Johns, Desmond JustineIn recent decades eutrophication has fueled intense phytoplankton blooms that frequently negatively impact ecosystems. Dramatic pH elevations are commonly overlooked during such blooms, which may also be harmful. Cyanobacteria blooms in the Sassafras River, MD were sampled during Autumn 2008 and measured for primary and bacterial production (PP and BP), and differences in bacteria communities composition were examined. pH elevations above 8.8 in the field corresponded to decreased PP, but had no effect on BP. Laboratory experiments demonstrated that negative effects of pH were dependant on light intensity; PP increased with pH at moderate light intensities, but decreased at low irradiance. There was some evidence that BP is affected by high pH, although bacteria community differences as determined by DGGE were not. Negative effects of high pH are probably most important during spring and summer in low salinity environments when pH fluctuations are more common and last longer.en-USThesisBiology, LimnologyBiology, Oceanographybacterial productionpHprimary production