TL: THE EFFECTS OF OZONE DEPLETION ON MARINE ORGANISMS (GP) SO: Greenpeace International DT: February 1992 Keywords: atmosphere ozone oceans effects gp greenpeace reports / 1. Introduction As stratospheric ozone is depleted increasing levels of UV-B radiation will reach the earth's surface. Ozone protects both marine and terrestrial organisms from UV-B, the most biologically damaging component of ultraviolet radiation, and concern is increasing over the implications of increased exposure for both human health and the wider environment. Laboratory studies have long established that UV-B has deleterious effects on a wide range of organisms, but only recently has the importance of the observations in the context of ozone depletion been recognised. Normal levels of UV-B act naturally as one of the constraints on ecosystem structure and function but this effect will be amplified as ozone depletion progresses. The Antarctic ozone hole has been present for almost a decade and this has led one author to observe that the impact of increased UV-B will have already been felt and have initiated ecological changes (Karentz, 1991). This paper reviews the means by which UV-B damages organisms and how this may affect the marine environment. Antarctica, where ozone depletion is most profound and UV-B levels are double normal at certain times, is used to illustrate the possible impact on marine food webs. In addition some of the possible interactions with global warming are mentioned. 1.1. Mechanism of action of UV-B UV-B may damage organisms in a variety of ways largely related to the UV-B absorbence characteristics of cellular components or effects on specific photreceptors. There may be direct damage to DNA with mutagenic or lethal consequences or damage to RNA, proteins and other molecules. Resulting from these effects will be damaging changes in the normal functions of the organism such as impairment in plants of respiration and photosynthesis. Reduced growth, production or death may then be a consequence. 1.2 Protection from UV-B Organisms do have mechanisms by which they can protect themselves from UV-B damage or respond to it. Protective pigments may be produced or avoidance behaviour may be shown and there are mechanisms to repair damaged DNA. However, such protection is not without its cost as energy is diverted from other processes such as growth (Smith, 1989). In addition, avoidance reactions depend on organisms having detection systems for UV-B and this may not necessarily be the case for all organisms (Smith, 1989). Acclimation to UV-B may also be an important protection mechanism. A gradually increasing exposure to UV-B may allow for the production of protective pigments by organisms or for avoidance mechanisms to operate. Some species, however are unable; some of these bacteria are extremely sensitive to UV-B, Worrest & Hader (1989) have argued that UV damage to these species could result in increased requirements for artificial fertiliser with severe consequences for less developed countries. Voytek (1990) considers that the effects on bacteria could result in other serious effects on nutrient budgets. 3. Phytoplankton Much attention has been paved to the effects of UV-B on phytoplankton as they are restricted to the upper layers of the ocean and therefore relatively highly exposed. Phytoplankton are primary producers, responsible for over half the world's total biomass and are fundamental to many food webs (Worrest & Hader, 1989). The threshold of sensitivity to UV-B is, for some species, close to the mid-summer levels they currently experience. Relatively small elevations in UV-B following ozone depletion may, therefore, have serious consequences (Worrest & Hader, 1989). UV-B affects photosynthesis, motility and orientation of phytoplankton. The effects on motility and orientation are seen at much lower levels of W-B than that which inhibits photosynthesis (Hater, 1986). Therefore not only may increased UV-B directly reduce phytoplankton production by inhibiting photosynthesis it may also do this indirectly by reducing chances for survival and growth by interfering with the mechanisms which optimise their position in the water column (Worrest & Hader, 1989). As well as reducing production, another of the most important consequences of damage to phytoplankton could be that certain, more resistant organisms may survive whilst others die or are present in reduced numbers. It is conceivable that the loss of some species may allow the predominance of other toxic or less palatable species and thereby negatively influence the development and growth of species higher in the food chain (Worrest, 1982). 