Which processes structure global pelagic ecosystems and control their trophic functioning? Insights from the mechanistic model APECOSM

Pelagic ecosystems are distributed throughout the world’s seas and oceans. They are characterized by strong vertical structuring, horizontal heterogeneity and temporal variability, which pose significant challenges for modelling them on a global scale. In this paper, we use the mechanistic high trophic level model APECOSM (Apex Predators ECOSystem Model) to assess how the physical and biogeochemical environment constrains the structure and trophic functioning of pelagic ecosystems worldwide. To this end, we configure the model to represent the three-dimensional and size-structured dynamics of six generic pelagic communities: small and medium epipelagics, tropical tunas, mesopelagic feeding tunas, small coastal pelagics, mesopelagic residents and mesopelagic migrants. We analyse their emergent three-dimensional spatial structuring on a global scale. We first show that the modelled horizontal and vertical distributions are consistent with the observed data. We then analyse the role of key environmental drivers, such as temperature, light, primary production, currents and oxygen on the response of the communities. Finally, we explore the trophic functioning of pelagic ecosystems, focusing on the emergent diets of communities and their variation with organism size. This study demonstrates the ability of a mechanistic ecosystem model to represent the multidimensional structural heterogeneity of marine ecosystems globally (encompassing three-dimensional distribution, size variations, and community composition) from a small set of universal principles and well-defined hypotheses. This approach helps to understand how the various processes at stake act and interact to shape the structure of global pelagic ecosystems, and eventually elucidate the heterogeneity of their trophic functioning. • By presenting the new 6-community configuration of the mechanistic model APECOSM, we explain the structure and function of global pelagic ecosystems. • APECOSM horizontal, vertical, size and community distributions align well with available observations. • The structure of pelagic ecosystems is determined by sea temperature, light, available food, currents, and oxygen concentration. • The analysis of emergent trophic interactions highlights the diversity of trophic linkages and shows the interdependence of different high and low trophic level communities and how this changes with size.