It’s getting tight – How colmation influences the sediment fauna and why we might fail in restoring our streams

Restoration of streams to a good ecological status, as required by the European Water Framework Directive (WFD), has been slow, if not failing. Colmation, the clogging of hyporheic interstices by fine sediments, could be one cause for this failure by negatively influencing hyporheic invertebrates. Therefore, this study analysed the hyporheic communities in six German rivers representing all five ecological status classes using ecological, colmation- and grain size analysis. The study revealed that I) the amount of fine sediment and level of colmation had a significant impact on the taxa number and the abundance of hyporheic meiofauna, II) increased colmation led to a shift from coarse- to fine sediment dweller-dominated hyporheic communities, III) hyporheic communities were impacted most by land use in catchments, fine sediment levels, and colmation, and IV) faunal patterns associated with colmation corresponded to the ‘general degradation’ classification defined by the WFD. Presumably elevated fine sediment loads from open land use types, and thereby increasing colmation-levels, altered the porosity and the size of voids. This led to a loss of habitats within the sediments which influenced hyporheic invertebrates. While substrate conditions are important for benthic communities, they are especially critical for hyporheic fauna, which responds particularly strongly to changes in sediment structure. Oxygen and nutrient provision are influenced by sediment structure and further impact the hyporheic organisms. Consequently, the loss of functioning communities led to the deterioration of the ecological status of rivers. Colmation should therefore be considered when implementing the WFD.