Environmental drivers of Northeast Atlantic food webs

Marine ecosystems are increasingly reshaped by interacting environmental stressors driven by global change, affecting the provision of ecosystem goods and services. This thesis adopts a multi-scale food web approach to quantify how increasing temperatures and commercial fishing alter trophic structure and function and ultimately ecosystem resilience across the Northeast Atlantic, progressing from pairwise interactions to food webs across large spatio-temporal gradients. Analysis of stomach contents records spanning 35 years demonstrates that an allometric constraint underpinning food webs, the predator–prey mass ratio, increases with temperature, which is amplified in areas under high commercial fishing, indicating that trophic size structure is dynamic across environmental conditions. These changes are driven primarily by physiologically driven declines in prey body mass. Scaling up beyond pairwise interactions, 4,728 spatially resolved empirical food webs were constructed across the North Sea from 1997 to 2015 using stomach content and survey data, providing an unprecedented representation of trophic interactions and energy flow. Ecological network analysis revealed that temperature and commercial fishing jointly reshape food web structure and function, with consequences for ecosystem resilience. Colder, less exploited systems retain greater trophic complexity, energy availability, and pathway redundancy, whereas warmer and heavily exploited systems exhibit simplified, more tightly organised trophic networks with reduced energy availability and pathway redundancy. Temporal analysis showed that North Sea food webs have become less complex, more structurally organised and energetically constrained over recent decades. These results reflect a shift in ecosystem resilience from ecosystems characterised by adaptive flexibility toward ecosystems structured for energetic efficiency and persistence. Localised analyses reveal substantial spatial heterogeneity, highlighting that regional trends can obscure localised variation in ecosystem resilience. By providing a spatially resolved empirical baseline, this work advances understanding of how environmental change reshapes marine ecosystem resilience, supporting spatially targeted ecosystem-based management informed by climate change and sustainable fisheries.