Dissolved Organic Carbon Accumulation in the North Atlantic Subtropical Gyre: Microbial Feedbacks and Ecological Controls Along Lagrangian Trajectories

With ca. 700 Pg C, dissolved organic carbon (DOC) represents the largest organic carbon reservoir in the ocean, serving both as a carbon storage and the base for the heterotrophic microbial food web. Despite its significance in the marine ecosystem, the mechanisms driving its spatial variability remain poorly understood. This insufficient quantitative understanding limits our ability to predict how DOC reservoirs will respond to changing ocean conditions under future climate scenarios. Subtropical gyres such as the North Atlantic Subtropical gyre (NASG) display elevated DOC concentrations, and several mutually not exclusive processes have been hypothesized to lead to the observed regional accumulation. In this study, we systematically investigate the factors controlling DOC accumulation in the NASG in a modeling framework accounting for microbial interactions with dissolved organic matter (DOM). We adopt a Lagrangian particle tracking approach to track environmental conditions along trajectories into the NASG, which are then fed into a biogeochemical box model that systematically incorporates implementations of different microbial‐DOM interactions. The results of our simulations suggest that DOC accumulation in the NASG is primarily driven by controls on bacterial degradation rather than production rates. By testing different models, we show that either top–down (e.g., grazing) or bottom‐up controls (e.g., nutrient availability) can explain the observed DOC concentration patterns. Our models offer mechanistic insight into how microbial‐DOM interactions influence organic matter dynamics, which may hold implications for environmental changes under future climate scenarios.