Testing the pressure‐specific invertebrate index (PSI) as a tool for determining ecologically relevant targets for reducing sedimentation in streams

Summary Sedimentation is a major cause of river impairment and water pollution worldwide. However, setting an ecologically meaningful sedimentation target is proving challenging due to significant gaps in the understanding of quantitative links between sedimentation and ecological response as well as variability between different types of surface waters. This study evaluates the utility of a new pressure‐specific macroinvertebrate index, Proportion of Sediment‐sensitive Invertebrates ( PSI ), to act as a simple tool for measuring sedimentation impacts and setting ecologically relevant sedimentation targets. Five macroinvertebrate indices were calculated from 51 samples taken from 13 sampling locations across two neighbouring, but contrasting study catchments in spring and autumn 2010 and 2011. For four of these, environmental quality indices ( EQI s) were also calculated as a proportion of observed to expected ( O : E ) macroinvertebrate scores, which were predicted for a theoretical pristine invertebrate community using the River Invertebrate Prediction and Classification System model. Principal component analysis has shown a clear hydromorphological and sedimentation gradient within the two study catchments. A generalised hierarchical mixed model with site as a random factor and % fine bed sediment as a fixed factor found a significant relationship between PSI and O:E PSI and % fine bed sediment cover at reach‐scale sampling resolution over a moderate gradient of impact. Lotic Index for Flow Evaluation ( LIFE ) scores and E phemeroptera– P lecoptera– T richoptera ( EPT ) % abundance were also related to % fine bed sediment cover. However, PSI was more strongly related to % fine bed sediment cover than either LIFE or EPT % abundance. While PSI and O:E PSI were correlated with LIFE and O:E LIFE , PSI was not correlated with EPT % abundance, which suggests a differentiated response of these metrics to multiple stressors. The relationship between PSI and other invertebrate metrics should be subjected to further testing along a pronounced gradient of multiple stressors, as our findings suggest that PSI and % fine bed sediment cover have the potential to provide simple, sensitive and effective tools for setting of ‘twin’ ecological and physical sedimentation targets and add additional exploratory power to the existing suite of macroinvertebrate indices.