A Novel Approach to Estimate Carbon and Nitrogen Flux from In Situ Optics: Application to Cyclonic Eddies off the Cape Verde Islands

Abstract. Mesoscale eddies are pervasive in the global ocean and are suggested to play a significant role in enhancing carbon export from the surface to the deep ocean. However, their dynamic nature and associated challenges of in-situ sampling have limited direct measurements of carbon flux within these features, leaving their contribution to carbon export uncertain. In-situ optical devices offer a promising solution by providing high-resolution data on particle abundance and size distribution (PSD) within eddies, both vertically and horizontally. Yet, converting PSD data into reliable carbon flux estimates is particularly challenging in dynamic features like eddies, and is further complicated by the need to account for key factors regulating carbon export. To address this, we present a novel mechanistic framework that optimally integrates PSD data with flux measurements, settling velocities and respiration rates of in-situ collected aggregates, together with water temperature and oxygen concentration to estimate both particulate carbon and nitrogen fluxes in the water column. This framework tackles primary challenges by incorporating contributions from suspended particles, size-dependent sinking velocities, and the degradation rates of settling particles with depth. Applied to a multi-instrument dataset from a high-resolution survey of eddies off the Cape Verde Islands, the presented framework reveals distinct funnel-shaped flux patterns for particulate organic carbon and nitrogen, with a twofold enhancement of flux within the examined cyclonic eddy. Furthermore, it identifies lower carbon-to-nitrogen ratios in settling organic matter at the deep eddy core, indicating the transport of fresher organic matter to the deep sea. These findings provide new insights into eddy-driven carbon export, highlighting the roles of settling velocity, water temperature, and oxygen concentration in shaping carbon flux and organic matter stoichiometry. While observed in a single eddy, this doubling of carbon flux underscores the potential for mesoscale eddies to locally enhance carbon export and, given their widespread occurrence, contribute significantly to the ocean's capacity to sequester atmospheric CO₂.