Particle-based representation of phytoplankton modulates emergent coexistence and biogeography in a model ocean
Phytoplankton communities sustain the marine food web through primary productivity. These communities sustain a high diversity of species, all of which must draw from the same finite nutrient supply. The mechanisms driving phytoplankton biogeography and succession, as well as those enabling high diversity, remain incompletely understood. Here, we compare emerging patterns of biogeography, richness, and diversity in a size-structured phytoplankton community as it is advected and diffused within an idealized North Atlantic double-gyre circulation, using both Eulerian and Lagrangian (particle-based) modeling (PBM) frameworks. When a sufficient number of particles is modeled, the PBM, which resolves mortality at the particle level, produces stronger seasonal variability in biomass in the northern gyre and less stable and higher biomass in the southern gyre, while reproducing large-scale chlorophyll patterns observed by satellite. These contrasting dynamics are accompanied by distinct regional diversity patterns: local richness and diversity are generally higher in the PBM than in the Eulerian model in the northern gyre, with differences depending on the mortality threshold, whereas in the oligotrophic southern gyre, PBM local richness increases with particle number, while Eulerian richness is more sensitive to the mortality threshold. The PBM also produces a dynamic and emergent biogeographical structure that is more robust to the mortality threshold than the Eulerian model. Finally, Lagrangian variability in environmental conditions experienced by particles shows a stronger impact compared to the local Eulerian variability in the Northern gyre and stresses variable sensitivity to environmental histories. Together, these results suggest that the discrete representation of biomass fundamentally alters mortality, biogeography, and emergent coexistence patterns in model plankton ecosystems.