A diagnostic biotic index for assessing acidity in sensitive streams in Britain

Despite the history of freshwater biomonitoring, there is still a dearth of proven indices that allow accurate status assessment while simultaneously diagnosing the cause of impairment, particularly when stressors are multiple. Here, we present an approach to developing diagnostic indices where the sensitivity of biota is quantified using multivariate ordination. We applied the approach to the development of an index to detect acidity in British streams. Using a 197-site calibration dataset, we quantified variation in macroinvertebrate assemblages and determined which environmental variables best described the pattern. We then ranked taxa along an acid–base gradient, having first considered the merits of factoring out confounding variation from natural environmental factors. The response of the new species-level Acid Water Indicator Community (AWICsp) index to variation in base-flow and storm-flow pH and acid neutralising capacity (ANC) was quantified using independent data. Performance was also compared with existing family-level and species-level indices. AWICsp was consistently the species-level diagnostic index most clearly related to base-flow pH, storm-flow pH and ANC, accounting for 38–56% of the variation in acid conditions among the 76 test sites. Given the need to develop bio-diagnostic indicators, these data illustrate how organisms can indicate causes of stream impairment using robust and objective procedures, and when applied to strong environmental gradients such as acid–base status. We suggest that given the necessary calibration data, this approach could be applied successfully to other widespread stressors with equally strong biological effects such as organic pollution and fine sediment deposition, particularly if used in combination with RIVPACS-type predictive bioassessment models