Thresholds in the dark – Colmation thresholds as a tool for identifying degraded streams

Despite great restoration efforts to achieve good ecological status in streams under the European Water Framework Directive (WFD), this objective is often unsuccessful. One stressor that could contribute to this failure is colmation, the clogging of hyporheic interstices by fine sediments. Colmation can impair hyporheic invertebrate communities and ecosystem functions. To assess biological responses to colmation, hyporheic communities were investigated in eleven rivers across Germany and Austria, covering all general degradation classes. Ecological assessments were combined with sediment clogging measurements using the Kolmameter, with outflow reduction serving as a proxy for colmation. Threshold Indicator Taxa Analysis (TITAN) was restricted to sites with very good or good general degradation to minimise the effects of additional stressors. Clear threshold responses emerged along the colmation gradient: I) sensitive indicator taxa Cyclopoida, Ostracoda, Chironomidae, Macroturbellaria and Nauplii declined above 23.1% outflow reduction; II) the tolerant taxon Parastenocaridae increased beyond 33%. Additionally, III) a practical TITAN-derived 30% outflow reduction threshold effectively separated sites along this gradient, and IV) river reaches can be classified as degraded when more than half the samples exceeded this 30% outflow reduction threshold. These patterns likely reflect increased fine sediment deposition, which reduces sediment porosity and outflow. Reduced porosity limits interstitial habitat availability and alters hyporheic community composition. Because these organisms are sensitive to subsurface habitat structure, colmation is a key driver of ecological impairment. The 30% threshold, and particularly the 50% cut-off for classifying degraded sections identified in this study, are important steps toward integrating colmation into WFD assessment frameworks.