The suspended small-particle layer in the oxygen-poor Black Sea: a proxy for delineating the effective N2-yielding section

The shallower oxygen-poor water masses of the ocean confine a majority of the microbial communities that can produce up to 90% of oceanic N-2. This effective N-2-yielding section encloses a suspended small-particle layer, inferred from particle backscattering (b(bp)) measurements. It is thus hypothesized that this layer (hereafter, the b(bp)-layer) is linked to microbial communities involved in N-2 yielding such as nitrate-reducing SAR11 as well as sulfur-oxidizing, anammox, and denitrifying bacteria - a hypothesis yet to be evaluated. Here, data collected by three BGC-Argo floats deployed in the Black Sea are used to investigate the origin of this bbp-layer. To this end, we evaluate how the key drivers of N-2-yielding bacteria dynamics impact the vertical distribution of b(bp) and the thickness of the b(bp)-layer. In conjunction with published data on N-2 excess, our results suggest that the b(bp)-layer is at least partially composed of the bacteria driving N-2 yielding for three main reasons: (1) strong correlations are recorded between bbp and nitrate; (2) the top location of the b(bp)-layer is driven by the ventilation of oxygen-rich subsurface waters, while its thickness is modulated by the amount of nitrate available to produce N-2; and (3) the maxima of both b(bp) and N-2 excess coincide at the same isopycnals where bacteria involved in N-2 yielding coexist. We thus advance that b(bp) and O-2 can be exploited as a combined proxy to delineate the N-2-yielding section of the Black Sea. This proxy can potentially contribute to refining delineation of the effective N-2-yielding section of oxygen-deficient zones via data from the growing BGC-Argo float network.