Lateral inputs sustains subsurface particle export during low productivity in Equatorial Atlantic

The equatorial Atlantic Ocean is characterized by seasonal upwelling and complex nutrient dynamics. These processes shape the temporal and spatial distribution of primary productivity causing a pronounced east-west chlorophyll-a gradient. But whether this surface biomass pattern is reflected in particle production and export remains uncertain. We analyze vertical high-resolution particle size distribution data from Underwater Vision Profilers and physical oceanographic observations collected during two trans-Atlantic cruises conducted in contrasting seasons. During the higher productivity fall season, upwelling and tropical instability waves enhanced local primary production and supported deep export of large particles. In contrast, during the lower productivity boreal spring, unexpectedly high concentrations of small particles (0.1-0.5 mm) were observed in the central basin, concentrated within the Equatorial Undercurrent (EUC), which is supplied by the energetic North Brazil Current retroflection. Backward trajectory analysis and biological and hydrographic evidence suggest that these particles originate from the South American continental margin and are laterally transported eastward along a short advective pathway into the EUC. Estimating Particulate Organic Carbon (POC) flux from the particle size distributions, we find that during the low-productivity period, between 15-35°W, over 70% of the POC flux in the upper 200 m might be attributed to small particles (0.05-0.1mm) carried within the EUC. These findings highlight the importance of lateral subsurface transport for equatorial carbon cycling, thereby influencing deep-sea and benthic ecosystem structure.