| Ultraviolet radiation (UV) is an important biological stressor in many aquatic ecosystems. The effects of UV on zooplankton communities may be modified by multiple factors. The chromophoric components (CDOM) of dissolved organic matter (DOM) are the primary attenuator of UV in many aquatic systems and can confer nutritional benefits to zooplankton. Temperature influences the UV response of organisms that utilize photoenzymatic repair of UV-induced DNA damage. Additionally, species interactions are potentially important, as UV-tolerant organisms may be indirectly affected by UV through interactions with UV-sensitive species. The purpose of my dissertation was to determine how these factors influence the response of zooplankton to UV in transparent lakes.; In Lake Giles, Pennsylvania, USA microcosm experiments with treatments differing in UV exposure and species composition demonstrated that UV had a positive effect on the abundance and reproduction of Leptodiaptomus minutus, a calanoid copepod, but this effect was not related to zooplankton species interactions. Chlorophyll stimulation in the presence of UV indicates that L. minutus may benefit from changes in phytoplankton abundance or composition. Results from another in situ experiment indicated that DOM benefits some zooplankton in a high-UV environment, especially the cladoceran Daphnia, but DOM may be detrimental under low-UV conditions. Large-scale in situ mesocosms were used to test the hypothesis that diel vertical migration (DVM) behavior of zooplankton is affected by DOM, as DVM is driven by light- and UV-avoidance in some species. The results indicate that Daphnia and diaptomid distribution are driven by light- or UV-avoidance, but DOM influences cyclopoid and nauplii distribution through other factors, potentially food distribution. Lastly, experiments were conducted in subalpine lakes of the Absaroka-Beartooth Wilderness Area to test the interactive effects of DOM and temperature on the UV response of a calanoid copepod. Reproduction of the copepod was lower in the presence of UV at colder temperatures and low DOM levels, but was not influenced by UV at warmer temperatures or elevated DOM levels. The results from these four studies suggest that DOM and temperature are important variables in the response of zooplankton to UV in transparent lakes. Climate change will likely alter these variables. |
