The primary motivation behind the considerable effort in studying stratospheric ozone depletion is the potential for biological consequences of increased solar UVB (280 -315 nm) radiation. Yet, direct links between ozone depletion and biological impacts have been established only for organisms of Antarctic waters under the influence of the ozone ''hole;'' no direct evidence exists that ozone-related variations in UVB affect ecosystems of temperate latitudes. Indeed, calculations based on laboratory studies with plants suggest that the biological impact of ozone depletion (measured by the formation of cyclobutane pyrimidine dimers in DNA) is likely to be less marked than previously thought, because UVA quanta (315-400 nm) may also cause significant damage, and UVA is unaffected by ozone depletion. Herein, we show that the temperate ecosystems of southern South America have been subjected to increasingly high levels of ozone depletion during the last decade. We found that in the spring of 1997, despite frequent cloud cover, the passages of the ozone hole over Tierra del Fuego (55°S) caused concomitant increases in solar UV and that the enhanced ground-level UV led to significant increases in DNA damage in the native plant Gunnera magellanica. The fluctuations in solar UV explained a large proportion of the variation in DNA damage (up to 68%), particularly when the solar UV was weighted for biological effectiveness according to action spectra that assume a sharp decline in quantum efficiency with increasing wavelength from the UVB into the UVA regions of the spectrum.cyclobutane pyrimidine dimer ͉ global change ͉ Gunnera ͉ Tierra del Fuego T he most important consequence of the depletion of stratospheric ozone is the increased transmission of solar UVB radiation to the Earth's surface. Present levels of stratospheric ozone are at the lowest point since measurements began in the 1970s (1). Ozone depletion is most pronounced over the Antarctic continent, where ozone levels commonly decline by more than 70% during late winter and early spring [data available in the NASA Total Ozone Mapping Spectrometer (TOMS) site: http:͞͞jwocky.gsfc.nasa.gov͞TOMSmain.html]. Acute effects of ozone depletion on native organisms have been documented only for marine ecosystems of Antarctic waters (for a review, see ref.2). For example, it has been shown that increased UVB can reduce phytoplankton photosynthesis in the marginal ice zone when the ozone hole is overhead (3), reduce phytoplankton cell densities (4), and increase the DNA damage burden in icefish eggs (5). Virtually nothing is known about the consequences of ozone depletion and increased solar UVB on natural ecosystems located outside Antarctica.We have set up a long-term experiment to study the responses of terrestrial ecosystems to elevated solar UVB resulting from stratospheric ozone reduction near Ushuaia, Tierra del Fuego, Argentina. The area, dominated by temperate forests, is on the southern tip of South America, across the Drake Passage from the Antarctic Peninsula, an...

Stratospheric ozone depletion occurs over Tierra del Fuego, southern Argentina and Chile, in the austral spring and summer due to the precession of the Antarctic ‘ozone hole’ and the general erosion of the ozone layer. Plots receiving either near‐ambient or reduced UV‐B radiation were established using different louvered plastic film filters over Sphagnum bog and Carex fen ecosystems in October 1996. In the Sphagnum bog system, growth measurements during the late spring and summer showed no significant differences in the moss Sphagnum magellanicum, or the vascular plants (Empetrum rubrum, Nothofagus antarctica, and Tetroncium magellanicum) between near‐ambient and attenuated UV‐B radiation treatments. In the Carex fen system, leaf length and spike height did not differ in the two dominant species, Carex decidua and C. curta, between UV‐B radiation treatments. The length of individual spikelets of C. curta under near‐ambient UV‐B radiation was less than under the reduced UV‐B radiation treatment, but this was not evident in C. decidua. No differences in seed number, seed mass, or viability were seen in either Carex species between the UV‐B treatments. Two important constituents of the microfauna that inhabit the Sphagnum bog are testate amoebae and rotifers. These both appeared to be more numerous under near‐ambient UV‐B radiation than under reduced UV‐B radiation. The subtle responses of the Sphagnum and Carex ecosystems may become more apparent in subsequent years as the treatments are continued. Trophic‐level changes, such as the differences in number of amoebae and rotifers, may be more sensitive to solar UV‐B radiation than growth and productivity of the vegetation.

Summary1 Plant growth and pigmentation of the moss Sphagnum magellanicum and the vascular plants Empetrum rubrum , Nothofagus antarctica and Tetroncium magellanicum were measured under near-ambient (90% of ambient) and reduced (20%) ultraviolet-B (UV-B) radiation for three growing seasons in a Sphagnum peatland in Tierra del Fuego, Argentina (55 ° S). 2 Reduction of solar UV-B increased height growth but decreased volumetric density in S. magellanicum so that biomass production was not influenced during the 3 years. The morphology of vascular plants tended not to respond to UV-B reduction. 3 A 10-20% decrease in UV-B-absorbing compounds occurred in T. magellanicum under solar UV-B reduction. No effects were seen on chlorophyll or carotenoids in S. magellanicum , although, for UV-B-absorbing compounds, a significant interaction between UV-B and year suggests some response to solar UV-B reduction. 4 The climate-related growth of the dwarf shrub E. rubrum was assessed retrospectively by correlating an 8-year record of annual stem elongation with macroclimatic factors including solar UV-B and visible radiation, precipitation and temperature. 5 No significant negative correlations were found between annual E. rubrum stem elongation and ambient solar UV-B, the ratio of UV-B : visible radiation, or the 305-nm : 340-nm irradiance ratio for an 8-year record (1990 -91 to 1997-98), nor was stem elongation affected by solar UV-B reduction in our experimental field plots after 3 years. 6 The role of solar UV-B radiation on plant growth in Sphagnum peatlands in Tierra del Fuego, Argentina, is likely to depend on the severity of stratospheric ozone depletion over the next several decades. The increases in ambient solar UV-B associated with ozone depletion over the last 20 years are less than the difference between our radiation treatments. Therefore, providing that the ozone layer substantially recovers by the middle of this century, only modest effects of increased solar UV-B on plant growth may be expected.