The suspended small-particle layer in the oxygen-poor Black Sea: a proxy for delineating the effective N<sub>2</sub>-yielding section

Abstract

Abstract. The shallower oxygen-poor water masses of the ocean 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 microbial communities involved in N2 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 N2-yielding bacteria dynamics impact 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 the bacteria driving N2 yielding 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; and (3) the maxima of both bbp and N2 excess coincide at the same isopycnals where bacteria involved in N2 yielding 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.

Biogeographical Classification of the Global Ocean From BGC‐Argo Floats

“…In the AR biome, weak correlations between particle stocks in the euphotic and mesopelagic zones are presumably attributable to the presence of mesopelagic biomass maxima as evident in Figure 5 at depths of ∼300 m. As previously reported, these maxima are likely caused by large populations of denitrifying bacteria at the depth of the oxygen minimum zone (Rasse et al, 2020;Roullier et al, 2014;Ward et al, 2009;Whitmire et al, 2009;Wojtasiewicz et al, 2020). This seems especially likely given the comparatively enhanced seasonality in the lower mesopelagic zone (b bp_m3 and b bp_m4 in Figure 10), and the significant correlation between b bp_pz and b bp_m3 in the AR biome (Table S7 in Supporting Information S1).…”

Using Argo Floats to Characterize Altimetry Products: A Study of Eddy-Induced Subsurface Oxygen Anomalies in the Black Sea

“…It would be relevant to apply a similar methodology to carefully calibrated additional Argo data variables such as chlorophyll (Ricour et al, 2021), nitrate, particulate organic matter, and hydrogen sulfide (Rasse et al, 2020). However, the lesser number of available profiles and the seasonality of biogeochemical cycles would prevent a direct application of the broad-scale, annual perspective adopted here.…”

The Synergy of Data From Profiling Floats, Machine Learning and Numerical Modeling: Case of the Black Sea Euphotic Zone

“…More details about the observations, their technical specifications, signal processing, quality control and definitions are given in Appendix and in previous publications (Kubryakov et al., 2020; Rasse et al., 2020, 2021; Roesler et al., 2017; Stanev et al., 2014, 2017, 2018; Xing et al., 2017). The number of floats operating simultaneously is shown in the upper‐right inset of Figure 1a.…”

“…Data with a quality control (QC) flag of 4 ("bad data") were removed (Argo Data Management Team, 2021;Schmechtig et al, 2018), while data with a QCD3 ("probably bad data") were retained. More details on the row data processing can be found in Rasse et al (2020). The vertical resolution of data acquisition was 10 m between 1,000 and 250 m, 1 m between 250 and 10 m, and 0.2 m between 10 m and the surface.…”

“…• Nitrate (NO 3 ) was measured using a Satlantic Submersible Ultraviolet Nitrate Analyzer (SUNA) sensor (Johnson & Coletti, 2002;Johnson et al, 2013). According to Rasse et al (2020), the respective detection limit is ∼0.5 μM.…”