EfIects of ultraviolet radiation and dissolved organic carbon on the survival of subarctic zooplankton

High intensities of ultraviolet radiation are known to be harmful to aquatic biota, especially for species living in shallow, clear water bodies. Zooplankton species from such habitats are good model organisms to study the effect of changes in UV radiation, and how animals deal with this. We tested experimentally the effect of natural UV radiation, which was controlled by different filters and varying concentrations of UV-screening dissolved organic carbon (DOC), on the survival of the cladocerans, Daphnia longispina and D. pulex, and a calanoid copepod, Eudiaptomus graciloides. All species originated from subarctic Fennoscandia where underwater UV intensity is influenced by ozone depletion, changes in DOC ana timing of ice break-up. Measured as mortality, all species were affected by both UVB and UVA radiation. Survival was highest and similar between species in the dark controls and photosynthetically active radiation exposures. Under each UV filter, the highest DOC concentration provided significant and best shelter from UV radiation and led to best survival. Variation in survival was observed between species. E. graciloides responded more readily to changes in UV radiation than did the daphnids. In natural environments, species' previous exposure to fight and different protection strategies (pigmentation, vertical migration) are probably as important factors controlling the survival of zooplankton as radiation intensity and optical properties of water.