Variable organic matter stoichiometry enhances the biological drawdown of CO2 in the Northwest European shelf seas

Variations in the elemental ratios of carbon, nitrogen, and phosphorus in marine organic matter (OM) and their influence on carbon cycling remain uncertain in both open and coastal oceans. While observations consistently show carbon enrichment and phosphorus depletion relative to elemental Redfield ratios, many biogeochemical models assume fixed Redfield stoichiometry. As a result, they often underestimate biological carbon fixation, limiting their ability to accurately represent carbon fluxes. Here, we provide a comprehensive assessment of the effects of variable OM stoichiometry on carbon cycling in the Northwest European shelf seas using the coupled 3D physical-biogeochemical modeling system SCHISM-ECOSMO-CO₂. For this, we integrate two pathways for variable OM stoichiometry into the ecosystem model component ECOSMO: a release of carbon-enriched dissolved OM under nutrient limitation and the preferential remineralization of organic nitrogen and phosphorus. We evaluate both their individual and combined effects compared to a reference configuration with fixed Redfield stoichiometry. The variable stoichiometry configurations result in a 10-33% increase in annual net CO₂-uptake. This additional uptake is driven by enhanced OM cycling, with greater surface net autotrophy and subsurface net heterotrophy. As a result, the seasonal biological drawdown of DIC increases, enhancing the biological contribution to pCO₂ changes and shifting the maximum CO₂-uptake from winter to spring and summer. These results underscore the crucial role of variable stoichiometry in accurately representing the shelf carbon pump mechanism in the Northwest European shelf seas, as it has a significant impact on the efficiency of carbon sequestration. They also highlight the need to incorporate variable OM stoichiometry into regional and global biogeochemical models for a more accurate representation of the marine carbon cycle.