Ecosystem Effects

What is the effect of UVB on the biosphere? While the UVB photobiology of individual species can be studied and characterized, evaluating the quantitative aspects of the ozone layer (with or without depletion) relative to species interactions or ecosystem-level processes has proven to be very difficult. First, concerted research efforts on ecological aspects of ozone depletion were initiated a decade after ozone depletion had begun (1970s). Organisms that are being studied today are already the result ofpopulation responses to increasing UVB. Second, isolating UVB effects from those of other solar wavelengths and the myriad of other environmental variables that regulate physiology, growth, and reproduction poses technological challenges. Third, is the issue of determining the proper biological endpoints to use as a measure of ecosystem UVB stress.

Characterizing UVB exposure in a particular environment is also not straightforward. Long-term studies are required to compensate for the high degree of temporal variability in UVB fluences caused by seasons and weather. There are limited data available on incident UVB fluences from before the 1970s, and it is only with the discovery of ozone depletion that networks for long-term monitoring are being established (see http:// www.biospherical.com/NSF/ for international listing of programs).

Biodiversity

Differential species and life-history responses to UVB exposure have dictated species distributions and shaped population and community structure over geologic time. One of the largest concerns about the impact of increased UVB caused by recent ozone depletion is that the changes in ambient UVB may have occurred on a much more rapid timescale that can naturally be accommodated by adaptation and natural selection. This would be especially true for unicellular organisms and those with short life-cycle times. Even now, UV-sensitive species may have already been replaced by more UV-tolerant taxa. Larger organisms are usually considered less susceptible to the direct effects of UVB, but more likely to be impacted through UVB-induced changes at lower trophic levels. Assessing changes caused by ozone depletion is a challenge as minimal baseline data (pre-1970s) relating to UVB effects are available.

The issue ofvariable species responses to UVB is more than expecting a shift in the taxonomic structure of communities. For example, in unicellular organisms, size can play a key role in UV tolerance. Smaller cells tend to be more sensitive than larger cells. In aquatic environments, many organisms are size-selective filter feeders. Restructuring the size distribution of microorganisms could have ramifications at all trophic levels and include functional aspects such as reorganization ofniches, alterations in trophic transfer, and changes in pathways of biochemical cycling.

Trophic Dynamics Primary productivity

The majority of ecological research on ozone depletion has focused on primary production in both aquatic and terrestrial systems. Numerous studies have demonstrated enhanced photosynthesis in algae and vascular plants when UVB wavelengths are excluded in experimental exposures. Conversely, reduced photosynthesis is observed in laboratory and mesocosm studies when UVB is enhanced. The results from field and lab clearly indicate that ambient levels of UVB limit global primary productivity and justify concerns that increasing UVB levels would lead to declines in primary production.

UVB limitation on photosynthesis can be manifested in a number of ways. Phytoplankton cells may maintain a constant cell size and divide less frequently, or maintain a constant division rate and produce smaller cells. Vascular plants can grow as tall, but have smaller leaves. The impact of UVB on primary productivity in terrestrial environments is often positively correlated to rainfall, with significant differences in the same area between wet and dry years. Secondary effects of UVB exposure can also be important. These include increased synthesis of UV-absorbing compounds, thickening of leaf cuticles and epidermal tissues, changes in morphology, and increased susceptibility to disease. Some of these physiological and structural alterations can result in reduced palatability and nutritional value, directly affecting the transfer of energy to higher trophic levels.

Consumers

Smaller consumer species may suffer from the direct effects of UVB and exhibit stunted growth, reduced reproduction, and increased fatalities. This is true in aquatic systems where some microheterotrophs are more sensitive to UVB exposure than their photosyn-thetic food source. Large consumers often have adequate size and external protective layers so that the direct effects of UV exposure are minimal with molecular damage limited to surface cells. In these organisms, deleterious UVB effects are more likely translated through the food web where lower trophic levels are negatively impacted.

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