Synergistic impacts of sediment contamination and dam presence on river functioning

Abstract

Summary 1. Dam presence is commonly associated with strong accumulation of polluted sediments. In spite of this context of multiple stressors, physical effects are often solely considered in the ecological assessment of the dam impacts. 2. We studied four ‘reservoir/downstream reach’ systems differing in levels of sediment contamination in reservoirs. Using assemblages and biotrait (i.e. ecological or biological attribute) responses of macroinvertebrate communities and leaf litter breakdown, we examined the individual effects and potential interactions between sediment contamination and dam presence along the gradient of ecotoxic pressure. 3. Leaf breakdown rates ranged from 0.0044° per day in the most contaminated reservoir to 0.0120° per day in the reference reservoir. Comparisons of community trait profiles among reservoirs highlighted a gradient of trait responses to sediment contamination. 4. In the absence of toxic contamination, the dam‐induced modifications in biotraits of invertebrate assemblages were not related to a reduction of leaf litter breakdown. Conversely, contaminated sediment in reservoir induced strong functional disturbances (i.e. bioecological shifts and reduction of leaf litter breakdown) downstream of dams. 5. Key biotrait categories positively related to leaf litter breakdown rate have been identified. They corresponded mainly to shredders and/or small‐sized (<0.5 cm) insects, using aquatic (e.g. crawlers) or aerial (e.g. fliers) active dispersal strategies. In addition, trait categories positively correlated to contamination level have been considered as ‘response’ traits. They corresponded to large‐sized (>4 cm) species, having several generations per year (polyvoltin), using asexual reproduction and/or disseminating by drift (aquatic, passive). 6. In the current context of ecological continuity restoration, this study has identified the risks associated with the presence of historical contamination in the run‐of‐river reservoirs for downstream ecosystem health.

Two microcrustaceans affect microbial and macroinvertebrate‐driven litter breakdown

“…Two species of shredder macroinvertebrate with different feeding strategies were used: the amphipod G. pulex as an opportunistic shredder and the trichopteran S. personatum as a selective shredder (Colas et al ., ). Individuals were sampled in a reference headwater stream located in the Pyrenees Mountains (01°05′35″E, 42°56′48″N) in winter 2013, sorted and counted back in the laboratory.…”

Dam removal and anadromous salmonid (Oncorhynchus spp.) conservation in California

“…Moreover, increased fine sediment deposition can decrease benthic invertebrate production in the short term (Orr et al 2008). Toxic materials (e.g., mercury) locked in reservoir sediments may become suddenly available to contaminate food webs and inhibit nutrient cycling (as in Colas et al 2013). …”

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

“…The removal of a dam typically causes an immediate but often temporary "pulse disturbance" (Van Looy, Tormos, & Souchon, 2014) via processes of sediment exposure, erosion and redistribution (Bednarek, 2001;Tullos et al, 2016), as well as sudden shifts in hydrology and temperature (Bednarek, 2001). Such dramatic changes potentially affect the biota in different ways: physical effects include scouring and modified bed forms and features (Wood & Armitage, 1997), while the release of pollutants associated with sediments can result in ecotoxic pressures (Colas et al, 2013). Furthermore, the environmental settings of the dam may influence organism response to such pulse disturbance, yet many are not commonly considered in the context of organism response to dam removal.…”