EFFECTS OF INCREASED SOLAR ULTRAVIOLET
RADIATION ON AQUATIC ECOSYSTEMS
D.-P. Häder (FRG), R.C. Worrest (USA), H.D. Kumar (India), and R.C. Smith (USA)
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 an 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°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 HFCs and 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.
Aquatic ecosystems balance terrestrial ecosystems in biomass production which are assumed to incorporate large amounts of atmospheric carbon into organic material with estimates between 90 and 100 gigatons (Gt, 109 tons) annually [Houghton and Woodwell, 1989; Siegenthaler and Sarmiento, 1993]. Therefore it is important to know what effect increased solar UV-B irradiation has on marine productivity and on the whole ecosystem depending on this productivity as well as climatological processes linked with it [Smith, 1989; Prézelin et al., 1993]. As only 0.5 % of the water surface is freshwater, the marine systems are by far the most important in the context of global carbon cycles. On the other hand, a recent workshop on freshwater ecosystems convincingly demonstrated that lakes are excellent model systems for studying larger marine environments, and many of these systems are locally important.
Since most macroalgae are restricted to coastal areas, the largest share in biomass production can be attributed to phytoplankton. Phytoplankton constitute the basis for the intricate food web in the oceans and thus are a prerequisite for the crop of fish, crustaceans and mollusks. Furthermore, as they are responsible for the uptake of half of the carbon dioxide from the atmosphere, any reduction in the uptake capacity would result in an increase in the greenhouse effect, with subsequent impacts on global climate change. Recent investigations indicate that many aquatic ecosystems are under considerable UV-B stress even at current levels [Häder, 1993a; Cullen and Lesser, 1991; Smith et al., 1992]. Being dependent on solar energy for photosynthesis, phytoplankton are restricted to the upper layers in the water column where they are exposed to high levels of short wavelength radiation. The subject of UV-B effects on aquatic ecosystems has been covered in a number of recent reviews [Häder, 1993b; Acevedo and Nolan, 1993; Weiler and Penhale, 1994; Cullen and Neale, 1994; U.S. DOE, 1993; SCOPE, 1992a,b; Holm-Hansen et al., 1993a,b; Tevini, 1993; Biggs and Joyner, 1994; Williamson and Zagarese, 1994; Karentz et al., 1994; Smith and Cullen, 1995].
Solar UV-B radiation has been found to affect DNA, to impair photosynthesis, enzyme activity and nitrogen incorporation, to bleach cellular pigments and to inhibit motility and orientation [Döhler et al., 1991; Häder et al., 1989, 1991; Worrest and Häder, 1989; Häder and Worrest, 1991]. DNA is one of the targets of radiation, but in addition a host of other chromophores and proteins are affected. Thus, UV-B does not damage one key target in phytoplankton but has many deleterious effects which differ in their action spectra. The action spectra are further complicated by antagonistic and repair processes stimulated by UV-A and visible radiation. Figure 4.1 shows the action spectrum based on irradiance response curves of UV inhibition of photosynthetic oxygen production in a mass biomass producer, the cyanobacterium Nodularia spumigena, isolated from the Baltic Sea.
In order to evaluate the effects of solar UV-B radiation on aquatic ecosystems a number of basic questions need to be answered:
Fig. 4.1. Action spectrum for the inhibition of photosynthesis in the cyanobacterium Nodularia spumigena.
Phytoplankton are not uniformly distributed in the oceans of the world (Figure 4.2); the highest concentrations are found in the high latitude regions while the tropics and subtropics show 10 to 100 times lower concentrations (with the exception of the upwelling areas on the continental shelves and near the equator). In addition to other factors including nutrients, light availability and water column stability, UV-B radiation may play a role in this, as irradiance here is about several times higher than in circumpolar areas [Häder, 1993b]. In temperate oceans phytoplankton blooms occur in spring and are reduced during summer. Sometimes there is a second bloom in autumn. Judging from this general pattern, significant increases in solar UV-B irradiation are expected to have detrimental effects on phytoplankton productivity. Field studies conducted under the Antarctic ozone hole have demonstrated some impact on primary productivity. Since depletions in global stratospheric ozone are expected to continue at all latitudes well into the next century [Stolarski et al., 1992] the effects of increased UV-B on marine primary productivity may also be relevant outside polar regions. The influence of UV-B on the abundance and distribution of phytoplankton remains a key uncertainty.
