Combining rapid bioassessment and field-based microcosms for identifying impacts in an urban river

Abstract

Rapid bioassessment indices based on macroinvertebrates are the most commonly used tools for assessing stream condition. However, once stream degradation has been detected, it is often difficult to identify which environmental stressors are most important because of changes in multiple correlated factors. In this study, we examined eight sites in an urban river watershed using a field-based microcosm experiment and the rapid bioassessment-based biotic index, SIGNAL. The experiment assessed the effects of polluted river sediment by examining the macroinvertebrate taxa that colonized sediments at an unpolluted wetland. Results were compared with an assessment of field-collected macroinvertebrates using SIGNAL, a biotic index that assigns pollution sensitivity scores to macroinvertebrate families, and environmental data, to determine whether sediment pollution or other factors such as habitat deterioration were likely to be influencing riverine macroinvertebrate communities. The microcosm results indicated that common species (Tanytarsus fuscithorax, Procladius paludicola, and Ablabesmyia notabilis) and the overall macroinvertebrate assemblage did not significantly change among sediments from different sites, with the exception of local effects on a few uncommon taxa (Chironomus pseudoppositus, Kiefferulus martini, Cladotanytarsus australomancus, Chaoboridae, Polypedilum "S1," and Tanytarsus belairensis). In contrast, SIGNAL showed a gradual trend of deterioration from upstream to downstream, decreasing from a score of 6.5 in upstream areas (unimpacted) to a score of 4.4 in the downstream sites (moderately impacted). This result combined with a significant correlation of SIGNAL scores to habitat data suggested that habitat deterioration rather than polluted sediment was likely to be responsible for the declining stream condition detected with the rapid bioassessment approach. The addition of the microcosms to other monitoring approaches could be useful for determining whether sediment pollution is influencing degraded urban streams. © 2010 SETAC.