Comparing apples with apples: use of limiting environmental differences to match reference and stressor‐exposure sites for bioassessment of streams

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.

Does water abstraction from unregulated streams affect aquatic macrophyte assemblages? An evaluation based on comparisons with reference sites

“…This analysis provided no evidence that water abstraction from these unregulated streams was impacting on assemblages of aquatic and amphibious macrophytes in contrast to a previous analysis for the same sites based on fish (Chessman et al, 2008). The average taxonomic richness of macrophyte assemblages was virtually identical between reference and exposure sites, and MRPP and NMS did not reveal any overall compositional difference between the two types of sites.…”

“…Hence, macrophytes may recover rapidly when flow resumes (Wright and Berrie, 1986;Holmes, 1999;Westwood et al, 2006a,b). Fish, for which we detected some apparent impact of water abstraction (Chessman et al, 2008), are not so well adapted to drought, since none of the freshwater fish species in northeastern NSW is known to be capable of aestivation. Disturbance by floods may be more limiting to aquatic macrophytes than drought, since a high frequency of high-flow disturbance can eliminate aquatic macrophytes altogether (Riis and Biggs, 2003).…”

“…Thus, large-scale spatial variation in macrophyte survey data was very high, even in the absence of appreciable upstream water abstraction. For fish, the average BC similarity among the same sites was somewhat greater (Chessman et al, 2008). This does not necessarily translate to lower spatial variability of fish because measures of biological similarity among sites are dependent on sampling method and effort (Kerans et al, 1992;Cao et al, 2002).…”

“…This comparison revealed an apparent impact of abstraction on fish at some sites (Chessman et al, 2008). Here, we use the same approach to assess the 68 B. C. CHESSMAN, M. J. ROYAL AND M. MUSCHAL effect of abstraction on aquatic and semi-aquatic macrophytes at the same sites.…”

“…We have recently described a new method to compare a biological assemblage between a stream reach exposed to a stressor of concern (an exposure reach) and reference reaches with little, or preferably no, exposure to that stressor, while taking account of the role of other environmental factors that are associated with spatial variation in the assemblage (Chessman et al, 2008). This method, named the limiting environmental difference (LED) approach, takes a set of reference sites and selects those that are most appropriate for comparison with a particular exposure site.…”