Climate-driven reductions in nutrient supply propagate through the ecosystem in the Ascension Island exclusive economic zone

Abstract

The Ascension Island exclusive economic zone (EEZ) was designated a no-take marine protected area (MPA) in 2019, creating the largest fully protected reserve in the Atlantic Ocean at the time. Despite its ecological importance, no previous study has projected physical–biogeochemical changes through the food web. Here, we developed a StrathE2E end-to-end model for Ascension Island EEZ to assess climate-driven changes across the biomass of 26 guilds, primary production and fishing. Climate forcing was derived from NEMO-ERSEM outputs based on two Earth system models (CNRM-CM6-1-HR and GFDL-ESM4) and two contrasting pathways (SSP1-2.6 and SSP3-7.0), comparing the 2010–2019 baseline with decadal projections from the 2020s to the 2060s. Output uncertainty was estimated using a likelihood-weighted Monte Carlo scheme. Across the four climate scenarios, projections indicate warmer, more oligotrophic, and more strongly stratified waters within the Ascension MPA over time. Net primary production (NPP) declined consistently across scenarios, with an 8%–13% decrease under SSP1-2.6 and 12%–15% under SSP3-7.0, and stronger reductions in CNRM-CM6-1-HR. Food web responses mirror these trends: all guilds exhibit biomass losses by the 2060s relative to the 2010s, with fish guilds showing the strongest proportional declines. The model parameters with the largest potential effects on NPP were boundary oceanic nitrate and vertical mixing (upwelling–downwelling), highlighting the importance of oceanographic drivers beyond temperature. Network indicators further show reduced structural organization of the food web, resulting in a less efficient and potentially less resilient ecosystem state. Nevertheless, climate change exerted a much stronger influence on ecosystem trajectories than fishing. Overall, we provide ecosystem-level climate projections for the Ascension Island MPA and demonstrate strong bottom-up vulnerability in this oligotrophic, low-fishing ecosystem. This model implementation also establishes a foundation for integrating climate considerations into adaptive management, as well as for further scenario testing.