Organic Matter Stoichiometry Regulates the Continental Shelf Carbon Pump Efficiency of the Northwest European Shelf Seas

Abstract Variations in the elemental C:N:P ratios of organic matter (OM) influence the coupling of carbon and nutrient cycles in the ocean. In this study, we assess the relevance of OM C:N:P stoichiometry for regional carbon cycling in the northwest European shelf seas (NWES). We build on previous work that incorporated two pathways for variable OM stoichiometry into the coupled physical–biogeochemical modeling system SCHISM‐ECOSMO‐CO 2 : extracellular release of carbon‐enriched dissolved OM under nutrient limitation, and preferential remineralization of organic nitrogen and phosphorus over carbon. This earlier study showed that enhanced biological drawdown due to variable OM stoichiometry increases oceanic CO 2 uptake by roughly 10%–30%. Here, we extend this analysis to the full organic and inorganic carbon budgets of the NWES to determine how additional carbon is redistributed. To assess the relative contributions of carbon export pathways, we evaluated the relevance of volume transport, cross‐shelf exchanges, benthic‐pelagic coupling, and their interannual variability. Our results link the previously identified increase in CO 2 uptake to a proportionate increase in the export of dissolved inorganic carbon, with approximately 60%–90% of the additional CO 2 exported across the shelf break, primarily via the Norwegian Trench. Changes in the organic carbon budget are on average weaker throughout the simulation, but show large interannual variability. These findings demonstrate that OM stoichiometry influences the continental shelf carbon pump efficiency of the NWES. This motivates regional assessments across other coastal regions and the incorporation of this variability into global coastal model representations.