Bioassessment in a harsh environment: a comparison of macroinvertebrate assemblages at reference and assessment sites in an Australian inland river system.

The Lachlan River system of inland New South Wales, which extends into semi-arid areas, is prone to natural extremes of climate and water quality and has been almost entirely modified since European settlement in Australia. We used this system as a proving ground for the mainly qualitative bioassessment metrics for river macroinvertebrates that are used widely in Australia--the EPT (Ephemeroptera, Plecoptera and Trichoptera) index, the SIGNAL (Stream Invertebrate Grade Number Average Level) biotic index and the AUSRIVAS O/E (Australian River Assessment System Observed over Expected) index--plus a recently developed qualitative index, the observed proportion of potential taxa (OPP). We tested these metrics on their ability to discriminate between sites judged to be less disturbed by human activities (reference sites) and sites selected by a semi-random process and therefore expected to have a higher average level of human disturbance (assessment sites). All metrics except the AUSRIVAS O/E index differed significantly between the two types of sites at higher altitudes, with SIGNAL showing the greatest discrimination. Assessment at these altitudes was more effective if based on composite data from multiple mesohabitats rather than data from single mesohabitats. No metric differentiated the two types of sites in the more arid, lowland, floodplain region of the river system. We suggest that Australia relies too heavily on bioassessment concepts developed to assess water pollution in well-watered regions of the Northern Hemisphere. Effective assessment of human impacts on macroinvertebrates in the rivers of inland Australia requires a better understanding of the roles of flow regimes, including flood and drought sequences, and of microhabitat structure and invasive alien species. Quantitative approaches may also be required.