Empirical relationships between particulate organic carbon (POC) and inherent optical properties (IOPs) are required for estimating POC from ocean-color remote sensing and autonomous platforms. The main relationships studied are those between POC and particulate attenuation (c p) and backscattering (b bp) coefficients. The parameters of these relationships can however differ considerably due to differences in the methodologies applied for measuring IOPs and POC as well as variations in particle characteristics. Therefore, it is important to assess existing relationships and explore new optical proxies of POC. In this study, we evaluated empirical relationships between surface POC and IOPs (c p , b bp and the particulate absorption coefficient, a p) using an extensive dataset collected during two Atlantic Meridional Transect (AMT 19 and 22) cruises spanning a wide range of oceanographic regimes. IOPs and POC were measured during the two cruises using consistent methodologies. To independently assess the accuracy of the POC-IOPs relationships, we predicted surface POC for AMT-22 using relationships developed based on independent data from AMT-19. We found typical biases in predicting POC ranging between 2-3, 4-9, and 6-13% for c p , b bp , and a p , respectively, and typical random uncertainties of 20-30%. We conclude that (1) accurate POC-c p and POC-b bp relationships were obtained due to the consistent methodologies used to estimate POC and IOPs and (2) a p could be considered as an alternative optical proxy for POC in open-ocean waters, only if all physiological variability in the POC:chl ratio could be modeled and used to correct a p .

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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 OMZs expand. However, little is known about how the POC fluxes of small particles—a significant component of the BCP—are attenuated inside OMZs. 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 particle during 3 years. This information was analyzed together with meteorological data and published POC fluxes of large particles and allowed us to conclude that (1) major pulses of surface‐derived small particles toward the OMZ interior coincided with seasonal changes in wind stress and precipitation; (2) a permanent layer of small particles, presumably linked to microbial communities, was found in the upper section of the OMZ which might play a key role attenuating POC fluxes; and (3) fluxes of large particles were attenuated less efficiently inside this poorly oxygenated region than above it, while attenuation of small‐particle fluxes were equivalent or significantly higher inside the OMZ. 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.