In the last few years the role of nano- and picoplankton have been investigated. In the past their existence was grossly underestimated because of their small size and technical problems during harvesting; today their contribution to the total biomass is estimated to be at least 40 %. A similarly important role and significant biomass productivity has been found for bacterioplankton which are responsible for degradation and cycling of organic matter in the sea. UV-B has been shown to strongly affect both bacterioplankton, as well as the extracellular enzyme activity of bacteria in subsurface waters [Herndl et al., 1993]. There are also indications of large populations of viral particles (107 per ml) in the oceans, the significance of which is not yet clear. The work of several workers [Karentz et al., 1994] suggests that small organisms (bacterial and microalgae), because of their size and short generation times, are likely to be more susceptible to UV stress than larger organisms. As a consequence, UV-B may play a key role in niche separation, lower food web processes and species composition shifts.
Fig. 4.2. Geographical distribution of global oceanic primary production based upon an optical model and satellite derived data (Byers, 1994).
The transparency of the water strongly depends on the water type [Piazena and Häder, 1994]: in coastal waters with high seston (particulate substances) and gelbstoff (yellow dissolved organic substances) concentrations UV-B may penetrate less than 1 m to the 1 % level; in contrast, in clear oceanic waters penetration to several tens of meters has been shown (Figure 4.3) [Smith and Baker, 1979; Smith et al., 1992]. Several recent papers contain information on bottom-ice algae and transmission of UV through ice [Ryan et al., 1992; Weiler and Penhale, 1994]. Until recently there were few in-water optical sensors capable of accurately measuring UV-B as a function of depth in aquatic systems. Recent comparisons by Kirk et al. [1994] suggest that several commercial instruments can, with care, be used to obtain quantitative underwater UV-B information. This is an important advance in studying the effects of solar short wavelength irradiation on aquatic ecosystems as it will permit more accurate estimates of the UV-B exposures that aquatic organisms receive.
Fig. 4.3. Penetration of solar radiation in Antarctic waters (65 ° 20' S, 73 ° 37' W) under thin overcast, solar zenith angle 49.2 °, ozone concentration 275 DU (Smith et al., 1992)Phytoplankton productivity is limited to the euphotic zone, the top layer of the water column, the lower limit of which is defined as the depth where photosynthesis balances respiration (typically the depth to which incident PAR (photosynthetic active radiation) irradiance is reduced to the 0.1 % level of surface radiation). The position of the organisms in the euphotic zone is influenced by physical processes, e.g., wind, waves and mixing. In addition, many phytoplankton are capable of active movements, and daily vertical migrations of up to 15 m have been measured. Light and gravity are employed to guide the organisms to depths of optimal irradiation resulting in typical vertical distribution patterns found in both freshwater and marine ecosystems [Lindholm, 1992; Eggersdorfer and Häder, 1991a,b]. Solar UV-B irradiation has been shown to affect both motility and the orientation mechanisms in phytoplankton [Häder, 1993a,b]. Organisms not actively motile, such as cyanobacteria, diatoms and even bacteria employ buoyancy to control their vertical position in the water column by producing gas vacuoles or oil droplets [Walsby, 1987; Walsby et al., 1992; Gosink et al., 1993].
Phytoplankton use various bands in the visible and UV-A range to orient with respect to light while UV-B is not used for photoorientation; thus, they are in a situation similar to humans. Phytoplankton are not able to perceive the detrimental radiation and cannot escape over-exposure if UV-B levels increase.
