Long‐term Taxon‐specific Responses of Macroinvertebrates to Dam Removal in a Mid‐sized Swedish Stream

Abstract

Dam removal to restore ecologically impaired rivers is becoming increasingly common. Although the target often is to facilitate fish migration, dam removal has also been assumed to benefit other types of organisms. Because few studies thus far deal with effects of dam removal on stream macroinvertebrates and because results have been equivocal, we investigated both short-and longer-term dam-removal effects on downstream macroinvertebrate communities. We did this in a before-and-after study of the removal of a dam located in a south Swedish stream. We sampled the benthic fauna 6 months before dam removal and both 6 months and 3.5 years after the dam was removed. We compared species composition, taxonomic richness, total densities and densities of macroinvertebrate groups before and after dam removal and between downstream and reference sites. We found that dam removal reduced some macroinvertebrate taxa at the downstream site, but we found no effect on community composition. Although this corroborates results from previous short-term studies, we also found a reduction of taxonomic richness and that some dam-removal effects persisted or even increased over time. The most likely explanation for the suppression of benthic macroinvertebrate richness following dam removal is a significantly increased sediment transport from the former reservoir and a subsequent loss of preferred substrates. Our results indicate that adverse dam-removal effects may be long lasting but taxon specific. We therefore call for longer-term studies on a variety of organisms to better understand how dam removal may influence downstream macroinvertebrate communities. Effects of dam removal on benthic macroinvertebratesBenthic communities are structured by factors such as hydraulic conditions, variation in discharge, water quality, temperature and channel bed morphology (Townsend et al.,

Seasonally distinct taxonomic and functional shifts in macroinvertebrate communities following dam removal

“…Benthic macroinvertebrate communities undergo seasonal shifts (Merritt, Cummins & Berg, 2008) and can respond rapidly to disturbances, including those driven by dam removal (Orr et al, 2008; Renöfält et al, 2013; Tullos, Finn & Walter, 2014). Macroinvertebrate densities and taxonomic richness have been shown to decline after discrete high-flow events (i.e., scouring floods; Gjerløv, Hildrew & Jones, 2003; Suren & Jowett, 2006) and dam removal (Orr et al, 2008; Hansen & Hayes, 2012; Renöfält et al, 2013).…”

“…Macroinvertebrate densities and taxonomic richness have been shown to decline after discrete high-flow events (i.e., scouring floods; Gjerløv, Hildrew & Jones, 2003; Suren & Jowett, 2006) and dam removal (Orr et al, 2008; Hansen & Hayes, 2012; Renöfält et al, 2013). Likewise, relative functional trait abundance can be affected by frequent high-flow events whereby taxa that are tolerant to unstable conditions become more established (Wallace, 1990; Townsend, Scarsbrook & Doledec, 1997; Suren & Jowett, 2006).…”

“…In contrast, dam-removal effects on macroinvertebrate taxonomic structure and functional traits can be idiosyncratic and less predictable. For example, Renöfält et al (2013) found that although some taxa were unaffected by dam removal or recovered quickly, others showed continuous declines, likely due to site-specific differences in tolerance to sediment deposition and recolonisation. Tullos, Finn & Walter (2014) observed that functional assemblages recovered within one year following lowhead dam removal, and trait modalities reflected both habitat stability (e.g., univoltinism) and disturbance (e.g., clinging habit).…”

Responses of macroinvertebrate communities to small dam removals: Implications for bioassessment and restoration

“…>40 months) negative responses in densities and taxa richness (e.g. Renöfält, Lejon, Jonsson, & Nilsson, ). These apparent discrepancies in the results from empirical case studies limit our ability to make general predictions on the spatial scale, magnitude, trajectory and duration of responses.…”

How to make sense of our rivers: using assemblage to understand angling

“…Freshwater habitats are one of the world's most threatened ecosystems and so the use and management of these habitats has become a critical focus for research . Yet, regulation and governance of rivers is dynamic and the ways in which management takes place in practice is continually being negotiated and constantly evolves (Box ).…”