Modelling the future of pelagic ecosystems in a changing climate

Pelagic ecosystems span the world's oceans and support a wide range of species, from iconic predators such as sharks and rays to essential fisheries resources, including offshore top predators such as tuna and swordfish, as well as small coastal forage fish such as anchovies and sardines. Equally important, though less visible, are mesopelagic organisms, which contribute to carbon export and regulate the entire pelagic ecosystem. Characterised by a strong vertical structure, horizontal variability, and temporal changes, pelagic ecosystems pose significant challenges for global-scale modelling, further complicated by the lack of comprehensive, synoptic observations to calibrate these models.As climate change rapidly alters ocean conditions, its impacts on pelagic ecosystems are becoming more pronounced, highlighting the urgent need to deepen our understanding of their response to environmental changes and human pressures. While current ecosystem models have predicted shifts in the spatial distribution of fish and declines in their global biomass, many uncertainties remain regarding the mechanisms that actually drive ecosystem structure and trophic interactions.Here, we use the mechanistic model APECOSM to simulate six generic pelagic communities spanning from the surface to 1,000 metres depth: small and medium-sized epipelagic, tropical tuna, mesopelagic migrant, mesopelagic resident, mesopelagic feeding tuna, and small coastal pelagic. We study how climate change could alter their three-dimensional spatial distribution, size structure, and trophic interactions by the end of the century. A set of sensitivity experiments further reveals the most influential environmental factors (e.g. primary production, temperature) driving these changes and how their importance may evolve over time. The results provide insights into the three-dimensional spatial structure of pelagic ecosystems, offering a clearer picture of their functioning and resilience in a changing climate.