³ IRD, UMR 6523 LPO, IUEM, Brest, France

Global changes are modifying oceanic ecosystems at an increasing speed, leading to large-scale modifications and potential no-analogue states with unknown effects on ecological services. In this context, modelling is of prime importance to infer future changes, identify potential tipping points and derive scenarios. However, the multi-stressor nature of changes in the ocean and the multiplicity of interacting physical, chemical, biological and ecological processes at stake at various organization levels turns the responses of ecosystems to environmental forcing into a highly multidimensional and non-linear problem. Understanding its complexity and delivering reliable predictions about global marine ecosystem’s future requires the development of comprehensive modelling frameworks addressing these daunting issues. These would furthermore provide theoretical basis to interpret and relate heterogeneous observations, while inspiring field studies in a focused and coordinated way. Such a mechanistic framework is not yet available however and marine ecology is still in search of its Navier Stokes equation. In this perspective, we propose a path towards the development of an OGEM (Ocean General Ecosystem Model) that consistently relates individual, population, community and ecosystem dynamics, from local to global scales. We provide an overview of APECOSM that represents the 3D Eulerian dynamics of interactive size-structured generic Open Ocean Pelagic Communities (OOPCs: epipelagic, mesopelagic and migratory) in the global ocean, based on individual processes. These include size-structured opportunistic trophic interactions; individual bioenergetics based on DEB theory (growth, maintenance, development, reproduction), physiology (respiration, vision) and behaviour (3D movements, schooling) and includes the effects of life-history diversity in communities using a trait-based approach. Depending on the configuration used, focus species can be represented explicitly and given more details. APECOSM is coupled to a hydrodynamic (NEMO) and a biogeochemical (PISCES) model, being used to provide physical (temperature, currents) and biogeochemical (light, oxygen, phytoplankton, zooplankton, particulate organic matter) forcing to the upper trophic levels. Symmetrically, feedbacks of upper trophic levels to lower trophic levels, detritus and nutrient pools can also be considered. As an illustration, we use of the model to investigate the effects of passive and active movements on global ecosystem distribution and size-structure. Three simulations are compared: one including both passive and active movements, one with active movements removed and one with both active and passive components of movement removed. The spatial distribution of different size of the three OOPCs is analysed and compared for the three simulations. To highlight the importance of movements and their interaction with bottom-up and top-down processes, simulated size spectrums are outputted for the three simulations in the eastern and western equatorial Pacific Ocean and in a specific region of the South Pacific subtropical gyre. We show that (1) passive movement have a major influence on the distribution of biomass for all organisms; (2) active movement have a greater impact on larger size classes; (3) movements have a strong region-specific influence on the size-spectrum; (4) movements allow wide areas of the global ocean to sustain important biomasses while they would have been desert otherwise; (5) active movement can lead to strong regional top-down trophic cascades.