High tropospheric ozone events are increasing in both frequency and intensity throughout rural and metropolitan areas.  The destructive impacts of high atmospheric ozone on terrestrial systems are well studied, but little is known about the effects of ozone on aquatic systems, beyond its use as a disinfectant.  We compared the effects of three levels of atmospheric ozone on benthic algae: 0ppb O₃; 80ppb O₃, chosen to simulate ambient summer conditions through much of the US; and 250 ppb, chosen as an extreme but not unseen atmospheric ozone level.  We compared algal biomass and species composition, bacterial cell density, and dissolved organic carbon, over a four week period, between experimental treatments.  Results indicate that DOC breakdown was present in both ozone treatments and algal communities in elevated ozone progressed more rapidly to a bluegreen dominant community.   This compositional switch appears to be greater in the high DOC environments and ANOVA analysis shows a significant interactive effect between ozone and DOC on algal community composition.  Previous studies have shown a similar shift toward green algae, suggesting that diatoms may be less tolerant of higher ozone atmospheres.