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Field testing the AWIC index for detecting acidification in British streams

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Using data from 132 acid-sensitive streams in Wales and Scotland from two available data sets, we tested a recently proposed indicator system that uses aquatic invertebrates to detect acidification in British rivers, the Acid Water Indicator Community (AWIC). Although many sites held less than a quarter of AWIC scoring taxa, index values at sites in both data sets correlated significantly with acid-base variables including pH, calcium concentration, alkalinity and dissolved aluminium. Relationships with acid-base status during acid episodes were particularly strong in the more comprehensive test set. Strong correlations between the AWIC index and pH in a subset of sites from Scotland indicated potential for some extrapolation beyond the initial calibration regions of England and Wales. Despite considerable pH variability within AWIC classes, measured base-flow pH was within 0.5 pH units of values expected from invertebrates at over 55–70 % of test sites, and within 1 pH unit at 87–100 %. This compares favourably with direct pH measurement, where samples at fortnightly-monthly intervals are typically required to confidently estimate mean pH within 0.7–1.2 pH units. These data, drawn from a realistic application, illustrate the potential accuracy and integrating value of invertebrate bioassessment in acid-sensitive waters. Further developments should involve more specific identification and targeted calibration to i) improve discrimination between sensitive streams of contrasting acidity; ii) further improve the accuracy of pH determination based on invertebrates; iii) avoid the apparent over-estimation of pH in the lower AWIC classes; iv) clearly differentiate between acid-sensitive and acidified streams.
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Document Type: Research Article

Publication date: 2006-05-01

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