No abstract

Human impacts on freshwater biota are often assessed by comparisons with regional reference sites judged to be free or nearly free from the influence of stressors of concern. These comparisons may involve predictive models that extrapolate from reference-site data to estimate the biota that would occur at each assessment site in the absence of the stressors. This extrapolation often involves selection or weighting of data from particular reference sites, but it is seldom demonstrated whether this process results in a more accurate, precise and sensitive assessment than would be achieved from comparison with a simple unweighted combination of data from all reference sites, that is a null model. In addition, predictive models often rely on additive combinations of environmental predictor variables, but such combinations may poorly represent the natural control of spatial variation in biological communities by multiple limiting factors. In this paper, we describe a different type of reference-site approach, based on the concept of limiting environmental differences (LEDs) as natural constraints on biological similarity among sites. In this method, an assessment site is compared with a sub-set of environmentally matched reference sites selected by application of LED criteria to individual environmental variables. We illustrate this approach by its application to a potentially subtle impact: the effect of water abstraction on fish assemblages in unregulated streams in northeastern New South Wales, Australia. We compared (1) a LED-based predictive model, (2) a null model and (3) a model based on cluster analysis and multiple discriminant analysis in the style of the widely followed River Invertebrate Prediction and Classification System (RIVPACS). The LED-based model was about as accurate as the RIVPACS-type model and the most precise and sensitive of the three, being best able to distinguish sites where fish assemblages departed from reference status.

Abstract. 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.

No abstract

Aquatic and amphibious macrophytes were surveyed during two seasons at 85 sites on unregulated streams in northeastern New South Wales, Australia, during a period of prolonged and recurring drought. Fifty-four of these sites were designated as reference sites with respect to water abstraction because upstream entitlement for abstraction was less than 1% of their mean annual flow (MAF). The remaining sites had an average of 4% of MAF licensed for upstream abstraction (range 1-20%). No statistically significant overall difference in macrophyte assemblage diversity (number of taxa) or composition was detected between reference and non-reference sites. When each non-reference site was compared with those particular reference sites that it most resembled in non-hydrological environmental features relevant to macrophyte assemblages, the similarity between observed and reference data was unrelated to the amount of upstream entitlement for abstraction. The lack of any evident impact on macrophyte assemblages was attributed mainly to the relatively small proportion of flow licensed for abstraction, the fact that the study sites did not dry completely, and the resilience of stream macrophytes to drought. However, the difficulty in distinguishing abstraction impacts from high background spatial variability in macrophyte assemblages may also have been influential.