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BackgroundTo date, only 8.2% of German surface waters achieve a good ecological status according to the European Water Framework Directive. This is primarily attributed to structural deficits, intensive land use, and chemical contaminations of water bodies. In this context, hydromorphological restoration measures are implemented with the aim to increase habitat and species diversity and thus improve the ecological status of water bodies. Nevertheless, existing studies show that restorations promote the reintroduction of individual species, but only in exceptional cases an improvement in the ecological status is achieved. Therefore, we examined the impact of the prevailing chemical contamination on the restoration success in the catchment of the river Nidda in Hessen (Germany) by comparing restored river sections at the rivers Nidda and Horloff with unrestored sections upstream (space-for-time-substitution) and a transect downstream the restoration measures. For this purpose, we conducted active biomonitoring campaigns with Potamopyrgus antipodarum and Gammarus fossarum and analyzed water and sediment samples with effect-based in vitro bioassays.ResultsAt the river Horloff, mortality of P. antipodarum and toxicity in water samples measured via the microtox assay were highest within the restoration. At the river Nidda, the reproduction of snails and gammarids significantly increased within the restorations, and reproduction of snails correlated positively and significantly with estrogenic activities. The microtox assay also exhibited the highest toxicities in water and sediment samples from the restorations and dioxin-like as well as estrogenic activities significantly increased compared to the unrestored reference site. On the basis of these results, the prevailing chemical contamination has negatively affected snails and gammarids in the active biomonitoring campaigns and consequently is likely to have also a negative impact on the local invertebrate community and thus endangers the restoration success.ConclusionHydromorphological restorations as a stand-alone measure are insufficient to improve the ecological status of a water body as long as the water and sediment quality remain deficient. Therefore, it is necessary to improve water and sediment quality in parallel with hydromorphological restoration measures to achieve the objectives of the EU-WFD.

Poison in paradise: increase of toxic effects in restored sections of two rivers jeopardizes the success of hydromorphological restoration measures