Ecosystem indicators: Predicting population responses to combined climate and anthropogenic changes in shallow seas

Crowded seas are becoming a pressing management problem with the increased development of offshore renewable energy (ORE) to combat climate change. Marine ecosystems are complex and varied; therefore, we need new tools to help rapidly increase our understanding of how they are likely to change with both climate and anthropogenic changes. This study uses a pragmatic data-driven Bayesian network approach to capture the patterns of ecosystem complexity and reveal trends of ecosystem drivers (i.e. indicators) important to ecosystem functioning across space and over time. The ecosystem approach assessed physical and biological indicators and their influence on population (abundance/productivity) trends in four regions with contrasting habitats of the North Sea within the last 30 years (1990–2019). What-if scenarios were conducted to examine species/functional group responses to physical (temperature and stratification) representing climate and large-scale ORE development effects, as well as anthropogenic (fishing) changes. Clear patterns were revealed, including temporal trends of the dynamic nature of bottom-up effects driven by physical change versus top-down effects driven by fishing across trophic levels and habitat types. All four regions are influenced by both effects; however, the dominance of effects was dependent on region: Shetland and Orkney (bottom-up driven), southern North Sea (top-down driven). In general, regions with stronger bottom-up effects showed increasing population trends whereas those with stronger top-down effects, decreasing trends. Our findings also suggest that some species are much better indicators of either bottom-up (e.g. zooplankton), top-down effects (e.g. fish) or both (e.g. grey seal), but the strength of indicator is dependent on habitat type. The habitat-specific results provide better understanding of what type of ecosystem change they are indicating (physical or biophysical) and therefore indicators that assess both ecosystem status and resilience, ensuring a more strategic and integrated evaluation of trade-offs for future sustainable management of our shallow seas.