Long-term recovery of benthic food webs after stream restoration

The assessment of restoration success often neglects trophic interactions within food webs, focusing instead on biodiversity and community structure. Here, we analysed the long-term recovery of food web structure based on stable isotopes (δ¹³C and δ¹⁵N) of benthic invertebrates and quantified responses of food web metrics to time since restoration. The samples derived from twelve restored sites with different restoration ages, sampled annually from 2012 to 2021, and covering an investigation period of up to 28 years after restoration for the whole catchment. Temporal developments of the restored sites were compared to the development of two near-natural sites. The restoration measures consisted of the cessation of sewage inflow and morphological restoration of the channels. As a clear and consistent result over almost all sites, trophic similarity (proportion of co-existing species occupying similar trophic niches) increased with time since restoration, and reached values of near-natural sites, suggesting an increase in the stability and resilience of the food webs. Surprisingly, resource diversity decreased at most restored sites within 10 years after restoration, probably due to the removal of wastewater-derived resources, and a shift towards leaf litter as the dominant resource following the regrowth of the riparian vegetation. Food chain length showed no consistent pattern over time at the different sites both increasing and decreasing with time since restoration. Overall, restoration had clear effects on the food web structure of stream ecosystems. While some effects such as the increase in trophic similarity were consistent at almost all sites, others such as response of the food chain length were context dependent. The study demonstrates the potential of utilizing food web metrics, particularly trophic similarity, in restoration research to achieve a more holistic understanding of ecosystem recovery.