The purpose of this study was to examine the direct and indirect effects of ultraviolet-B (UV-B) radiation on freshwater protists. Laboratory experiments were conducted in order to determine the importance of photoenzymatic repair (PER) of UVBinduced DNA damage in the heterotrophic nanoflagellate Paraphysomonas sp. Investigations into the combined effects of UV-B and chromophoric dissolved organic matter (CDOM) were conducted in laboratory experiments in which protist cultures were exposed to UV-B radiation in the presence and absence of water amended to a higher CDOM concentration in order to determine the ability of CDOM to act as a UV-B filter and as a potential nutrient source. Field experiments examined the responses of natural communities of protists and bacteria to ultraviolet radiation (UVR) in the presence of high and low concentrations of CDOM. Ultraviolet-B radiation (UV-B; 280 – 320 nm) negatively affects many aquatic organisms, including heterotrophic flagellates, by directly damaging DNA. The quantity of UV-B reaching the surface of a lake varies with atmospheric chemistry, including stratospheric ozone, and the presence of large holes (Alldredge 1977) in this ozone shield during the last decade resulted in historically high UV-B levels. In aquatic systems, the nature of the damage to organisms depends on the intensity and duration of solar radiation, plus its attenuation in the water column. The amount of UV-B damage is highly dependent on the concentration of CDOM in the water column because CDOM strongly absorbs UV-B radiation. This protective role of CDOM is likely to be reduced in areas where warmer, drier climate decreases watershed runoff, which ultimately results in acidification and increased CDOM photodegradation. However, CDOM also may also act as an organic carbon source for bacteria and stimulate growth of the microbial food web, including bacterivores like heterotrophic flagellates. The effect of UV and CDOM interactions on the microbial food web is not well understood, but climate-related increase in CDOM in an oligotrophic lake could increase the heterotrophic microbial food web impact by reducing UV-B damage and increasing available resources. Since aquatic organisms, including protists, are differentially susceptible to UV-B radiation, climate change effects on CDOM and UV-B attenuation are likely to alter the ecology and community structure of aquatic systems. This thesis describes investigations into the direct and indirect effects of UV-B radiation on freshwater protozoa. The role of PER of direct UV-B induced DNA damage was examined in laboratory experiments that compared the survival and population growth of the heterotrophic nanoflagellate Paraphysomonas sp. at two environmentally relevant temperatures. The results from these experiments demonstrated the reliance of Paraphysomonas sp. on PER, with 100% mortality in the absence of the photorepair radiation that activates photoenzymatic repair enzymes. The ability of Paraphysomonas sp. to recover from exposure to UV-B radiation declined in flagellates adapted to 15°C relative to the same exposures at 20°C. Experiments examining the direct and indirect effects of UV-B radiation and CDOM on freshwater protists conducted in the laboratory and in an oligotrophic lake in the Pocono Mountains showed that potential DNA damage resulting from UV exposure is reduced and microbial growth may be enhanced with an increase in CDOM concentration. / Biology