Seasonal upwelling–dust controls on export production in the Canary Current System revealed by Lagrangian particle tracking

Abstract

The Canary Current System (CCS) is a major eastern boundary upwelling system where intense nearshore productivity, dynamic offshore transport, and Saharan dust deposition jointly shape biogeochemical cycling. Understanding how these physical and atmospheric forcings regulate particulate export is crucial for assessing the biological carbon pump under ongoing North Atlantic warming. Here we combine Lagrangian backtracking of satellite-derived chlorophyll-a (Chl-a), particulate inorganic carbon (PIC), and primary production (PP) with one year of sediment trap fluxes of coccolith species, biogenic particles and lithogenic material (proxy for aeolian dust fluxes) from moorings CB (21° N, 20° W) and M1 (12° N, 23° W), representing distinct open-ocean settings offshore of NW Africa. These fluxes are further integrated with data from satellite-derived upwelling indices, sea surface height (SSH), aerosol optical depth (AOD), and in situ water-column observations collected at the trap locations. The results reveal strong seasonal connectivity between coastal upwelling, offshore transport, and deep export. Late winter–spring intensification of mixing, upwelling, and filament/eddy activity sustained elevated Chl-a, PP, PIC, and high CaCO3 fluxes, with sinking assemblages dominated by fast-blooming (r-selected) placolith-bearing coccolithophores – especially at CB. Lagrangian trajectories further show that this connectivity weakens offshore, with strong coast-to-open-ocean declines in Chl-a, PP, and PIC – particularly along pathways to M1 – highlighting reduced cross-shelf transfer and a stronger year-round influence of stratified tropical waters at that site. During summer–autumn, weakened upwelling and intrusions of warm Mauritanian Current waters reduced surface productivity at both trap sites, yet deep organic matter export remained high – most prominently at CB but also at the persistently oligotrophic M1. Across this period, elevated Saharan dust deposition coincided with enhanced particle fluxes. Multivariate statistical analyses show strong negative correlations between dust and all upper photic zone (UPZ) productivity indicators including Chl-a, the ratio of surface-dwelling species to lower photic zone taxa (UPZ / LPZ), and carbonate fluxes associated with the placolith-forming species Emiliania huxleyi and Calcidiscus leptoporus. In contrast, dust showed positive associations with warm, stratified conditions dominated by tropical, non-blooming LPZ species (Florisphaera profunda and Gladiolithus flabellatus), suggesting that mineral ballasting was the dominant seasonal dust effect. At CB, where summer cross-shelf transfer weakened, the persistence of high export regardless of low surface Chl-a suggests that lateral and subsurface supply of previously produced organic matter played a major role, with Saharan dust further enhancing its downward transfer through ballasting. Nonetheless, several dust-associated export pulses also displayed increases in coccolith UPZ / LPZ ratios, suggesting episodic fertilisation responses by fast-blooming taxa superimposed on a broader ballasting-driven regime. Importantly, dust contributed under both windy, high-productivity late winter–spring conditions and during the stratified summer–autumn phase, sustaining downward particle flux even when local surface productivity was low. The weak relationship between AOD and measured dust flux reflects cloud-induced suppression of satellite AOD retrievals during wet deposition rather than reduced dust deposition. Altogether, these results demonstrate a dual physical–atmospheric control on export in the central–southern CCS. Upwelling and cross-shelf transport fuel the winter–spring CaCO3-rich export regime, whereas Saharan dust plays a particularly important role in maintaining organic-matter fluxes under summer–autumn stratification through ballasting, alongside episodic fertilisation responses. These findings contribute to refine the mechanistic understanding of coast-to-ocean and vertical export pathways and help constrain how dust–upwelling interactions will shape the biological carbon pump under future climate forcing.