Zonal variability in organic matter distribution along the equatorial atlantic ocean: insigths for vertical carbon export

Abstract

The Equatorial Atlantic is subject to seasonal wind-driven upwelling of nutrient-rich waters, enhancing primary production. To understand the link between surface productivity and vertical carbon export in the region, we measured suspended and dissolved organic C and N (POC, PON, DOC and DON) distributions in the water column, and compared it with surface productivity and particle flux estimates obtained with an Underwater Vision Profiler 5 along a zonal equatorial section extending from 5°E to 44.5°W, during the TRATLEQ1 cruise (Sep.-Oct. 2019). Epipelagic DOC concentrations were higher in the western half of the section, contrasting with Chl-a values, which were higher in the central eastern region. In the meso- and bathypelagic layers, high DOC values tended to coincide with areas of elevated particle numbers, but in general did not match with highly productive surface waters. Although epipelagic PON matched Chl-a distribution, POC did not show clear zonal trends and no clear relationship was found with DOC or particle flux estimates in the entire water column. Coloured DOM (CDOM) levels in epipelagic waters decreased from east to west, but with marked variability, possibly arising from near-surface meridional advection associated with tropical instability waves. Below 200 m, water mass aging and local organic matter remineralisation processes were the main drivers of CDOM levels and spectral characteristics. Occasionally, coupled CDOM and sinking particle abundance signals were found, highlighting the possibility that intense events of vertical export of particles impact the transport and distribution of DOM. Our results show that organic matter distribution in the Equatorial Atlantic is influenced by the vertical flux of sinking particles, although there is not a clear association with near-surface productivity, possibly due to transient surface conditions and redistribution of dissolved and suspended particulate matter via the equatorial current system