The impact of anthropogenic drivers on benthic ecosystem functions in a changing North Sea

Macrozoobenthos play a crucial role in soft-sediment ecosystems like the North Sea by driving processes such as bioturbation (physical mixing of sediments), bioirrigation (active flushing of water into sediments), and trophic interactions (e.g., predation). These functions are essential for nutrient cycling, primary productivity, and benthic–pelagic coupling. However, benthic communities face various anthropogenic drivers, including rising temperatures, increasing organic enrichment, and indirect impacts of commercial fishing such as mesopredator release. Given that these drivers alter benthic activity, they likely also influence the ecosystem functions that benthos facilitate. Accordingly, this thesis addresses empirical knowledge gaps by integrating experiments and surveys using benthic model organisms to elucidate mechanistic links between these drivers and ecosystem functioning. Manuscripts I and II examined how temperature affects bioturbation, bioirrigation, and related nutrient fluxes. Using luminophore and bromide tracers inside incubation cores in a mesocosm tank setup, the studies quantified sediment reworking and irrigation rates of key benthic species. Warmer temperatures increased bioturbation, boosting oxygen consumption within the sediment and nutrient effluxes (Manuscript I). The abundant lugworm Arenicola marina also shifted from dominant bioirrigation activity at 15 °C to bioturbation at 20 °C, indicating a sensitivity of ecosystem processes to warming (Manuscript II). These findings suggest that rising sea temperatures may initially enhance Wadden Sea ecosystem functioning by intensifying faunal activity and strengthening coupling between the sediment–water interface. Manuscripts III and IV explored how organic enrichment influences benthic ecosystem functions using in situ microcosm incubations and different species combinations. Algal material represented coastal enrichment, while particulate organic matter (POM) reflected offshore enrichment conditions. Elevated temperature and algal enrichment enhanced bioturbation by the intertidal amphipod Corophium volutator. However, an upper threshold was reached under 15 °C, beyond which further organic enrichment likely results in a suppression of activity (Manuscript III). In parallel, POM enrichment indirectly boosted bioturbation by Corophium multisetosum by providing the shrimp predator Crangon crangon with an alternative food source, reducing predation pressure on the amphipod (Manuscript IV). These findings underscore both direct and indirect enrichment effects on bioturbation, with potential cascading impacts on broader benthic ecosystem functioning. Finally, to assess the impact of mesopredator release on benthic communities, Manuscript V examined the diet of the flatfish Buglossidium luteum. Its population has grown due to milder winters and overfishing of predators and competitors, making it a significant mesopredator likely exerting strong pressure on the benthos. Stomach content analysis, combining DNA metabarcoding and traditional morphological methods, was paired with benthic infauna data from sampling sites. Results revealed a diverse diet of 164 taxa, mainly crustaceans and polychaetes, with clear spatial variation in prey composition. This highlights B. luteum as a highly opportunistic feeder, which likely facilitates its ongoing success. Correlations with infauna data also suggest lower mobility than previously assumed. Given continued warming and fishing pressure in the North Sea, this small flatfish is poised to maintain or increase its predation pressure, tightening trophic linkages in benthic food webs. Overall, this thesis demonstrated that anthropogenic drivers (rising temperatures, enrichment, and mesopredator release) intensify ecosystem functions by boosting bioturbation, nutrient cycling, and trophic transfer. This creates a positive feedback loop; enhanced bioturbation drives organic matter remineralization and primary productivity, which, in turn, fuels more benthic activity. Essentially, these animals sustain their own productivity by reinforcing bentho-pelagic coupling while also serving as abundant resources for higher trophic levels. While the benthic system is generally robust, signs of emerging thresholds were observed under combined drivers, with species like lugworms and corophiid amphipods showing stress at high temperatures or enrichment levels. Rather than a clear tipping point, the soft sediment environment of the North Sea demonstrates partial compensation through behavioural and functional adjustments, highlighting its nonlinear response to change. This integrated perspective calls for multi-driver experiments and continuous environmental monitoring, and provides a compelling glimpse into the future of North Sea benthic ecosystem dynamics.