Reconstructing the Global Vertical Structure of Marine Phytoplankton Using a Two-Community Model

The vertical distribution of marine phytoplankton plays a crucial role in oceanic primary production and the carbon cycle. Satellite observations of surface chlorophyll (Chl) and particulate organic carbon (POC) capture large-scale surface patterns but are limited to roughly one diffuse attenuation depth (K_d), representing approximately the top one-fifth of the euphotic zone. Consequently, subsurface phytoplankton structures, including the deep chlorophyll maximum (DCM), remain poorly represented at the global scale, while in situ vertical profile observations are too sparse to provide continuous three-dimensional coverage. To address this, we employ a two-community parametric model that represents the vertical structure of phytoplankton as two functional groups. and examine their physiological characteristics, specifically their responses to light adaptation and nutrient availability. The model is tuned to global 4D fields of Chl and POC spanning 1998-2022 derived from satellite ocean color merged with BGC-Argo profiles. We reveal the relative contributions of the two communities to the total water column phytoplankton biomass, study their physiological dynamics, and their relationship with light and nutrient conditions, in different regions of the ocean. Globally, these findings clarify how environmental conditions shape phytoplankton vertical structure, particularly the seasonal dynamics of the DCM, advancing understanding of global subsurface phytoplankton patterns and their impact on the marine carbon cycle.