Field studies in Ghana showed that the percentage of motile filaments and linear velocity of several cyanobacteria as well as their orientation mechanisms with respect to light are affected within minutes by solar radiation. The damage was only partially repaired and only after short exposure times. In contrast, longer exposure times even resulted in increasing damage over a 24-h period [Donkor et al., 1993a,b]. Work carried out in India has indicated that solar UV-B affects the nitrogenase activity and carbon dioxide uptake in rice paddy cyanobacteria [Tyagi et al., 1991, 1992]. Some cyanobacteria, however, characterized by a brown color, seem to be better adapted to high solar radiation than closely related green forms.
The oceans play a key role with respect to global warming. A long-term global warming of surface air temperature by 1.5 - 4.5°C is predicted for a doubling of the CO2 concentration accompanied with a 1-m rise in sea level by 2080 [IPCC, 1990, Weaver, 1993]. As marine phytoplankton are a major sink for atmospheric CO2 they have a decisive role in future trends of carbon dioxide concentrations in the atmosphere as well as in terrestrial and aquatic ecosystems (Figure 4.4) [Bowes, 1993; Melillo et al., 1993].
Before the onset of anthropogenic carbon release the uptake and release of atmospheric carbon were balanced and the concentration of CO2 in the atmosphere was constant for extended periods of time. The increase in fossil fuel burning and deforestation results in an additional release of about 7 Gt of carbon into the atmosphere. However, long-term measurements indicate an annual deposit in the atmosphere of only 3 Gt. It can be assumed that the remaining 4 Gt are taken out of the global atmosphere by a net uptake by the biological pump in the ocean, a net uptake of CO2 by the terrestrial biosphere or a combination of both. The relative importance of the two uptake modes is the topic of much current research [Lampitt et al., 1993; Toggweiler, 1993].
Fig. 4.4. Annual carbon fluxes (in Gt) of natural and anthropogenic origin and the sizes of major reservoirs [after Houghton and Woodwell, 1989]Historical Secchi disk measurements (qualitative estimate of light transmission in the water) in the North Pacific show no evidence of a significant increase in phytoplankton biomass during this century [Falkowski and Wilson, 1992]. Though Secchi disk measurements are a surprisingly sensitive measure for phytoplankton biomass they do not reflect phytoplankton productivity. In contrast to terrestrial systems phytoplankton possess a very low standing crop with a high productivity. Under optimal conditions daily productivity may even equal the existing biomass. Therefore it is difficult to exactly predict the loss in carbon sink from decreases in phytoplankton biomass, and this remains a key uncertainty.
Smith et al. [1992] directly measured the increase in and penetration of UV-B into Antarctic waters and provided the first conclusive evidence of direct ozone-related effects on natural phytoplankton communities. Making use of the space and time variability of the UV-B front [Smith and Baker, 1989] associated with the polar vortex-driven ozone hole, they quantitatively evaluated phytoplankton production inside (ozone column thickness as low as 150 Dobson units, DU) compared to outside (300 DU) the hole. These workers suggest that the ozone-related damage to phytoplankton result in reduction in primary productivity by 6 - 12 % within the marginal ice zone of the Southern Ocean, and other estimates of different Antarctic areas range from 6 - 23 % [Weiler and Penhale, 1994; Holm-Hansen et al., 1993a]. The work of Smith and coworkers is distinguished by the inclusion of direct in-situ measurement of both incident and in-water UV-B so that quantitative evaluation of biological dose as a function of depth and time could be made [Smith et al., 1992; Prézelin et al., 1994]. An important result from this work is the quantitative measurement of a direct ozone-related negative impact on phytoplankton production in the Southern Ocean which is directly linked to human activities in the Northern Hemisphere. Recently, there has also been increasing interest in the Arctic as there are indications for a potentially developing Arctic ozone hole [Manney et al., 1994].