4. Zooplankton Zooplankton are generally more resistant to UV-B than phytoplankton. However, the larval forms of some species are killed, adult mortality may be increased and reproduction rates of survivors decreased by UV-B (Voytek, 1990). In some species the threshold sensitivity to UV-B is well within current mid- summer levels of UV-B and predicted UV-B levels in predicted ozone depletion scenarios. This situation has led Worrest & Hader (1989) to conclude: "Given a 16% ozone decrease over temperate pelagic waters, UV-B radiation levels at a depth of one metre would reach a lethal (50% mortality) cumulative radiation dose in fewer than five summer days for about half the zoo plankton species examined. Perhaps even more importantly, the threshold levels of W-B exposure would occur. Some observers consider that the overall effect of an elevated W-B irradiance on eggs and larvae would be small in comparison to losses from other causes (Hunter et al, 1982). However others consider that there would be a major effect through a knock on impact on the food-web. The assessment of Worrest & Hader (1989) is that: "Based on one assessment, a 5% decrease in primary production (estimated for a 16% ozone depletion) would yield reductions in fish yield of approximately 6 to 9%. A reduction in fish yield, if it occurred on a global basis, would then represent a loss of about 1010 kg of fish per year." Such a reduction in fish would not only affect availability for fisheries but would also affect those top predators such as seals and whales which depend upon them. It has been reported that UV radiation can cause cataracts in adult farmed fish (Allen, 1980). The significance and incidence of such cataracts in wild fish has not been described. 7 Marine Mammals and Birds Marine mammals and birds will be protected by their coat, skin pigmentation or plumage from directly damaging effects of UV-B. As in terrestrial animals it will be those unpigmented, exposed regions which could suffer damage such as the eyes. It is conceivable that the eyes of some bird and seal species which spend long periods on land could have an increased likelihood of cataract formation. The most likely impact, however, on marine mammals and birds would be via food-web effects as primary production is reduced. More time would have to be spent foraging for food and reproduction and growth may be affected (Voytek, 1990). The long generation times of higher species means that they will be less able to evolve adaptation strategies to cope with increased levels of UV-B in comparison to lower species. This is particularly important when the speed of progression of ozone depletion is considered. Antarctica Most information on the effects of UV-B radiation on ecosystems relates to Antarctica. The level of ozone depletion which occurs during the austral spring has led to markedly elevated levels of UV-B. In the spring W-B levels can reach levels expected in mid summer and in 1990 W-B levels were recorded in December that were twice those expected at a summer solstice with a normal ozone level (Frederick & Alberts, 1991). Species diversity in Antarctica may be altered (Worrest, 1982) as well as primary production. i) Although it has been postulated that rising levels of CO2 may stimulate phytoplankton and plant growth by increasing photosynthesis, which in turn could help remove excess CO2 from the atmosphere, any such effect may be counteracted by the damaging effects of UV-B on the same and other processes. It has been calculated that a loss of 10% of marine phytoplankton alone would reduce ocean uptake of carbon dioxide by an amount (5Gt) equal to annual emissions from fossil fuel consumption (Hater et al., 1991). ii) Coral reefs appear to be under particular stress at the moment. Although this probably results from a variety of causes increases in sea temperature are known to be damaging. Since W-B is also damaging to corals there may be an amplification of this effect as ozone depletion gets worse. iii) Increased UV-B may significantly reduce dimethyl sulphide (DMS) production by phytoplankton. DMS plays an important role in cloud formation and reduced formation may result in decreased cloud formation and thereby increase UV-B reaching the earth (a positive feed back). However, some global warming scenarios include increased cloud formation as a consequence of increased evaporation as global temperatures rise. 10. Conclusions i) Ultraviolet radiation can be directly damaging to all classes of organisms. Effects include reduced photosynthesis, survival, growth and reproduction. ii) Those organisms at the bottom of the food-web, for instance bacteria and phytoplankton, appear to be particularly sensitive. Reduced production at this level will have serious implications for species higher in the food chain. The effect is analogous to removing cans from the bottom row of a pyramid or stack - the whole stack collapses. iii) Decreased primary production associated with increased UV-B levels will affect human food availability as fish stocks are reduced and, because of reduced bacterial nitrogen assimilation, rice production will be reduced. iv) Some species have evolved protection mechanisms to avoid damage by UV-B. However, these mechanisms will have an energy cost and growth or reproduction may be reduced as a consequence. v) The selection pressure exerted by increased UV-B levels in Antarctica will already have initiated changes in the ecosystem. Damaging effects of UV-B on Antarctic species of micro plankton have been demonstrated and biological effects of UV-B can be detected at depths of 30 meters in Antarctic waters.