Comparative assessment of stream acidity using diatoms and macroinvertebrates: implications for river management and conservation

1. Macroinvertebrates and phytobenthic organisms (e.g. diatoms) are frequently used as bioindicators of water quality, yet few studies compare their effectiveness despite both being emphasized in the EC Water Framework Directive. 2. Here, as a case study, the efficacy of each group in assessing acid–base status in the catchment of the Welsh River Wye was evaluated from surveys in 2 years. 3. Ordination showed that both diatom and macroinvertebrate assemblages varied highly significantly with pH, alkalinity and calcium concentrations. Moreover, ordination scores were highly inter‐correlated between these groups in both study years. 4. There were also contrasts, with diatoms and macroinvertebrates changing in differing ways with catchment land‐use and channel hydromorphology. These differing responses suggest complementary indicator value, while variation in generation times between diatoms and macroinvertebrates suggests potentially contrasting speeds of response to variations over different timescales. 5. These data reveal that significant water quality problems in the River Wye, a proposed Special Area of Conservation, are generated from the continued acidification of low‐order, headwater streams and this has considerable significance for the objectives of the Water Framework Directive, and the EC Habitats Directive. Copyright © 2006 John Wiley & Sons, Ltd.