In situ incubations of natural phytoplankton assemblages in Antarctic waters indicated that UV-B under the ozone hole (150 DU) impaired photosynthesis by about 4.9 % while UV-A was responsible for about 6.2 % inhibition [Holm-Hansen et al., 1993a]. Similar ratios were found for tropical waters [Helbling et al., 1992], and screening of most UV <378 nm results in an increase in photosynthesis by 10 to 20 %. However, no significant decreases in stratospheric ozone have been detected in the tropics. Phytoplankton from below the mixing layer in tropical waters were very sensitive to solar radiation while surface plankton showed a high adaptation.
Recently McMinn et al. [1994], using high-resolution stratigraphic sequences from anoxic basins in Antarctic fjords, present findings suggesting that there have been no compositional changes in diatoms during the past 20 years of ozone hole development. They add that their findings are not necessarily applicable to the marginal ice edge and sea-ice communities nor are they relevant to non-diatom components of Antarctic phytoplankton communities. The ecological consequences of UV-B on phytoplankton communities remains a key uncertainty, and high latitude regions with relatively large decreases in ozone provide opportunities to quantitatively explore these consequences.
The induction of screening pigments has been found in marine and freshwater organisms [Karentz et al., 1991]. The pigment scytonemin has been isolated from cyanobacteria where it is induced by UV [Garcia-Pichel and Castenholz, 1991]. In addition, cyanobacteria as well as eukaryotic phytoplankton use several water soluble, UV-absorbing mycosporines as screening pigments. Other phytoplankton use carotenoids to dissipate the excess radiation energy from the photosynthetic pigments, and some have been found to even tolerate the unfiltered solar radiation at the water surface in tropical oceans.
In recent years there has been an increased interest in UV-B effects on macroalgae and seagrasses. In contrast to the phytoplankton, most macrophytes are attached to their growing site; therefore they are restricted to certain depth zones above, below or within the tidal zone. It is thought that this zonation is mainly caused by the visible light penetrating to this depth [Lüning, 1985]. If the UV-B/PAR ratio increases, the algae will be exposed to enhanced short wavelength radiation to which they may not be adapted. In recent studies no effect on respiration was found while photosynthesis was inhibited in many red, brown and green benthic algae. When using PAM (pulse amplitude modulation) fluorescence measurements, deep-water benthic algae were most sensitive while intertidal algae were least sensitive [Häder et al., 1994a,b; Larkum and Wood, 1993; Maegawa et al., 1993]. As in phytoplankton, the occurrence and induction by UV of screening pigments has been recorded in tropical red algae [Wood, 1989].
Phytoplankton are the basis for the intricate marine food webs; thus, any losses in biomass production necessarily cause decreases in biomass at the next higher trophic levels. Eventually these losses are relayed through all levels of the food web, ultimately leading to losses in fisheries yield [Nixon, 1988; Gucinski et al., 1990]. In addition to these indirect effects, solar UV-B radiation has been found to cause damage to early developmental stages of fish, shrimp, crab and other animals. The most severe effects are decreased reproductive capacity and impaired larval development [U.S. Environmental Protection Agency, 1987]. Even at current levels, solar UV-B radiation is a limiting factor, and small increases in UV-B exposure could result in significant reductions in the size of the consumer community [Damkaer, 1982].
In a recent ecosystem study an interesting effect was encountered: After some lag time, algal growth in an artificial stream was higher under UV-B than in the control. The explanation of this surprising result was that the grazers, larval chironomids, were more sensitive to UV-B than their food, the algae [Bothwell et al., 1994]. The result of this experiment reinforces the fact that predictions of responses by ecosystems to elevated UV-B exposure should not be based solely on single-species assessments.
