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. UV is now known to penetrate deeply through the water column of many freshwater and marine systems, and exposure to UV can lead to a decrease in the survival, growth, and reproduction of both freshwater and marine zooplankton.

My dissertation research examined the relationship between UV and the distribution and abundance patterns of freshwater zooplankton inhabiting lakes of northeastern Pennsylvania, USA. 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 UV 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 UV were suspended in the epilimnion of high UV lakes to examine zooplankton vertical responses to UV. Cladocerans (Daphnia spp.) consistently displayed a distinct avoidance of the surface waters in the presence of UV but often showed a preference for the surface in the absence of UV. Rotifers and copepods displayed little to no avoidance of UV with similar distribution patterns in both light treatments. While negative phototactic responses to UV have been demonstrated in the laboratory, my experiments provided the first evidence of a zooplankton behavioral response to UV in nature.

A three year, multi-lake database was analyzed to further investigate the relationship between 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. Among lakes, the relationship between UV avoidance and taxon was evident only in the highest UV lake. These results are consistent with the hypothesis that UV influences the zooplankton community structure and dynamics of UV-transparent systems.