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Phytoplankton

Considerable recent work, covering a wide range of aquatic ecosystems, have contributed to an increased consensus that environmental UV-B, independent of ozone related increases, is an important ecological stress that influences the growth, survival and distributions of phytoplankton. On a global scale, phytoplankton is the most important biomass producer in aquatic ecosystems. The organisms populate the top layers of the oceans and freshwater habitats where they receive sufficient solar radiation (photosynthetic available radiation, PAR) for photosynthetic processes. This layer is called the euphotic zone, the base of which is defined as the depth where gross daily photosynthetic carbon fixation balances phytoplankton respiratory losses over a day (typically a depth to which 1% to 0.1% of PAR penetrates). Within this zone, phytoplankton is simultaneously exposed to solar UV radiation, in addition to longer-wavelength radiation. Information required to quantitatively estimate ozone-related UV-B damage to phytoplankton include: the spectral characteristics of solar radiation penetrating to depth and its space-time variability; a biological weighting function (BFW) of the biological effect; an exposure-response curve (ERC); a quantitative model of phytoplannkton inhibition by UV-B; and an assessment of how vertical mixing influences variable irradiance exposures of phytoplankton and their physiological response. We are far from a full quantitative understanding but advances in each of these areas have been made during the past few years and recent reviews include (Smith and Cullen, 1996; Hader, 1997c; Cullen and Neale, 1997; Vernet and Smith, 1997).

    While it has long been known that DOC influences the penetration of UV-B, recent work has provided quantitative data which permits a more accurate estimated of DOC breakdown by UV-B, the resultant reduction of absorption in the UV region, and consequent increased penetration to depth. Among other things, this recent work permits a more accurate estimation of penetration of UV-B to depth based upon knowledge of in-water DOC concentrations. This may be especially important in assessing the potential influence of increased UV-B on freshwater ecosystems.

    Biologically weighting functions, describing the spectrally weighted sensitivity of phytoplankton photosynthesis to UV and visible irradiance, have recently been determined by a number of workers (Cullen and Neale, 1996; Boucher and Prezelin, 1996; Neale et al., 1998a). There is general agreement that the biological weighting, while highest in the UV-B, also contains a significant UV-A component. However, in spite of this broad agreement, biological weighting functions have been shown to vary by species, region, mixing characteristics of the water column and, perhaps, other environmental variability. As a consequence, it is now recognized that a single, or even a few, BWFs may be inadequate for a complete description of an ecosystem thus making quantitative analysis more complex.

    Exposure-response curves (Cullen and Neale, 1996, 1997a,b) have recently been determined. To estimate the ERC one must determine if the measured damage is a function solely of cumulative exposure or whether it is a function of exposure rate. As noted by these authors, this difference is fundamental, and it has an important impact on both the design and interpretation of experiments and on the extrapolation of experimental results to real-world predictions. Further, the shape of the ERC influences model accuracy. In addition, the ERC which is important for accurate modeling, shows a range of experimental variability (Neale et al., 1998a), likely dependent on the balance between damage and repair and, thus, on the time-scale considered. Recent work has shown both forms of ERC in phytoplankton from different hydrographic environments thus making accurate modeling of ozone-related impacts more complex.

    Models have been developed (Arrigo, 1994; Cullen and Neale, 1994; Boucher and Prezelin, 1996; Cullen and Neale, 1996a; Neale et al., 1998a) in an effort to estimate the impact of ozone depletion. These efforts represent important advances in our effort to quantify possible impacts, identifying the most significant processes and unknowns and evaluating uncertainities. However these recent advances continue to underscore the difficulty of using short-term observations to estimate longer-term (days to years) ecological response (Smith et al., 1992; Vincent and Roy, 1993; Bothwell et al., 1994; Cullen and Neale, 1994; Holm-Hanson, 1997; Neale et al. 1998a).

    Solar UV affects growth and reproduction, photosynthetic energy harvesting enzymes (Vassiliev et al., 1994; Herrmann et al., 1995a, 1996, 1997; Giacometti et al., 1996; Figueroa et al., 1997; Gieskes and Buma, 1997), and other cellular proteins, as well as photosynthetic pigment contents (Gerber and Häder, 1995a,b; Buma et al., 1996a; Peletier et al., 1996; Häder, 1997a). The uptake of ammonium and nitrate is affected by solar radiation in phytoplankton (Behrenfeld, 1995; Döhler,1996, 1997; Döhler and Hagmeier, 1997), as well as in macroalgae (Döhler et al., 1995a). Phytoplankton responds with the production of heat-shock proteins, as well as changes in the cellular amino acid pools. One of the major targets is the DNA, which strongly absorbs in the short-wavelength range of solar radiation. Solar UV-B has been found to induce DNA damage and DNA synthesis delay in many organisms (Scheuerlein et al., 1995; Buma et al., 1995, 1996b, 1997). UV-B effects have also been studied on the ecosystem level using mesocosms (Santas et al., 1996; Wängberg and Selmer, 1997).


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