Ozone is a highly reactive oxidant and is the primary constituent of photochemical smog. Since the industrial revolution, ozone levels in the troposphere have been rising and, during warm seasons, regions all over the world reach concentrations that exceed the US EPA's national health-based standard of 75 ppb. In the most polluted areas, maximum levels have reached over 400 ppb. Numerous studies have shown acute and chronic impacts of ozone pollution on the health of terrestrial plants and animals. Ozone is readily soluble in water and is often gravitationally deposited onto surface waters. This research is the first to examine the potential of tropospheric ozone as an aquatic pollutant by focusing on the effects of atmospheric ozone levels on algal and heterotrophic bacterial assemblages. Algae and bacteria were grown in three different ozone atmospheres (0, 80, and 250 ppb) and biomass and assemblage composition were measured. Individual experiments focused on 1) the community-level responses of natural, multi-division periphytic algal assemblages to these different ozone levels and the interactive effect of dissolved organic carbon (DOC), 2) the effect of elevated atmospheric ozone levels on heterotrophic bacteria within the above periphyton matrices and in the absence of algal interactions, and 3) the independent responses of diatom and cyanophyte communities to ozone pollution in the absence of interdivisional interactions. Ozone had both negative direct effects and positive indirect, interactive effects on algal biomass and assemblage composition. Within the natural periphyton assemblages ozone effects varied with algal division and DOC concentration. In the low DOC water, ozone effects were minimal. However in the high DOC water, the interactive effects of ozone and DOC were great. Diatom biomass was maintained at lower levels in ozone treatments but cyanophytes colonies increased by two orders of magnitude. This DOC and ozone interaction therefore led to a shift of assemblage dominance from diatoms to cyanophytes. Heterotrophic bacterial density in these periphyton films was closely correlated with algal biomass. Responses changed when each group was treated independently. The cyanophyte-only assemblages were directly affected by the oxidative stress created by ozone treated environments and biomass was significantly lower in the ozone treatments. Diatoms, in independent cultures, were unaffected by ozone treatment and heterotrophic bacterial growth was facilitated. This study indicates the potential of atmospheric ozone to cause ecologically significant changes to aquatic systems and highlights the need to consider direct and indirect effects of any potential ecosystem stressor, species interactions, and effects in different environments. Integrating the results of my experiments indicates that ozone has greater effects on algae and bacteria in high than low DOC waters, and that ozone may cause a shift toward cyanophytes in high DOC waters. I hypothesize this is due to ozone oxidation of DOC and release of organic and inorganic resources that stimulate growth, and diatom mucilages mediated oxidative stress of ozone on bacteria in the periphyton mat. Because ozone effects differed among algal divisions and heterotrophic bacteria, with different roles, atmospheric ozone may change microbial food webs and biochemical cycling within ecosystem, and these effects are likely more important in high than low DOC waters.
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