A global re-assessment of surface bio-optical properties in Case 1 waters using Biogeochemical-Argo profiles

Surface bio-optical relationships linking chlorophyll-a concentration to light attenuation and euphotic depth are widely used in ocean color applications across the open ocean, yet their global performance remains poorly constrained by existing observational frameworks. To address this limitation, we present a re-assessment of commonly used surface bio-optical relationships based on a globally distributed dataset from Biogeochemical-Argo (BGC-Argo) floats equipped with multispectral radiometric sensors and chlorophyll-a fluorometers in Case 1 waters. We show that surface chlorophyll-a concentration averaged over the upper 5 m provides a robust proxy for chlorophyll-a within the first optical depth across a wide range of radiometric conditions, supporting its use as a consistent surface variable for bio-optical applications. Building on this result, empirical relationships linking surface chlorophyll-a to diffuse attenuation coefficients and euphotic depth are evaluated and found to be broadly consistent with established global empirical models, while showing clear differences across Case 1 optical regimes. Although globally refitted models explain much of the total variability, systematic biases reveal the limited transferability of given global parameterizations within Case 1 waters. Indeed, water-class-specific parameterizations reduce bias, highlighting the importance of accounting for bio-optical regime dependence when estimating light attenuation and euphotic depth from surface chlorophyll-a. These results provide a globally consistent, radiometry-based benchmark for commonly used surface bio-optical models and support further applications across the expanding BGC-Argo array.