Measurements in polar firn air (Butler et al. 1999) have shown that gases with the largest potential to deplete ozone (CFCl3, CF2Cl2, CF2ClCFCl2) are definitely released by anthropogenic sources. Current surface measurements of these compounds show that concentration growth rates have decreased since the Montreal Protocol (WMO 1999; Prinn et al. 2000). However, initial recovery of stratospheric ozone will not be detected much before 2010, and depletion due to halogens will probably recover during the next 50-100 years (Hofmann and Pyle 1999). These forecasts are uncertain due to the increasing consumption of O3-depleting substances by developing countries. Moreover, with respect to chlorine compounds, increasing concentrations of CO2, CH4 and N2O and the impact of climate change on stratospheric temperature and circulation may cause even larger changes in stratospheric O3 (IPCC 2001).
In this scenario, considering that C fixation plays a central role in ecological and climatic processes, the biological effects of springtime stratospheric O3 depletion (as much as 50 % during the last decade) and the increase in UV-B radiation (290-320 nm) reaching the surface of the Southern Ocean are of particular concern. During the austral spring and summer, high nutrient and sunlight availability at the receding edge of the pack ice promotes phyto-plankton blooms which account for a large proportion of total primary production in the Southern Ocean. Sea-ice meltwaters form an upper layer of relatively fresh water over a saltier deeper one, and this stratification concentrates algal blooms in near-surface waters. This highly productive upper layer is therefore most at risk from enhanced UV-B radiation, especially during the period of maximum O3 depletion.
The Antarctic O3 hole was first reported in 1985, but observations at Halley Station have shown that depletion began in the 1970s (Farman et al. 1985). Research on the biological and ecological consequences of O3 depletion for Antarctic ecosystems began some years later (e.g. Bidigare 1989; Voytek 1990; Karentz 1991) and were therefore hindered by the lack of baseline data; the response of Antarctic organisms was tempered by about 20 years of adaptation and species selection under enhanced seasonal UV-B radiation (Karentz 1994).
As the biological effects of UV radiation strongly depend on wavelength, and even differences of a few nanometres are important (Cullen et al. 1992; Helbling et al. 1994), small changes in O3 concentrations may disproportionate harmfulness of incident UV-B radiation. As a rule, DNA is the primary lethal target of UV-B but RNA, proteins and other molecules are also adversely affected by exposure. UV radiation may catalyse photochemical reactions in seawater and within algal cells, causing oxidative stress and impairing nutrient uptake, membrane transport and photosynthesis, thereby inhibiting growth and reproduction, and ultimately leading to death (Vincent and Roy 1993). The sensitivity of algae show large inter- and intraspecific variations, depending on avoidance strategies, number and efficiency of repair systems, physiological state and genotypic differences (Karentz 1994). During the last decade, several studies on the Southern Ocean have concentrated on O3-dependent shifts in in-water spectral irradiance and on alterations to spectrally dependent phytoplankton processes (photo-inhibition, -reactivation, -protection, and -synthesis; Smith et al. 1992; Helbling et al. 1994; Arrigo 1994; Boucher and Prezelin 1996; Neale et al. 1998; Bracher and Wiencke 2000). The results of these studies have often been used to estimate the loss of primary production. However, spatio-temporal variations in ozone depletion, cloudiness, sea-ice cover and vertical mixing make it difficult to reliably estimate the temporal pattern of phytoplankton exposure. The reported decreases in annual primary production therefore span quite a large range (from 0.1 to 12 %).
Recent research suggests several factors and processes which can contribute to a relatively small loss of primary production as a result of O3 depletion. Helbling et al. (1994), for instance, found that flagellates were much more sensitive to UV than diatoms, and that the latter tended to dominate the phy-toplankton crop in areas with a shallow upper mixed layer, while flagellates dominated crops at stations with deep mixed layers (more than 40 m). Mycosporine-like amino acids (MAAs), which occur in several algae taxo-nomic groups, play a photo-protective role against UV-B exposure. Bracher and Wiencke (2000) studied the effects of spectral exposure at normal and depleted stratospheric O3 concentrations on photosynthesis and MAA contents in natural phytoplankton communities; they found that only samples outside the phytoplankton bloom showed a significant decrease in the photo-synthetic production rate due to enhanced UV-B radiation. Models describing UV-influenced photosynthesis in the presence of vertical mixing (Neale et al. 1998) show that O3 depletion can inhibit primary productivity in Southern Ocean open waters, but the natural variability in exposure of phytoplankton to UV radiation, associated with vertical mixing and cloud cover, can enhance or diminish the effect on water-column photosynthesis. Interactions between vertical mixing and UV radiation have direct effects on photosynthesis and can also influence the acclimatisation and selection of phytoplankton. Thus, despite the small loss in primary production, there is no doubt that O3 depletion and UV-B radiation in the Southern Ocean constitute significant environmental stress. Possible cumulative impacts on interspecies variations and the pelagic food web cannot be excluded, although these impacts are difficult to foresee in communities which were tempered by about 20 years of adaptation and species selection under increased seasonal UV-B radiation.
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