EVALUATING AMMONIA TOXICITY IN SEWAGE EFFLUENT TO STREAM MACROINVERTEBRATES .1. A MULTILEVEL APPROACH

A multi-level approach incorporating instream biological response, wat er quality, and toxicity testing was used to evaluate the toxicity of ammonia in sewage effluent to macroinvertebrates in two Ozark border s treams. Macroinvertebrate community compositions at sites upstream fro m effluent discharge were most similar, while communities at upstream vs downstream sites were least similar. Upstream sites had abundant ma yflies, blackflies, and caddisflies, which were absent immediately bel ow effluent discharge. Water quality was also different at upstream vs downstream sites and was significantly correlated with differences in macroinvertebrate communities (r(2) = -0.66, P<0.05) when data for al l months were combined. However, differences in water quality could no t consistently be explained by ammonia, which contributed from <1% to 31% of total variation in water quality among sites. The failure of ob served NH3-N concentrations to consistently explain differences in wat er quality and macroinvertebrate community composition among sites in field studies was corroborated by results of 96-h, static-renewal, amm onia toxicity tests conducted on Chironomus riparius in undiluted sewa ge effluent and well water. Ammonia concentrations measured in-stream were not toxic to C. riparius in toxicity tests. By using a multi-leve l 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.