Effect of weir removal on the aquatic macroinvertebrate community of an urban river in the UK

Weirs are a globally abundant type of low-head dam that negatively alter river ecosystems. Their removal is an increasingly popular river restoration measure in many countries. However, it is unclear what benefits weir removal might provide for the typically depauperate macroinvertebrate communities of urban rivers, which are usually subject to a range of anthropogenic pressures. This Before-After Control-Impact Paired study tested whether the removal of a weir on an urban river in the UK resulted in a shift in the aquatic macroinvertebrate community towards a more natural state. Macroinvertebrates were sampled before and after removal, above and below the structure and at control sites over four years. Following weir removal, the proportion of macroinvertebrate families that were made up of mayfly, stonefly or caddisfly families (an indicator of stream health) underwent a statistically significant increase at the upstream impact sites. There was a concurrent statistically significant increase in two indices, the Lotic-invertebrate Index for Flow Evaluation and the Empirically-weighted Proportion of Sediment-sensitive Invertebrates index, suggesting weir removal had an ameliorating effect on flow and sediment stressors acting on the macroinvertebrate community upstream of the weir. ‘Reference samples’ for the study location were generated using the River Prediction and Classification System, which are predictions of what the macroinvertebrate community would likely be if the study site was in a near natural state. Non-metric multidimensional scaling ordination showed that the macroinvertebrate community upstream of the weir became more similar to the ‘reference samples’, suggesting that it had shifted towards a more natural composition.