Identification of macro-invertebrate taxa as indicators of nutrient enrichment in rivers

Eutrophication of fresh waters, especially from diffuse sources, is often a priority environmental issue for industrialised countries. Understanding the relationships between nutrient pressures and their impacts on ecology is essential for predicting the likely benefits of a programme of remedial measures to return nutrient concentrations to former levels. The aim of this study was to use mutual information to analyse the strength of association between macroinvertebrate families and nutrient levels (Total Oxidised Nitrogen and Total Reactive Phosphorus) in data covering rivers in England and Wales. Prior to the analysis the dataset was screened to minimise the confounding effects of organic pollution and split according to site type and season. Significance thresholds for the values of mutual information were calculated and the most significant indicator taxa were identified for each site type, season and nutrient pressure. It was found that in upland rivers the most significant indicator taxa were generally positive indicators, that is, their presence is indicative of high levels of at least one nutrient. In addition the number of significant indicators was greatest in upland rivers and least in lowland rivers.