Functional responses of oligochaetes and chironomids to restoration-induced changes in connectivity- A case study in the Donau-Auen National Park

Lateral connectivity, as a cornerstone of floodplain ecosystems, shapes hydrogeomorphological features, supports floodplain functions, and initiates new habitat formation processes such as fine sediment and deadwood dynamics. However, anthropogenic activities have increasingly disrupted connectivity in large-river floodplains, leading to terrestrialization processes and significant declines in freshwater biodiversity. Restoration efforts in the Upper Danube River aim to enhance hydrological connectivity within the river-floodplain system to mitigate habitat isolation and terrestrialization. Evaluating the outcomes of these efforts requires understanding the interplay between connectivity, environmental factors, and freshwater biodiversity responses.

Using the available information from a river-floodplain stretch in the Donau-Auen National Park, we compared the responses of oligochaetes and chironomids to side-channel reconnection measures across control and impacted sites before (reference period), in the short term and the long term after restoration. We applied a Before-After x Control-Impact (BACI) design to analyse the direct effect of restoration-induced habitat changes on the taxonomic and functional composition and diversity of these indicator groups. A graph theoretical approach followed by applying Partial Least Squares Regressions was used to determine the overall effect of connectivity change on the functional diversity of the indicator groups.

The BACI analysis revealed the positive effects of restoration on oligochaete taxonomic and functional diversity. However, we observed that terrestrialization processes dominate over the long term, outweighing the impacts of restoration. Variations in species traits such as longitudinal zonation, body size, dispersal strategy, drift propensity, and adult lifespan showed short-term restoration effects for both groups, returning to pre-restoration conditions in the long term. For oligochaete functional diversity, connectivity was influential shortly after restoration, while environmental factors became more significant over time.

Our findings underscore the importance of incorporating functional trait responses into restoration assessments to inform the management of protected areas. We highlight the need for restoration measures to refine strategies that enhance floodplain connectivity in the long term to ensure lasting effects on aquatic biota and recommend continuous monitoring to understand better the role of connectivity in influencing ecological processes and their cascading effects on freshwater communities.

 

This research acknowledged support from the EU Projects i-CONN’ H 2020 research and innovation programme under the Marie Skłodowska-Curie (grant agreement number 859937), DANSER (grant agreement No 101157942), H2020 MERLIN (grant agreement No 101036337), HEU DANUBE4ALL project (grant agreement no. 101093985), and AquaINFRA (grant No 101094434). Furthermore, the Austrian Federal Ministry for Digital and Economic Affairs and the Christian