Factors controlling UV injury

Injuries induced by exposure to UVR are dose-dependent. A number of factors can influence dose including intensity of exposure, spectral composition of the irradiance, and duration of exposure. Each of these may be influenced by climate and habitat.

13.5.1 Climate conditions

Stratospheric ozone depletion has been the focus of many investigations concerning UV impacts. Depletion of ozone concentration in the stratosphere reduces the filtering capacity of the ozone layer resulting in increased UV irradiance reaching the Earth's surface. Ozone depletion of about 5% has occurred in the past decade over the United States, but has been considerably greater in the Southern hemisphere [75,76]. In addition, brief periods (1-2 days) of elevated UV can occur over localized regions as a result of irregularities in atmospheric ozone concentrations.

Increased UV dose can occur as a result decreased cloudiness. For example, the number of sunny days in Central America has increased significantly over the past decade [77]. Such changes in weather patterns were found to significantly correlate with the loss of amphibian species. Although clear sky irradiance would not likely vary greatly from day to day, aside from changes associated with solar angle, the cumulative UV dose would increase considerably compared to doses that occur under cloudy conditions. In addition, temperature and soil moisture would likely decline under such conditions. Individually and in combination, these stressors could be harmful and contribute to population decline.

Increased surface temperatures, possibly as a result of global warming trends, could lead to the stratification of the water column. For example, nutrients released from submerged aquatic vegetation could stay in the lower portion of the water column. This could then result in fewer algae in the upper water column. This could result in greatly increased water clarity and UV irradiance of the upper water column where a majority of juvenile forms of aquatic organisms occur. Surface waters are highly productive and numerous organisms would be affected by increases in surface irradiance.

13.5.2 Habitat characteristics

Degradation and destruction of aquatic habitats often include the removal of trees, rocks, aquatic vegetation, and other structures that protect aquatic organisms from solar radiation. There would be a greater solar impact in the water column in these habitats and greater potential of increased exposure of aquatic organisms to solar UV-B. Habitat degradation and destruction that include the loss of shading structures may be more important than ever with expected increases in solar UV-B. Thus, it is important that aquatic organisms have shade that protects them from much of the solar radiation spectrum.

Aquatic organisms have adapted to certain levels of UV-B and exhibit different levels of tolerance to UV-B. Species naturally adapted to high levels of solar radiation exposure would be more tolerant to high levels of UV-B than species not naturally adapted to high levels, especially in clear, shallow water. The presence of shading structures, including vegetation in riparian and littoral zones, is important. There should be sufficient numbers of trees along a stream bank or pond edge to provide sufficient canopy shading over the water column. Floating and submerged aquatic vegetation also provides protection from solar UV-B. Dewatering of the water column by irrigation or other diversion, or channelization results in erosion of stream banks and excess deposition in side channels can result in overexposure of organisms to solar UV-B. Aquatic organisms can also be exposed to sudden and intense levels of UV-B when they inhabit water that is turbid a large part of the year and then clears up during the summer. In general, any activity that increases water clarity, such as the release of reservoir water or the presence of introduced zebra mussels (Dreissena poly-morpha), could increase exposure of the vast majority of freshwater aquatic organisms to harmful levels of solar UV-B.

Water quality characteristics of the habitat can also affect the UV-B dose received by aquatic organisms. DOC concentration plays a major role in limiting UV-B in the water column and has been extensively investigated [78,43,79]. Investigations in Minnesota show that slight reductions in DOC resulted in dramatic increases in water column UV-B irradiance (Figure 8). DOC originates from diverse sources, especially terrestrial vegetation. The chemical composition of DOC may vary considerably among watersheds, reflecting the unique vegetation and soil chemistry of the site [80]. Therefore, it is likely that the UV filtering characteristics will vary as well. Moreover, UV irradiance in the aquatic habitat is considerably more dynamic than at the Earth's terrestrial surface, and can vary by orders of magnitude depending on DOC concentrations. Thus, conditions

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Figure 8. Average total UVB irradiance at a 10 cm water depth in amphibian habitats in Minnesota having dissolved carbon concentrations ranging from 6 to 26 mg L_1.

Figure 8. Average total UVB irradiance at a 10 cm water depth in amphibian habitats in Minnesota having dissolved carbon concentrations ranging from 6 to 26 mg L_1.

that influence DOC will significantly influence UV dose. Acidification from acid deposition breaks down humic and fulvic acids making them less effective in screening UV [79].

In aquatic habitats UV can interact additively or synergistically with certain contaminants, increasing their toxicity and severity of injury ([81], see Chapter 7). Chemicals of anthropogenic origin that have molecular characteristics similar to photoprotective substances may be altered by absorbed UV. This interaction may generate free radicals or singlet oxygen that can alter DNA, enzymes, or lipoproteins leading to cellular injury and rapid death. For example, polycyclic aromatic hydrocarbons (PAHs) and other components of crude and refined petroleum increase in toxicity by as much as 10000-fold in the presence of UV [82]. Unexpected acute mortality occurred among bluegill sunfish treated with the PAH anthracene and exposed to solar UV in outdoor artificial streams [83]. It was concluded that solar UV significantly enhanced the toxicity of anthracene to the fish. In a subsequent study with juvenile sunfish (Lepomis spp.), the acute toxicity of anthracene was photoenhanced by simulated UVR [84]. The authors also observed severe necrosis and loss of epidermal cellular layers in affected fish. UV may also change the chemical structure of the substance to a more toxic form. UV breaks down ferrocyanide compounds to release free cyanide, which is toxic to fish and amphibians [Figure 9 - 85,86]. Pesticides, plastics, and pharmaceuticals may also be transformed into more toxic substances [87], Thus, photo-

Figure 9. Average mortality of rainbow trout (Oncorhynchus mykiss) exposed to a fire-retardant chemical with and without ferrocyanide (YPS) alone and in the presence of UV.

[From Little and Calfee 85.]

Figure 9. Average mortality of rainbow trout (Oncorhynchus mykiss) exposed to a fire-retardant chemical with and without ferrocyanide (YPS) alone and in the presence of UV.

[From Little and Calfee 85.]

sensitization and sunburn-like lesions can occur at solar irradiance levels that would otherwise be harmless.

It is apparent that a variety of factors, acting singly or as multiple stressors, can contribute to UV-induced injury in freshwater organisms. Each species of freshwater organism will be susceptible to harmful levels of solar UVR depending on the conditions present at a certain point in time. If conditions are appropriate for UVR to penetrate sensitive cellular molecules, and if cellular repair mechanisms are unable to keep up with the rate of cellular damage, UV-induced injury is inevitable.

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