Do oceanic hypoxic regions act as barriers for sinking particles? A case study in the eastern tropical north Atlantic

In oxygen minimum zones (OMZs) the attenuation rates of particulate organic carbon (POC) fluxes of large particles are known to be reduced, thus increasing the efficiency with which the biological carbon pump (BCP) transfers carbon to the abyss. The BCP efficiency is expected to further increase if the OMZs expand. However, little is known about how POC fluxes of small particles – a significant component of the BCP – are attenuated inside OMZs. It is thus important to assess small-particles fluxes in these regions. In this study, data collected by two BGC-Argo floats deployed in the hypoxic OMZ of the eastern tropical north Atlantic, were used to estimate net instantaneous fluxes of POC via small particles during three years. This information, together with meteorological data and published POC fluxes of large particles, allowed us to conclude that: (1) major pulses of surface-derived small particles towards the OMZ interior coincided with seasonal changes in wind stress and precipitation, (2) a permanent layer rich of small particles was found at the upper section of the OMZ and this might play a key role attenuating POC fluxes, and (3) inside poorly oxygenated regions, large particles are attenuated less efficiently, while small-particles attenuation rates fluxes were equivalent (ρ>0.05) or significantly higher (ρ<0.05) relative to those in well oxygenated regions.These results highlight that more information about the processes controlling the fluxes of small and large particles in hypoxic OMZs is needed to better understand the impact of hypoxic OMZs on the BCP efficiency