Irradiances

Solar radiati«»n

Figure 3. Schematic illustration of potential effects of solar radiation on phytoplankton, MPB and macroalgae photosynthesis. For benthic w algae, three zones are defined in term of the irradiance received: high, medium and low. While the high irradiance zone is clearly defined as the g intertidal zone, the boundaries between medium and low radiation are uncertain and will vary among different water bodies, species considered, q etc. The graphs are examples of UVR-induced photoinihibition in the three groups of algae considered here.

[135], simulating mixing conditions for Patagonian waters, showed an UVR-induced reduction of photosynthetic rates when the UML extended to a relatively small portion of the euphotic zone (ZUML/ZEph <0.5). When mixing was deep (•^UML/^Eph >0.8), and mean PAR levels were low, phytoplankton were able to use UV-A radiation for carbon fixation. The use of solar UV-A at low PAR irradiances has also been observed in Californian waters, with a 10-20% increase in photosynthesis due to this effect [136].

In terms of photosynthesis, studies have demonstrated that tropical phyto-planktonic species are more resistant to UVR than those from polar environments [43,83,111], probably due to their evolutionary light history with naturally high radiation levels. In addition, tropical organisms had a higher irra-diance threshold for photosynthesis inhibition [83] than polar species [43,137], thus providing an additional evidence of their resistance to high UVR levels. Solar radiation increases with altitude [138] and thus photosynthesis in lakes located at high altitudes might exhibit enhanced inhibition. The inhibition of photosynthesis, however, depends not only on the irradiance received at the lake surface, but also on the differences on water temperature, attenuation coefficients and phytoplankton composition among other variables [139]. Biological Weighting Functions (BWFs) [71,106] had also implied the higher resistance of tropical organisms to UVR [83] as compared to those from polar environments [109,137]. Figure 4 shows a comparison of different BWFs calculated for different geographic locations - Arctic, Antarctica, tropical lakes and temperate latitudes.

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Wavelength (nm)

Figure 4. Representative biological weighting functions of a laboratory culture and phytoplankton assemblages from different environments. The numbers in the labels indicate the reference from where the data were obtained.

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Wavelength (nm)

Figure 4. Representative biological weighting functions of a laboratory culture and phytoplankton assemblages from different environments. The numbers in the labels indicate the reference from where the data were obtained.

11.3.2 Long-term effects

During long-term experiments, species have the potential to acclimate to new radiation conditions, and processes such as DNA repair and synthesis of photo-protective compounds may occur [107,118,140,141] (see also Chapter 10). One of the best ways to test UVR effects on aquatic autotrophic organisms on a long-term basis is by using a "model ecosystem" or mesocosms [129], in which a parcel of the aquatic body is isolated and allowed to progress under similar conditions as in the natural environment. The main restriction of these experiments is that it is not possible to completely simulate natural conditions - e.g., water movements are restricted and larger organisms are normally excluded. Hence, one should be cautious when interpreting results obtained in these experiments, as other factors (e.g., immigration) are important components when addressing UVR effects from an ecological point of view (see also Chapter 4). Experiments carried out in polar areas [137] showed that, at the beginning of experimentation, both Arctic and Antarctic phytoplankton cells were significantly inhibited by UVR. This inhibition, however, did not increase as the experiment progressed, and growth rates (based either on chl-a content or carbon incorporation) were not significantly different between the UVR -I- PAR and the PAR treatments [137]. Kim and Watanabe [79] found that even though short-term exposure to UVR provoked a significant decrease of chl-a and photosynthetic rates in two freshwater phytoplankton species, Melosira sp. and Chlorella ellipsoidea, under prolonged UV-A exposure; however, the algae acclimatized by reactivation of the photosystem and enhanced cellular chlorophyll synthesis. Results from long-term exposure of freshwater phytoplankton are also very variable with no effects determined in an Alpine location [142], low impact of UVR in a community from a Canadian lake [143] and significant changes in phytoplankton composition in a lake from the Andes region [144].

Interactive effects of UVR with other ecological variables are important when addressing photosynthetic inhibition on a long-term basis. In particular, temperature seems to play a crucial role. For example, the temperate dinoflagellate Prorocentrum micans had a maximum decrease in photosynthetic rates after 21 days of exposure to solar UVR [145], whereas Antarctic phytoplankton had this maximum inhibition after 9 days [115]. In addition, research has been conducted to address the interactive effects of UVR and nutrient limitation. There was variability in the responses, with studies that revealed that nutrient-limited cultures were more sensitive to UV-B than those nutrient-replete [105,146]; however, Behrenfeld et al. [147] did not find growth inhibition produced by UV-B in nitrogen-limited cultures. Bergeron and Vincent [148] determined growth rates in different phytoplankton size categories present in a P-enriched system in a Subarctic lake and found different responses according to the wavebands to which cells were exposed.

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