Effects of increasing acidity on aquatic protozoan communities

Laboratory tests were conducted to evaluate the chronic effects of increased acidity on protozoan communities. Taxonomic richness of the protozoan component of these communities was severely affected at pH ≤ 5.33. Taxonomic composition was significantly affected at much higher pH. A permissible concentration was defined as the pH affecting no more than 20% of the species and estimated from concentration-response regression as 6.92 (5.92 to 8.04) pH units. A similar value estimated from more conventional chronic toxicity tests was 6.28 (6.16 to 6.45) pH units. Bacterial abundance decreased at pH levels ≤ 5.34 pH units. Other nontaxonomic responses (e.g., biomass, algal biomass, fungal biomass, net oxygen metabolism) were not sensitive to increases in acidity.