Marine invertebrates differ greatly in their sensitivity to UV-B radiation [Hunter et al., 1982]. One crustacean has been found to suffer about 50 % mortality at current UV-B irradiances at the sea surface. Other shrimp larvae tolerate irradiances higher than those predicted for a 16 % ozone depletion [Damkaer and Dey, 1983]. The adult crustacean Thysanoessa raschii has a threshold sensitivity exceeding levels expected for anticipated ozone levels in spring. However, in summer a 50 % mortality cumulative radiation dose for about half the species examined would be reached in less than 5 days assuming a 16 % ozone depletion. UV-B kills most individuals of the common copepod Acartia clausii in culture and also reduces fecundity in the survivors. Similar inhibitions have been found in shrimp-like crustaceans and crab larvae in the Pacific Northwest. At high latitudes (over 40°N) the recently recorded late-spring increases in UV-B may affect some species as the UV-B enhancement occurs at critical phases of their development. Even sustained small increases or temporary fluctuations in UV-B may affect especially sensitive species.
Benthic organisms are also affected by UV-B radiation: cleavage in sea urchin eggs is impaired by ultraviolet radiation [El Sayed, 1988a]. Marine organisms associated with coral reefs, such as sponges, bryozoans and tunicates are similarly impaired. Melanins seem to serve as absorbing pigments since, e.g., several colored corals withstand high levels of radiation by production of the protective pigment S-320. Corals differ in their UV-B sensitivity depending on the depth at which they grow [Siebeck and Böhm, 1987]; also, the amount of the UV absorbing pigments decreases with depth [Maragos, 1972; Jokiel and York, 1982]. UV-B radiation seems to exert an oxidative stress on these invertebrate organisms as shown in sea anemones and octocorals [Shick et al., 1991].
Gleason and Wellington [1993] observed coral bleaching in the Bahamas that is not explained by increases in seawater temperature. Instead they showed that UV radiation under calm, clear water column conditions induced bleaching of reef-building corals. With increasing depth, colonies of Motastrea annularis showed a gradual reduction in mycosporine amino acids, indicating that deeper water colonies may be particularly vulnerable to sudden increases in UV radiation. Transplant experiments indicated that the organisms could not adapt to higher UV levels within a period of 21 days.
Freshwater crustaceans are also affected by solar UV-B. It is interesting to note, however, that Daphnia species from an alpine lake, where UV-B radiation is higher than in lowland lakes, are more intensely colored and tolerate higher UV-B doses [Siebeck and Böhm, 1987; Hessen, 1994].
Enhanced solar UV-B radiation directly reduces the growth and survival of larval fish [Hunter et al., 1982]. Based on these data for a region of the North American Pacific coastal shelf in June, a 16 % ozone reduction would result in increases in larval mortality of 50 %, 82 % and 100 % at the 0.5-m depth for anchovy larvae of ages 2, 4 and 12 days, respectively. Anchovy larvae occur in many regions coincident with high radiation levels between June and August with a peak in July. Because virtually all anchovy larvae in the shelf areas described occur within the upper 0.5 m, a 16 % ozone reduction level could lead to large increases in larval mortality.
Little and Fabacher [1994], comparing the sensitivity of rainbow trout and two threatened salmonids to UV-B radiation, have shown that UV-B is an additional, and often important, environmental stress. Inhabiting shallow headwater streams and lakes in high altitude locations, these fish showed considerable variability in response to simulated increases in UV-B with some showing skin injury as well as apparent suppression of their immune system.
Williamson et al. [1994], studying the impact of short-term exposure to UV-B radiation on zooplankton communities in north temperate lakes, suggest that UV-B in relatively clear lakes may prevent some species of zooplankton from fully exploiting warmer surface waters during periods of summer stratification. As a consequence, UV-B may be responsible for altering their ecological interactions with food resources, predators and other environmental variables.
Increased UV-B radiation may be responsible for recent population declines of some amphibians [Blaustein et al., 1994]. Species of amphibians differ in their ability to repair UV-B induced damage to their eggs or oocytes. The eggs of some frog species withstand exposure to sunlight better than those of some other frog species. The populations of certain species that lay their eggs in open water, strongly exposed to sunlight, have suffered drastic declines [Blaustein et al., 1994]. The egg-laying behavior and enzymatic repair activities suggest that certain amphibian species have become adapted in such a way as to minimize exposure of their eggs to UV-B radiation.
