Tolerance and behavioral responses of freshwater zooplankton to ultraviolet radiation

While investigators agree that multiple factors, both abiotic and biotic, influence the distribution and abundance of zooplankton, less attention has historically been given to UV radiation (UVR). UVR is now known to penetrate deeply through the water column of many freshwater and marine systems, and exposure to UVR can lead to a decrease in the survival, growth, and reproduction of both freshwater and marine zooplankton.; This dissertation examines the relationship between UVR and distribution and abundance patterns of freshwater zooplankton inhabiting lakes of northeastern Pennsylvania. Laboratory experiments were first conducted to examine UV tolerance across zooplankton taxa. Results showed that, in general, cladocerans tend to be less UV-tolerant than rotifers and copepods, regardless of the UVR transparency of their source lake.; Based on these observations, it was then predicted that UV avoidance behavior would be greatest in species with low UV tolerance. More specifically, cladocerans should exhibit greater UV avoidance than rotifers or copepods. Cylindrical acrylic columns that either transmitted or blocked UVR were suspended in the eplimnion of high UV lakes to examine zooplankton vertical responses to UVR. Cladocerans (Daphnia spp.) consistently displayed a distinct avoidance of the surface waters in the presence of UVR but often showed a preference for the surface in the absence of UVR. Rotifers and copepods displayed little to no avoidance of UVR with similar distribution patterns in both light treatments.; A three year, multi-lake database was analyzed to further investigate UV tolerance and zooplankton seasonal and vertical distribution and abundance. UV exposure was modeled as a function of solar irradiance, lake transparency, and mixing depth, and linear regressions were performed to test for correlations between UV exposure and zooplankton distribution and abundance. A higher frequency of statistically significant relationships was observed in zooplankton taxa with low UV tolerance. Specifically, cladocerans tended to avoid the surface waters more during periods of high UV in the UV-transparent lake while rotifers and copepods showed little to no evidence for avoidance of high UV habitats. These results are consistent with the hypothesis that UVR influences the zooplankton community structure and dynamics of UV-transparent systems.