No abstract

Hydromorphological rehabilitation through installing large woody material (LWM) is increasingly being used to reverse degradation of stream ecosystems. There have been many criticisms of stream rehabilitation projects, because many have not met their goals and many others have not been monitored well enough to assess whether their goals were met. In a before–after–control design (with samples collected 1 year before and two successive years after LWM installation), instream biotopes and their macroinvertebrate assemblages were used as structural and functional units to assess the effectiveness of LWM installed at the Rolleston Brook, a headwater tributary of the River Welland in Leicestershire, UK. The project was successful in enhancing the coefficient of variation of channel water depth and width, wetted surface area, number of instream biotopes, and the biotope diversity in the rehabilitated reach. LWM installation led to significant increases in macroinvertebrate total density, total biomass, and taxon richness. Macroinvertebrate community composition was also enhanced, so that it became more similar to that of the control reach. Small increases in the number of instream biotopes (appearance of gravel and leaf litter) and changes in biotope proportions (decreasing percentage of silt) were significantly related to changes in the macroinvertebrate community metrics in the rehabilitated reach. The results show that using macroinvertebrate community composition is more effective than only using taxon richness and/or diversity metrics for understanding the relationship between LWM installation and macroinvertebrate community responses. To be effective, samples must also be collected in a predefined sampling protocol stratified at the instream biotope level. This approach would be of great benefit in evaluating biodiversity conservation value, and could be incorporated into the advice provided by Natural England concerning restoration and protection of English rivers that are designated as Sites of Special Scientific Interest under UK legislation or Special Areas of Conservation under European legislation.

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.

Many river rehabilitation projects have been criticized for failing to meet their goals or for being insufficiently monitored. There is, therefore, an urgent need to develop robust approaches for assessing treatment efficacy and thus guide the increasing investment in rehabilitation. Instream biotopes (formerly called ‘functional habitats’ or ‘mesohabitats’ by different authors) and their macroinvertebrate assemblages were used to assess the effectiveness of entire‐channel hydromorphological rehabilitation of a 1.8‐km stretch of a lowland stream through the town of Market Harborough, Leicestershire, UK. The project successfully enhanced the physical diversity, measured in the rehabilitated reach as the coefficients of variability for channel water depth and width, wet surface area, number of instream biotopes, and biotope diversity. The project also enhanced the biodiversity conservation value, measured as macroinvertebrate total density, total biomass, richness, diversity, Ephemeroptera, Plecoptera, and Trichoptera (EPT) richness, EPT diversity, EPT count%, and EPT biomass%, all of which significantly increased following rehabilitation. Chironomidae count% and biomass% significantly decreased following rehabilitation. Rehabilitation was also successful in significantly increasing macroinvertebrate shredder, scraper, and filter‐feeder group density. Changes in the macroinvertebrate community metrics in the rehabilitated reach were related significantly to changes in the percentages of cover of instream biotopes and increases in biotope diversity in the rehabilitated reach. Macroinvertebrate structural and functional metrics can provide quantitative data for assessing reach‐level rehabilitation outcomes, if samples are collected in a pre‐defined sampling protocol stratified at the instream biotope level. The practical implications of this work are that the design of rehabilitation projects, if based upon the recreation of biotope heterogeneity, will succeed in improving biological value and restoring the area to near naturalness if a suitable upstream source of macroinvertebrates for natural recolonization is available. The study shows that the concept of biotopes has an important role to play in river conservation management.