Some HFCs and HCFCs, notably HFC134a, HCFC123 and HCFC124, are degraded generating trifluoroacetic acid (TFA, CF3COOH) as their main product. TFA is neither photolyzed nor does it undergo any other physicochemical degradation. It is apparently not metabolized by plants, but microorganisms such as methanogens, sulfate reducers and aerobic soil bacteria have been shown to degrade TFA [Visscher et al., 1994]. Thus, TFA may be a global contaminant persistent over long times. Current levels in an industrialized area are 0.01 - 0.05 ng/m3 in air (Southern Germany) and 40 times these values are predicted for the year 2010. Further research is currently being conducted to determine whether these results reflect the global situation. Even with the event of continued production well into the next century, TFA is unlikely to reach toxic levels for phytoplankton in the oceans. In environmental niches such as vernal pools, TFA could be expected to accumulate to higher levels than elsewhere. The magnitude of this concentration effect should be similar to that observed for other solutes, which is in the range of 5 - 10 fold [Chumley, 1994].
TFA is mildly toxic to most marine and freshwater phytoplankton tested so far (EC50 1200 to 2400 mg/l). However, the freshwater green alga, Selenastrum capricornutum, showed an EC50 of 4.8 mg/l [Groeneveld, 1992]. Bioaccumulation factors have not been established for phytoplankton organisms; thus, it is still uncertain whether or not TFA is concentrated in the food web.
Though there is overwhelming evidence that increased UV-B is harmful to aquatic ecosystems, quantitative estimates are rudimentary at this stage. There is pressing need to increase our efforts to understand the possible long-term effects on aquatic ecosystems on a global scale. In order to evaluate the current productivity in the oceans and a possible decrease in the future, combined satellite and surface data are important tools. Additional focus on important biomass producers such as diatoms, dinoflagellates and cyanobacteria is necessary. 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 would cause a reduction in fishery and aquaculture yields of about 7 % which equals a loss of about 7 million tons of fish per year [Nixon, 1988]. Consequences of increased solar UV-B levels may be further complicated by unpredicted feedback loops and other changing factors such as temperature, salinity, CO2 concentration and different irradiation patterns caused by changing cloud cover.
However, biological effects of small changes in UV-B may be difficult to determine because the biological uncertainties and variations are large and furthermore the baseline productivity for pre-ozone-loss eras is not well established. Figure 4.5 summarizes the effects of UV-B on phytoplankton with their expected ecosystem consequences.
Fig. 4.5. Effects of enhanced solar UV-B on phytoplankton.The second major impact of decreased phytoplankton productivity may be a reduced sink capacity for atmospheric carbon dioxide which results in a faster development of the greenhouse effect and global climate change.
Prokaryotic microorganisms such as cyanobacteria and root nodule bacteria are capable of fixing atmospheric nitrogen in contrast to higher plants which can only utilize ammonia, nitrate or nitrite. Decreased nitrogen assimilation by prokaryotic microorganisms may lead to a nitrogen deficiency for higher plant ecosystems, such as rice paddies. Consequently, losses in nitrogen fixation due to increases in UV-B radiation may need to be compensated for by artificial nitrogen fertilization.
Both macroalgae and phytoplankton release organic sulfur compounds such as dimethylsulfide (DMS) which enter the atmosphere and serve as cloud condensation nuclei. Changes in DMS production may affect the atmospheric radiation balance. The time frame of the predicted changes in the ozone layer may not be sufficient for genetic adaptation to higher UV-B levels. Since different species differ in their sensitivity toward solar short wavelength radiation, shifts in species diversity may be a consequence. As a general rule, UV seems to affect smaller phytoplankton more than larger organisms. As primary feeders prey by size and not by species preference, this effect may also alter the subsequent links in the food web.
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