The suspended small-particles layer in the suboxic Black Sea: a proxy for delineating the effective N<sub>2</sub>-yielding section
Résumé
Abstract. Upper suboxic water masses confine a majority of the microbial communities that can produce up to 90 % of oceanic N2. This effective N2-yielding section encloses a suspended small-particle layer, inferred from particle backscattering (bbp) measurements. It is thus hypothesized that this layer (hereafter, the bbp-layer) is linked to N2-yielding microbial communities such as 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 key drivers of anammox-denitrifying bacteria dynamics impact on the vertical distribution of bbp and the thickness of the bbp-layer. In conjunction with published data on N2 excess, our results suggest that the bbp-layer is at least partially composed of anammox-denitrifying bacteria for three main reasons: (1) strong correlations are recorded between bbp and nitrate; (2) the top location of the bbp-layer is driven by the ventilation of oxygen-rich subsurface waters, while its thickness is modulated by the amount of nitrate available to produce N2; (3) the maxima of both bbp and N2 excess coincide at the same isopycnals where denitrifying-anammox bacteria coexist. We thus advance that bbp and O2 can be exploited as a combined proxy to delineate the N2-yielding section of the Black Sea. This proxy can potentially contribute to refining delineation of the effective N2-yielding section of oxygen-deficient zones via data from the growing BGC-Argo float network.