Ecological assessment of high sediment loads based on macroinvertebrate communities in the Bohemian Massif in Austria – A sensitivity analysis

Sediment dynamics and composition in streams are key factors influencing the habitat quality of aquatic organisms, consequently playing an important role in terms of river restoration. The superposition of the river-bed with fine sediments is an increasing global stressor affecting local habitat diversity and leading to changes of benthic communities. We analysed benthic macroinvertebrates in five streams of the Bohemian Massif from morphologically unaffected reference sites, and impacted sites influenced by fine sediment deposition (D = 1 mm – 10 mm) of different intensity. Siltation resulted in a gradually decline in biodiversity, abundance and biomass of macroinvertebrates linked with increasing fine sediment deposition. While for reference sites, dominated by coarse substrates, a specific community was allocated, no characteristic fauna was identified for the impacted sites. However, the results of the EU Water Framework Directive compliant Austrian national method for assessing the ecological status did not react accordingly. Solely streams highly impacted by fine sediment deposition in combination with low river-bed heterogeneity and non-natural riparian conditions indicated status shifts. Based on positive faunal responses to micro-habitats such as woody debris we recommend to support the introduction of small-scaled river restorations and anthropogenic initiated river-type specific stable structural elements (e.g. large stones) serving as refugial habitats, which can result in a significant improvement of macroinvertebrate diversity and composition.