sea, Baltic Sea, K attegat, East and Western Mediterranean, North Atlantic), establishing the first network of underwater dosimeters for continuous monitoring. Bacteria play a vital role in mineralization of organic matter and provide a trophic Link to higher organisms. New techniques have substantially chang ed our perception of the role of bacteria in aquatic ecosystems over the re cent past and bacterioplankton productivity is far greater than previously thought, having high division and turnover rates. It has been shown that ba cterioplankton play a central role in the carbon flux in aquatic ecosystems by taking up DOC and remineralizing the carbon. Bacterioplankton are more prone to W-B stress than larger eukaryotic organisms and, based on one stud y, produce about double the amount of cyclobutane dimers. Recently, the mec hanism of nitrogen fixation by cyanobacteria has been shown to be affected by UV-B stress. Wetlands constitute important ecosystems both in the tropic s and at temperate latitudes. In these areas, cyanobacteria form major cons tituents in microbial mats. The organisms optimize their position in the co mmunity by vertical migration in the mat, which is controlled by both visib le and W-B radiation. Cyanobacteria are also important in tropical and sub- tropical rice paddy fields, where they contribute significantly to the avai lability of nitrogen. Solar UV radiation affects growth, development and se veral physiological responses of these organisms. On a global basis, phytoplankton are the most important biomass producers i n aquatic ecosystems. The organisms populate the top layers of the oceans a nd freshwater habitats where they receive sufficient solar radiation for ph otosynthetic processes. New research strengthens previous evidence that sol ar UV affects growth and reproduction, photosynthetic energy-harvesting enz ymes and other cellular proteins, as well as photosynthetic pigment content s. The uptake of ammonium and nitrate is affected by solar radiation in phy toplankton, as well as in macroalgae. Damage to phytoplankton at the molecu lar, cellular, population and community levels has been demonstrated. In co ntrast, at the ecosystem level there are few convincing data with respect t o the effects of ozone-related W-B increases and considerable uncertainty r emains. Following UV-B irradiation, shifts in phytoplankton community struc ture have been demonstrated, which may have consequences for the food web. Macroalgae and seagrasses are important biomass producers in aquatic ecosys tems (but considerably smaller than phytoplankton). In contrast to phytopla nkton, most of these organisms are sessile and can thus not avoid exposure to solar radiation at their growth site. Recent investigations showed a pro nounced sensitivity to solar W-B radiation, and effects have been found thr oughout the top 10-15 m of the water column. Photoinhibition can be quantif ied by oxygen exchange or by PAM (pulse amplitude modulated) fluorescence. Surface-adapted macroalgae, such as several brown and green algae, show a m aximum of oxygen production at or close to the surface; whereas algae adapt ed to lower irradiances usually thrive best when exposed deeper in the wate r column. Mechanisms of protection and repair are being investigated. UV effects on aquatic animals are of increased interest. Evidence for UV ef fects has been demonstrated in zooplankton activity. Other UVB-sensitive aq uatic organisms include sea urchins, corals and amphibians. Solar UV radiat ion has been known to affect corals directly. In addition, photosynthesis i n their symbiotic algae is impaired, resulting in reduced organic carbon su pply. Amphibian populations are in serious decline in many areas of the wor ld, and scientists are seeking explanations for this phenomenon. Most amphi bian population declines are probably due to habitat destruction or habitat alteration. Some declines are probably the result of natural population fl uctuations. Other explanations for the population declines and reductions i n range include disease, pollution, atmospheric changes and introduced comp etitors and predators. W-B radiation is one agent that may act in conjuncti on with other stresses to affect amphibian populations adversely. The succession of algal communities is controlled by a complex array of ext ernal conditions, stress factors and interspecies influences. Freshwater ec osystems have a high turnover and the success of an individual species is d ifficult to predict, but the development of general patterns of community s tructure follows defined routes. There is a strong predictive relationship between DOC concentration and the depth to which UV radiation penetrates in lakes. Since DOC varies widely, freshwater systems display a wide range of sensitivity to UV penetration. In these systems, increased solar UV-B radi ation is an additional stress factor that may change species composition an d biomass productivity. The Arctic aquatic ecosystem is one of the most productive ecosystems on ea rth and is a source of fish and crustaceans for human consumption. Both end emic and migratory species breed and reproduce in this ocean in spring and early summer, at a time when recorded increases in W-B radiation are maxima l. Productivity in the Arctic ocean has been reported to be higher and more heterogeneous than in the Antarctic ocean. In the Bering Sea, the sea-edge communities contribute about 40-50% of the total productivity. Because of the shallow water and the prominent stratification of the water layer, the phytoplankton are more exposed and affected by solar W-B radiation. In addi tion, many economically important fish (e.g., herring, pollock, cod and sal mon) spawn in shallow waters where they are exposed to increased solar UV-B radiation. Many of the eggs and early larval stages are found at or near t he surface. Consequently, reduced productivity of fish and other marine cro ps is possible but has not been demonstrated. There is increased consensus, covering a wide range of aquatic ecosystems, that environmental W-B, independent of ozone-related increases, is an impor tant ecological stress that influences the growth, survival and distributio n of phytoplankton. Polar ecosystems, where ozone-related UV-B increases ar e the greatest and which are globally significant ecosystems, are of partic ular concern. However, these ecosystems are characterized by large spatial and temporal variability, which makes it difficult to separate out UV-B-spe cific effects on single species or whole phytoplankton communities. There i s clear evidence for short-term effects. In one study a 4-23% photoinhibiti on of photosystem II activity was measured under the ozone hole. However, e xtrapolation of short-term effects to long-term ecological consequences req uires various complex effects to be accounted for and quantitative evaluati on remains uncertain. (C) 1998 UNEP. Published by Elsevier Science S.A. All rights reserved.