CIESIN Thematic Guides

Ultraviolet-B Effects on Aquatic Ecosystems

Ozone depletion results in increased ultraviolet-B (UV-B) doses at all depths of the seas and oceans, thereby extending the depth to which higher than ambient levels of biologically effective radiation penetrate. Studies conducted on the effects of UV-B on aquatic ecosystems have focused on the Antarctic region, where the largest losses of ozone have been measured. UV-B is known to affect adaptive strategies in phytoplankton, cause damage in DNA and proteins, and affect organisms in their developmental stages. These direct effects may lead to indirect effects, such as decreased primary productivity, changes in biodiversity, decreased nitrogen uptake by microorganisms, and reduced capacity for oceans to fix carbon dioxide. Häder, Worrest, and Kumar (1991) review these effects in the chapter "Aquatic Ecosystems" of the United Nations Environment Programme (UNEP) publication Environmental Effects of Ozone Depletion.

The main area of study related to UV-B effects on aquatic ecosystems has been phytoplankton, the primary producer at the base of the Antarctic food web. Decreases in mobility and orientation of phytoplankton have been observed, and impairment of these processes can affect growth and survival rates. Smith et al. (1992) report on a reduction in primary productivity of phytoplankton in the Antarctic region during the spring of 1990 in "Ozone Depletion: Ultraviolet Radiation and Phytoplankton Biology in Antarctic Waters." Because these organisms are at the bottom of the food chain, any decrease in productivity or population would likely affect higher species and trophic levels. Karentz, Cleaver, and Mitchell (1991) note in "DNA Damage in the Antarctic," however, that no catastrophic events have occurred in the decade-plus presence of the ozone hole.

Herndl, Muller-Niklas, and Frick (1993) discuss evidence of detrimental effects on bacterioplankton in "Major Role of Ultraviolet-B in Controlling Bacterioplankton Growth in the Surface Layer of the Ocean." Decreased bacterioplankton activity may lead to an increase in dissolved organic matter (DOM) in ocean waters as bacterial assimilation of DOM is reduced. Cyanobacteria, an organism important in nitrogen fixation, is also at risk. In "Effects of Stratospheric Ozone Depletion on Marine Organisms," Worrest and Häder (1989) report that cyanobacteria transform dissolved nitrogen in ocean water to nitrates and other forms that are accessible by higher plants. Reduction in cyanobacteria's ability to fix nitrogen is likely to affect growth of higher plant species such as rice, thus impacting the biochemical cycling of nitrogen.

Research on higher trophic levels has shown direct effects of UV-B on juvenile fish species, shrimp, and crab larvae. These effects include reduced reproductive capacity, growth, and survival rates. Direct effects on higher animals such as seabirds, seals, and whales are expected to be minimal. In "Addressing the Biological Effects of Decreased Ozone on the Antarctic Environment," Voytek (1990) discusses potential indirect effects on higher trophic levels based on decreasing productivity of the lower producers. Changes in species composition and biodiversity may result.

Climatic feedbacks may also be associated with ozone depletion. In "Ocean Biogeochemistry and Air-Sea CO2 Exchange," Williamson and Platt (1991) relate that a decrease in phytoplankton growth would reduce the uptake of carbon dioxide by the oceans, thus leaving more CO2 in the atmospheric reservoir. Increased atmospheric CO2 would have implications on global warming scenarios. In addition, planktonic production of dimethylsulfide (DMS), an important source of cloud condensation nuclei (CCN), may be altered. Charlson et al. (1987) address the potential impact on cloud formation and the radiative balance of the atmosphere due to changes in DMS in "Oceanic Phytoplankton, Atmospheric Sulfur, Cloud Albedo, and Climate." As with direct effects on marine ecosystems, however, such indirect effects are the result of many variables that need to be addressed before quantitative results can be predicted.