Abstract

  The ecological impact of discharge to streams of domestic sewage waste stabilization lagoon effluent was investigated. Benthic invertebrates were compared upstream and downstream of discharges to eight New Zealand streams where effluent dilution ranged from 6- to 484-fold. The percentage of common invertebrate taxa whose density changed significantly (ANOVA, P < 0.05) downstream declined in proportion with the log of the effluent dilution (r = -0.87) and increased with downstream increase in benthic respiration (r = 0.91) and several inter- correlated indicators of organic enrichment (log biochemical oxygen demand, r = 0.91; log suspended solids (SS), (r= 0.84). However, these changes in invertebrate densities did not always reflect degraded community structure. The nature and direction of changes suggests a subsidy-stress gradient of responses. Increases in SS of > 4 g m-3 were associated with significant changes in density of > 50 percent of the common taxa and > 50 percent reduction of the densities of the sensitive Ephemeroptera, Plecoptera, and Trichoptera (EPT). However, EPT densities increased by up to 50 percent at lower organic solids loadings. No general relationships were found between relative densities of functional feeding groups and metabolic or water quality variables. These findings confirm that early warning of enrichment stress is more easily seen at the species level than at the functional level.

Keywords: streams, benthic invertebrates, stress ecology, ammonia toxicity, suspended solids, benthic respiration