Duplicate, 8-compartment, continuous-flow microcosms were used to study the effects of acid addition on community function, algal community structure, and degradation of a plasticizer, diethyl phthalate. Inputs of HCl decreased the alkalinity (measured as CACO3) from 25 to 8 mg/l, creating diurnal H+ activity curves that indicated that the ecosystem was being severely stressed. Removal of excess acid was accompanied by a return to a normal diurnal pH cycle. Nutrient concentrations and O2 production did not give a definite indication of stress resulting from the addition of acid. Algal community structure and total biomass were not affected by acid inputs. Also, degradation rates of diethyl phthalate by the aquatic bacteria were similar for the control and the acid-stressed systems.