The decrease in stratospheric ozone concentration has received wide attention because the ozone layer protects the earth from harmful ultraviolet-B radiation (UVB, 280-320 nm). UVB radiation is harmful for organisms, and therefore scientific research into how UVB radiation affects organisms and ecosystems receives great interest. This thesis describes the effects of UVB radiation on interactions between phytoplankton (algae) and zooplankton (waterfleas) in freshwater systems. Research was conducted in laboratory experiments, and in field experiments in the Netherlands, Norway, and Canada.

The effects of UVB on the phytoplankton-zooplankton interactions were present, detectable, and mostly negative. The magnitude of the UVB effects was not large. It was not possible to make generalizations because of the species-specific reactions to UVB. The experiments were all short-term, and it may be erroneous to make long-term predictions based on these results. However, the subtle differences found in this study may be important in affecting ecosystem functioning. UVB radiation interacts with other environmental variables (such as temperature, nutrient concentrations, and vertical mixing), and, independent of the ozone hole, it is already playing a role in the functioning of an aquatic ecosystem.