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 from Cape Blanc (CB) and M1, integrating lithogenic, organic, and coccolith species export with satellite-based upwelling, sea surface height (SSH), aerosol optical depth (AOD), and in situ water-column observations.

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 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 yearlong 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 traps, yet deep organic matter export remained high—most prominently at CB and even 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, and positive associations with warm, stratified conditions dominated by tropical, non-blooming lower photic zone (LPZ) taxa typical—suggesting that mineral ballasting was as the dominant seasonal dust effect. At CB, where summer cross-shelf transfer weakened, the persistence of high export under 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 mineral 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 predominantly 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, with secondary episodic fertilisation. 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.