Evaluating ammonia toxicity in sewage effluent to stream macroinvertebrates: I. A multi-level approach

A multi-level approach incorporating instream biological response, water quality, and toxicity testing was used to evaluate the toxicity of ammonia in sewage effluent to macroinvertebrates in two Ozark border streams. Macroinvertebrate community compositions at sites upstream from effluent discharge were most similar, while communities at upstream vs downstream sites were least similar. Upstream sites had abundant mayflies, blackflies, and caddisflies, which were absent immediately below effluent discharge. Water quality was also different at upstream vs downstream sites and was significantly correlated with differences in macroinvertebrate communities (r²=–0.66, P<0.05) when data for all months were combined. However, differences in water quality could not consistently be explained by ammonia, which contributed from <1% to 31% of total variation in water quality among sites.The failure of observed NH₃-N concentrations to consistently explain differences in water quality and macroinvertebrate community composition among sites in field studies was corroborated by results of 96-h, static-renewal, ammonia toxicity tests conducted on Chironomus riparius in undiluted sewage effluent and well water. Ammonia concentrations measured in-stream were not toxic to C. riparius in toxicity tests. By using a multi-level approach, a more realistic evaluation of ammonia toxicity in sewage effluent to macroinvertebrates was obtained than by using methods that focus on only one aspect of effluent toxicity.