Microbial Controls on Carbon Pump Partitioning in the Subtropical North Atlantic: Stoichiometry and Nutrient Limitation Across a Basin‐Scale Transect

Abstract The subtropical North Atlantic is a key region for ocean carbon cycling despite its relatively small spatial extent. Here we present a high‐resolution transect (23°–27°N; ∼2° longitude spacing) across the North Atlantic Subtropical Gyre, integrating physical, chemical, and microbial observations to examine how hydrography and nutrient limitation structure organic matter stoichiometry and carbon pump partitioning. The region was strongly stratified, with a mean mixed layer depth of 125 ± 25 m deep nutriclines (>180 m), limiting nutrient resupply to surface waters in the gyre interior. Boundary regions influenced by western boundary currents and eastern upwelling exhibited higher nutrient availability and distinct microbial communities. Across this gradient, particulate and dissolved organic matter stoichiometry varied systematically with nutrient supply and microbial structure. Particulate organic matter exhibited elevated C:P and N:P ratios in the gyre interior, consistent with stronger phosphorus limitation under Prochlorococcus ‐dominated conditions, while boundary regions approached Redfield‐like stoichiometry. Dissolved organic matter showed a similar spatial structure, with higher inferred C*:N and C*:P ratios in the interior relative to the margins. C* was derived from dissolved nitrogen and phosphorus using literature‐based stoichiometric relationships and evaluated across multiple endmember assumptions. Although absolute values varied, spatial patterns remained consistent, indicating robust gradients despite stoichiometric uncertainty. Statistical analyses identified phosphorus availability and microbial community composition as key predictors of organic matter stoichiometry, with particulate and dissolved pools showing differing predictability. Together, these results suggest strong coupling between stratification, nutrient supply, microbial community structure, and carbon pump partitioning across the subtropical North Atlantic.