Rapid biological assessment charts the recovery of stream macroinvertebrate assemblages after sewage discharges cease

Rapid biological assessment (RBA) methods involving non-quantitative sampling of aquatic macroinvertebrates, sub-sampling of only a proportion of the organisms collected, and identification only to genus-level were used to assess responses over up to 7 years following the cessation of discharges from five sewage treatment plants into small streams in the Blue Mountains, Australia, and an increased volume of discharge into another stream. Multivariate analysis (ordination and analysis of similarities) did not always show an obvious recovery after discharge cessation: assemblages sampled upstream and downstream of discharge points were not always clearly closer in ordination space after cessation than before and statistically significant differences between upstream and downstream assemblages generally remained after cessation, apparently because of natural medium-scale spatial variability. However, the tolerance-based SIGNAL-SG biotic index demonstrated some recovery in all cases of discharge cessation, and a lag in recovery of about a year. A lesser recovery in the more urbanised catchments was attributed to persistent pollution from sources other than treatment plant discharges. The biotic index also revealed subtle biological change that was not predictable from monitoring data on nutrient concentrations. The SIGNAL-SG biotic index derived from RBA sampling provides an easily communicated measure of the effectiveness of investment in abatement of sewage impacts on stream macroinvertebrates in the Blue Mountains.