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

Abstract. Variations in the elemental ratios of carbon, nitrogen, and phosphorus in marine organic matter (OM) and their influence on the marine carbon cycle remain poorly understood for both the open and coastal oceans. Observations consistently show an enrichment of carbon and a depletion of phosphorus relative to elemental Redfield ratios. However, many biogeochemical models are constrained to Redfield stoichiometry, neglecting the effects of variable stoichiometry on carbon cycling and typically underestimating biological carbon fixation. This impedes the accurate representation of OM cycling and the resulting carbon fluxes, especially in productive temperate shelf seas such as the Northwest European shelf seas (NWES). Here, the efficiency of oceanic CO₂-uptake strongly depends on the biological uptake of inorganic carbon and its export to the North Atlantic, both of which are influenced by OM stoichiometry. In this study, we provide a first comprehensive and quantitative assessment of the effects of variable OM stoichiometry on carbon cycling in the NWES. For this purpose, we integrate two pathways for variable OM stoichiometry, motivated by observational and experimental results, into the regional high-resolution coupled 3D physical-biogeochemical modeling system SCHISM-ECOSMO-CO₂: first, the release of carbon-enriched dissolved OM under nutrient limitation, and second, the preferential remineralization of organic nitrogen and phosphorus. With these extensions we reproduce the observed OM stoichiometry and evaluate its impact on marine carbon cycling with a focus on OM cycling and the resulting air-sea CO₂-exchange. Compared to the reference simulation with fixed Redfield stoichiometry, the variable stoichiometry configurations show an increase of the annual net CO₂-uptake in the NWES by 10–33 % , depending on the relative contribution of the two new implementations. As the main driver of the additional CO₂-uptake, we identify a corresponding intensification of annual and seasonal OM cycling, resulting in higher net autotrophy in surface waters and higher net heterotrophy in sub-surface layers. This enhanced gradient in net community production leads to an increased biological drawdown of inorganic carbon, most pronounced in the Norwegian Trench. By increasing the biological control on the surface partial pressure of CO₂, this leads to higher summer and lower winter uptake. Our results highlight the importance of variable stoichiometry for an accurate representation of the shelf carbon pump mechanism in the NWES, as it significantly influences the efficiency of carbon sequestration. Since the response depends largely on regional physical conditions and pre-existing carbon export mechanisms, regional assessments are essential to understand the sensitivity of the carbon cycle to OM stoichiometry, which should be included in global models to accurately represent the coastal carbon cycle.