There is nearly undisputed evidence that human activities have caused a diminution of stratospheric ozone, especially in the south polar vortex region, and this diminution has led and will continue to lead to increased levels of UV-B incident at the surface of the earth. There is also widespread agreement that ozone-related increases in UV-B has the potential to cause wide-ranging direct and indirect effects on aquatic ecosystems [SCOPE 1993, Hader Worrest Kumar Smith UNEP 1994 irradiation] as enumerated in the introduction. Damage at the molecular, cellular, population and community levels has been demonstrated (at least in phytoplankton). At the ecosystem level there are few, if any, convincing data with respect to effects of ozone-related UV-B increases, considerable uncertainty remains, there is no consensus, and much future work remains.
An important area of convergence concerns the BWF for photoinhibition of photosynthesis. While all results are not in exact agreement, the work of several groups are consistent with the high-resolution work of Cullen and co-workers (Fig. 1) in showing a function heavily weighted in the UV-B but with a significant component in the UV-A region. Thus, ozone-related UV-B increases are an environmentally relevant issue with respect to phytoplankton, but the magnitude of the potential impact is less than what would be inferred by use of a DNA (deoxyribonucleic acid) action spectrum. This convergence on an accurate BWF is an important step toward quantitative predictive modeling of potential UV-B effects on phytoplankton.
Another important area of agreement is that individual phytoplankton species demonstrate differential sensitivity to UV-B as was suggested by early workers [Calkins Thordardottir 1980, Worrest 1983]. Virtually all recent UV-related aquatic research results show, or are consistent with, this hypothesis of differential sensitivity. While there is general agreement on the existance, there is little agreement over the possible consequences, of differential sensitivity. That a community might show variable response to increased UV-B creates a key unpredictable factor: a `wild card' that could dramatically influence (through altered species composition, food web structure, differential impact on predator/prey components, etc.) ecosystem structure. Alternatively, differential sensitivity could merely be a demonstration of the high level of resilience and community-level adjustment of phytoplankton to increased UV-B levels. The uncertainties associated with the ecological consequences of possible altered species composition on communities and ecosystems impinges on two other important considerations: short-term variability vs. long-term consequences and trophic level interactions.
Smith, Prezelin and co-workers [Ray Smith Prezelin macintyre 1992] conclusively measured an impact on an Antarctic phytoplankton community within the ozone hole. They also pointed out that the magnitude of this impact was less than the presumed natural variability in southern ocean phytoplankton productivity. While the ecological significance and magnitude of this impact continues to be debated, the fact remains that chlorofluorocarbons (CFCs) generated primarily in the Northern Hemisphere have been linked to a measurable impact on a Southern Ocean phytoplankton community. Karentz [Karentz ozone depletion 1991] has observed that the ozone hole has now been in existence for over a decade, thus any potential ecological effects may have already been initiated, yet none have been conclusively documented. She also observed that baseline (pre ozone hole) photobiological data are lacking so that a quantitative assessment of any possible change is difficult. Smith and Baker [Ray Smith Baker science 1980] pointed out that productivity estimated using short-term incubations (e.g., 14C technique) are inadequate for assessing longer-term processes. At the ecosystem level, the work of Bothwell and co-workers [Bothwell Sherbot Roberge Daley 1993, Bothwell Sherbot Pollock 1994], demonstrates the complexity of trophic-level response to added UV-B and they emphasize the need for long-term (multigeneration times) autecological studies to assess ecosystem structure. Thus while uncertainties persist and the need for process oriented laboratory studies remains, there is a growing consensus that long-term ecosystem studies are required in order to answer ecosystem related questions. It is at the ecosystem level that the greatest interest, uncertainties and challenges remain.
Acknowledgments. Supported by NSF/DPP Grants No. DPP8917076 (RCS) and DPP901844 (JJC), NASA Grants No. NAGW-290-10 (RCS) and NAWG3549 (JJC) and NSERC Canada (JJC).