In the following paragraphs we summarize the status of our knowledge about both short-term and long-term effects upon phytoplankton photosynthesis. However, several reviews dealing with the impact of UVR on phytoplanktonic organisms have been published [45,127-130], so we encourage the reader to refer to them for more specific details that are not addressed here.
One of the best-known effects of solar radiation upon phytoplanktonic organ isms is photoinhibition, which refers to the reduction of photosynthetic rates at relatively high irradiances . Many studies have used production-irradiance (P-I) curves to determine phytoplankton photoinhibition due to high PAR levels [132,133]; in addition, research has been carried out to determine the additional effects of UVR, not only in tropical  but also in temperate  and polar regions [19,133]. Interestingly, it has been shown that the relative effect of UVR (i.e., as compared to the PAR control) is sometimes higher at lower irradiances. On bright days, when high PAR levels already inhibit the photosystem, UVR produces a relatively lesser effect. This observation, however, depends on many variables, such as the light history of the cells and species composition. In addition, when phytoplankton cells are exposed to increased levels of solar radiation they may show a threshold for inhibition, which is followed by a steep increase in photosynthetic inhibition at mid-irradiances, levelling off at higher irradiance values [43,83]. However, in some cases no discernible threshold was determined [17,108].
In general, when in situ incubations are done, UVR causes a sharp decrease in photosynthetic rates (as compared with the PAR-only treatment) especially in surface waters (Figure 3). Even though UV-B radiation is more effective per unit energy (see Chapter 2), and hence potentially more damaging than those at longer wavelengths, many studies conducted in different locations have shown that UV-A is responsible for most of the photosynthetic inhibition, just because their natural levels are much higher [19,79,111]. Photosynthesis inhibition decreases with depth, depending, among other things, on water transparency, presence of microorganisms, as well as on incident radiation. The depth distribution of photosynthesis inhibition is highly variable and hence, surface values are not good indicators of the total inhibition in the water column as it has been demonstrated in a comparison between freshwater and seawater environments from mid-latitudes and sub-Antarctic areas . Furthermore, when evaluating the integrated photosynthesis inhibition, it is more important to consider the extent of the euphotic zone that is inhibited (e.g., optical depth), rather than the physical depth at which the inhibition is observed.
Inhibition of photosynthesis due to UVR is highly variable, depending on the irradiance/doses received by the cells, their specific sensitivity and acclimation potential, as well as the interaction with other variables that can mask the observed effects (mixing, temperature, pH, etc.). The daily integrated loss of carbon fixation in the euphotic zone in Antarctic waters was calculated to be about 4.9%, under normal ozone column concentrations . At the time of ozone depletion events, which are responsible for a relative increase in incident solar UV-B (see Chapter 2), there was a greater photosynthetic inhibition -reducing daily aquatic primary production by an additional ~4-12% [18,19]. However, taking into consideration the magnitude and timing of ozone depletion events, the yearly loss of carbon fixation in the Southern Ocean due to these processes was estimated to be <0.15% . In addition, some studies [21,22] have demonstrated that the effects of mixing, i.e., fluctuating radiation regimes (Chapter 4), are more important in affecting photosynthesis than the variations in ozone levels. Studies conducted with temperate phytoplankton
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