Today much information exists on the probability of DNA damage induction in aquatic organisms as a result of natural UYR exposure. It is now generally believed that UVR-induced DNA damage is a common feature in aquatic ecosystems based on studies dominated by measures of CPD induction. The present chapter addresses some key issues related to in situ DNA damage induction and repair in a variety of aquatic organisms. The penetration of DNA effective UVR will be addressed for various water types. Furthermore, factors that may control damage induction and repair will be considered. These include mixing, ozone depletion and long term acclimation. Species specific differences, cell size effects and interactions with other environmental (stress) variables, such as nutrient limitation are also considered. Consequences of DNA damage accumulation for organisms, community structuring and trophic interactions will be discussed. Finally, some suggestions are given with respect to future research directions.

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