Effects of ammonia on periphytic communities.

Laboratory tests were conducted to evaluate the chronic effects of ammonia on periphytic communities. Species richness of the protozoan component of these communities was affected at un-ionized ammonia concentrations of </= 0.01 mg NH3 litre(-1). A biologically important concentration was defined as the concentration of ammonia affecting 20% of species and was estimated from a concentration-response regression as 0.011 mg litre(-1). A comparable value based on literature reports of chronic toxicity to fish and invertebrates was 0.0126 mg litre(-1). Other non-taxonomic responses were equally sensitive to ammonia. Biomass (ash-free dry weight) and algal biomass (in vivo fluorescence) were significantly reduced even at the lowest tested ammonia treatment, 0.01 mg litre(-1), but the abundance of bacteria was reduced only in the highest treatment group, 0.43 mg litre(-1). Net community metabolism was reduced in all ammonia treatments. Periphyton communities were affected at levels below the USEPA chronic criterion of 0.027 mg litre(-1) (temperature = 8.8 degrees C and pH = 8.1). Successional maturity or age of the periphytic community affected the amount of biomass and algal biomass, but did not modify sensitivity to ammonia.