Influence of hydro-morphological quality and pollution pressure on macroinvertebrate assemblages in restored streams

Stream restoration constitutes a tool to mitigate hydro-morphological degradation, which affects a substantial proportion of European streams and rivers. However, improvement of local habitat quality does not always result in the recovery of macroinvertebrate assemblages, mostly due to persisting catchment-scale stressors or colonization constraints. In this study, we aimed to explore the effect of pollution pressure on selected biotic indices in restored sites, and the role of hydro-morphological quality of upstream reference sites on macroinvertebrate colonization. We utilized a paired comparison design and included 24 streams restored before up to 32 years. Using linear regression models, we tested the effect of factors such as water chemistry, land use, hydro-morphological parameters, distance between restored and reference sites, and time since restoration on diversity in restored streams. Alpha diversity parameters at both restored and reference sites displayed comparable biotic trends along all tested predictors, with water chemistry being the strongest predictor. This was also the case for beta diversity parameters, but here, interestingly, streams with higher pollution pressure exhibited higher dissimilarity between restored and reference sites. This dissimilarity was governed by turnover, indicating that restoration resulted in greater assemblage change in polluted streams. This suggests that restoration may, to an extent, increase diversity even in disturbed agricultural landscapes. However, we did not confirm the role of hydro-morphological quality of upstream reference site on macroinvertebrate colonization. Large-scale factors such as water chemistry and land use play a pivotal role in structuring macroinvertebrate assemblages.