Secondary production of macroinvertebrates as indicators of success in stream rehabilitation

Hydromorphological rehabilitation is increasingly being used to reverse degradation of stream and river ecosystems. River rehabilitation projects have nevertheless been criticised for not meeting their goals or for not being monitored sufficiently well to assess whether their goals were met. There is therefore an urgent need to develop robust approaches to assessing treatment efficacy and to thus guide the increasing investment in rehabilitation. A headwater tributary of the River Welland in Leicestershire, UK was rehabilitated in summer 2014. Rehabilitation included removal of weirs and the creation of a distinct and meandered low-flow channel with pool-riffle sequences. Macroinvertebrates were collected in random sampling protocols stratified at in-stream biotope level. The rehabilitated reach was compared with a semi-natural upstream reach as a reference, and a similarly degraded reach as a control. This study demonstrated that rehabilitation of the River Welland had clear beneficial effects on ecological processes. It demonstrated that active re-creation of lost biotope heterogeneity improved macroinvertebrate biodiversity and secondary production, which is the formation of heterotrophic biomass through time. The rehabilitated reach yielded higher production estimates for Gastropoda, Bivalvia, Malacostraca, Odonata, Arachnida and Ephemeroptera, Plecoptera and Trichoptera, and a lower estimate for Chironomidae relative to the control reach. The rehabilitated reach also had higher estimates of production for Shredder, Scraper, Filter-feeder and Predator feeding groups relative to the control reach. Rehabilitation works transformed the reach from a condition more akin to the degraded reach to being more similar to the semi-natural reach. These outcomes demonstrate recovery of the reach's entire macroinvertebrate community structure and function after rehabilitation. The results provide a clear message to river rehabilitation practitioners: rehabilitation of the function of a physically degraded river ecosystem is possible if the rehabilitation is planned to actively restore the lost in-stream biotope diversity.