Fast but transient recovery of aquatic and terrestrial communities after a large dam removal

Abstract

IntroductionEcological restoration through dam removals receives increasing attention from scientists, environmental managers and policy makers. However, most dam removal projects focus on small structures (< 10 m in height) and on few ecosystem compartments at a time (e.g. river morphology, reservoir sedimentation, aquatic invertebrates, terrestrial vegetation), but rarely aquatic and riparian ecosystems simultaneously.MethodsWe explored the joint taxonomic recovery (temporal patterns in α- and β-diversity) of three communities after the removal of the Vezins dam (36 m in height; Sélune River, France): aquatic benthic invertebrates, riparian invertebrates, and the riparian vegetation. These communities were monitored yearly, during three years after the dam removal, on sites located within (n = 3; restored sites) and outside (n = 3; two upstream and one downstream; non-impounded sites) the former reservoir.ResultsResults showed a fast recovery of the three ecological communities, as patterns in α-diversity between restored and non-impounded sites were not necessarily different from each other 1.5 years after complete reservoir dewatering. The mean number of species or morphospecies (± standard deviation) reached up to 28.2 ± 5.2, 17.0 ± 2.3 and 77.5 ± 11.2 for the aquatic invertebrates, and the riparian vegetation and invertebrates, respectively. Relative to the sampled area, the riparian invertebrates were the most diversified of all ecological communities with > 500 taxa (i.e., pooling all sites and years). In addition, in some restored sites, α-diversity kept increasing over time while species turnover (β-diversity) remained high after three years for all ecological communities suggesting a transient recovery (i.e., still facing temporal changes in species diversity and composition). This recovery was mediated by the identity of the ecological community as inter-annual changes in α- and β-diversity of the riparian vegetation were less pronounced compared to those of aquatic and terrestrial invertebrates. This recovery depended also on site-specific features as the most distant restored site from the former dam had more time for recovery following the slow reservoir dewatering and through increased sedimentation in the downstream site.DiscussionDifferential patterns of recovery in α- and β-diversity found in this study are discussed in light of species functional traits and ecosystem functioning.

Slow drawdown, fast recovery: Stream macroinvertebrate communities improve quickly after large dam decommissioning

“…Remarkably, as reported by others (Dézerald et al, 2023;Mahan et al, 2021), the invertebrate communities in the area formerly drowned by the Enobieta Reservoir were by the end of the experiment very similar to the communities found in the rest of the sites. This result indicates not only a very high re-colonization capacity but also that the physical habitat had recovered enough for these organisms to live there.…”

Persistent disconnect between flow restoration and restoration of river ecosystem functions after the removal of a large dam on the Sélune River

“…In terms of temporal dynamics, most biological and physico-chemical components (Foley et al, 2015) respond much faster than geomorphological ones (Major et al, 2017). In large dam removal case studies, the subsequent responses of the fish and invertebrate communities take between a few months to 1-3 years after dam removal (Mahan et al, 2021;Dézerald et al, 2023), and the same is true for small dam removal (Carlson et al, 2018).…”

“…A small part of the reservoir was contaminated with heavy metals (Andrade et al, 2020), but these sediments were dredged and safely stored outside the riverbed before the removal. The most upstream dam-Vezins Dam (36 m height)was removed in 2019 after 1 year of regular decrease of the water level in the reservoir, whereas the second dam-Roche-qui-Boit Dam (16 m height)-was completely removed in 2022 (see Dézerald et al, 2023 for details). To keep the massive amount of sediments stored in reservoirs from clogging the downstream section of the river, much of it was dredged and stored in ponds built with gabions and dykes made with local sediments (Berrée, 2019).…”