The fate of biodiversity following regulation and restoration of a Lowland River of Danish and European importance

Historically, the lower River Skjern (Denmark) was a biodiversity hotspot of national and European importance, but in the 1960s this part of the river was channelized and the wetlands drained to improve conditions for agriculture. However, to regain former biodiversity, a major restoration project was conducted in 2000–2002, which transformed a 19 km straight channel into a 26 km meandering river. In this study, we use historical data to evaluate how the channelization of the river and its restoration 40 years later affected the aquatic biodiversity. Generally, and as expected, we observed a significant reduction in the taxon richness of aquatic plants and EPT taxa (Ephemeroptera, Plecoptera, and Trichoptera) in response to the channelization, especially taxa associated with backwaters. As hoped, the restoration mediated an increase in taxon richness for both aquatic plants and EPT taxa. This positive response took place immediately after the restoration (1 year) and persisted throughout the study period (10 years). However, taxon richness did not reach the pre-channelization levels. Especially, we observed that the recovery of backwater-associated taxa was limited. The main reason for this is likely that only 5.8 % of the original backwater area was recreated and, additionally, that natural regeneration of these areas relies on natural hydromorphological processes that might take several decades. Furthermore, the possibility of several taxa to recolonize from other and distant areas is poor, reflecting today's rarity of backwater habitats in Danish rivers. Full recovery of the river's former biodiversity might therefore take decades or more without implementing active measures. • The study of a former near-pristine lowland river is unique in size and details. • Largest N-European physical restoration project. • Taxon richness of aquatic plants and EPT taxa is assessed on species level. • Evaluate effects of both channelization (30 year period) and restoration (1–10 years). • Link recolonization of taxa to river area, physical habitats, and dispersal range.