Changes in macroinvertebrate and fish assemblages in a medium‐sized river following a breach of a low‐head dam

Abstract

1. Dam removal has great potential for restoring rivers and streams, yet limited data exist documenting recovery of associated biota within these systems following removals, especially on larger systems. This study examined the effects of a dam breach on benthic macroinvertebrate and fish assemblages in the Fox River, Illinois, U.S.A. 2. Benthic macroinvertebrates and fish were collected above and below the breached dam and three nearby intact dams for 1 year pre-and 3 years post-breach (2 years of additional pre-breach fish data were obtained from previous surveys). We also examined the effects of the breach on associated habitat by measuring average width, depth, flow rate and bed particle size at each site. 3. Physical habitat at the former impoundment (IMP) became comparable to free-flowing sites (FF) within 1 year of the breach (width and depth decreased, flow rate and bed particle size increased). We also found a strong temporal effect on depth and flow rate at all surveyed sites. 4. Following the breach, relative abundance of Ephemeroptera, Plecoptera and Trichoptera (largely due to hydropsychid caddisflies) increased, whereas relative abundance of Ostracoda decreased, in the former IMP to levels comparable to FF sites. High variation in other metrics (e.g. total taxa, diversity) precluded determination of an effect of the breach on these aspects of the assemblage. However, non-metric multidimensional scaling (NMDS) ordinations indicated that overall macroinvertebrate assemblage structure at the former IMP shifted to a characteristically FF assemblage 2 years following the breach. 5. Total fish taxa and a regional fish index of biotic integrity became more similar in the former IMP to FF sites following the breach. However, other fish metrics (e.g. biomass, diversity, density) did not show a strong response to the breach of the dam. Ordinations of abundance data suggested the fish assemblage only slightly shifted to FF characteristics 3 years after the breach. 6. Effects of the breach to the site immediately below the former dam included minor alterations in habitat (decreased flow rate and increased particle size) and short-term changes in several macroinvertebrate metrics (e.g. decreased assemblage diversity and EPT richness for first post-year), but longer-term alterations in several fish metrics (e.g. 1055 decreased assemblage richness for all three post-years; decreased density for first two postyears). However, NMDS ordinations suggested no change to overall assemblage structure for both macroinvertebrates and fish following the breach at this downstream site. 7. Collectively, our results support the effectiveness of dam removal as a restoration practice for impaired streams and rivers. However, differences in response times of macroinvertebrates and fish coupled with the temporal effect on several habitat variables highlight the need for longer-term studies.

Impact of habitat diversity on the sampling effort required for the assessment of river fish communities and IBI

“…For eurytopic and rheophilic species, the availability of sheltered diversified habitats with a diverse food supply is essential as nursery and feeding grounds (Karr & Dudley, 1981). A decrease of natural river systems or habitat diversity will therefore be reflected by an impoverished fish diversity (Brooks et al, 2004;Sindilariu et al, 2006;Lester & Boulton, 2008) or native species richness (Koel, 2004;Tales & Berrebi, 2007) often related to the decline of rheophilic species (Kruk, 2007;Maloney et al, 2008;Slawski et al, 2008). The presence of these structures is therefore crucial in homogenous reaches but structures are different and vary markedly Fig.…”

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

“…From this perspective, the effects of dam removal are two-fold, in terms of the initial effects on the physical structure of the river (pulse disturbance; Tullos, Finn & Walter, 2014; Dorobek, Sullivan & Kautza, 2015; Gartner, Magilligan & Renshaw, 2015) and the reestablished seasonal disturbances from high flows over longer timescales (press or ramp disturbance; e.g., Maloney et al, 2008). Despite evidence that dam removal effects can occur at both short and longer time scales, few studies have examined the trajectory of ecological responses over multiple years and across different seasons within the same system (but see Hansen & Hayes, 2012).…”

Ecological effects of small dams on benthic macroinvertebrate communities of mountain streams (Córdoba, Argentina)

“…Then, a better understanding of the effects of dams, particularly across a range of dam sizes, is needed to guide management decisions and maximize the effectiveness of river restoration projects. Some studies have evaluated the ecological effects of small dams (<10 m height) to be able to predict how streams and watersheds were likely to respond to the potential removal of these dams (Ambers, 2007;Maloney et al, 2008;Skalak et al, 2009). Dam removal has become an increasingly practiced form of restoration, which has been proposed as a method to restore connectivity and to improve fluvial health, but it has been mainly applied to small and obsolete dams with poor structural conditions (American Rivers, Friends of the Heart and Trout Unlimited, 1999).…”