Since the discovery of the Antarctic ozone "hole", many studies have been conducted to determine the effects of enhanced UV-B (280-315 nm) on photo-synthetic rates of autotrophic organisms. It is accepted now that even natural levels of UVR (280-400 nm) are stressful for some autotrophic organisms. In this chapter we will summarize what we know about the effects of UVR on the photosynthesis of aquatic organisms. Here we consider three major groups - phyto-plankton, microphytobenthos (MPB), macroalgae/marine angiosperms - which differ in many ways, especially in regard to their habitats. While phytoplankton live in the water column, MPB and macroalgae occupy the benthic environment. This creates substantial differences with respect to the amount and quality of radiation that they receive. Thus, although there is a common and general response to UVR of these autotrophic organisms - i.e., inhibition of photosynthesis - there are differences among the groups studied. These are mainly due to differences in the radiation conditions to which cells are exposed, as well as to the specific sensitivity/acclimation of the organisms under study. To evaluate the overall response of aquatic primary producers to UVR, it is crucial to consider the temporal scale of experimentation, to allow enough time for repair mechanisms and acclimation to UVR. Thus, short-term experiments frequently give an insight about the worst-case scenario for UVR effects on photosynthesis. We also review in this chapter the effects of UVR upon some related physiological processes (e.g., nutrient incorporation/assimilation, pigment synthesis/bleaching) and morphology (e.g., cell size) that may in turn affect the photosynthetic performance. Finally, to determine the impact of natural and increased levels of UVR upon aquatic ecosystems, we consider the interactive effects of other variables (pH, carbon dioxide concentrations, temperature, etc.) with UVR. Consequences for aquatic autotrophic organisms of increased UV-B levels due to ozone depletion events are still uncertain, but changes in biogeochemical cycles, community structure, and trophic web dynamics can be expected.

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