Here, data collected by BGC-Argo floats are used to investigate the origin of the suspended small-particle layer inferred from optical sensors in the oxygen-poor Black Sea. Our results suggest that this layer is at least partially composed of the microbial communities that produce dinitrogen. We propose that oxygen and the optically derived small-particle layer can be used in combination to refine delineation of the effective N₂-yielding section of the Black Sea and oxygen-deficient zones.