Detection of Transient Denitrification During a High Organic Matter Event in the Black Sea

Abstract

N2 production by denitrification can occur in anoxic water or potentially inside organic particles. Here we compare data from the Black Sea, a permanently anoxic basin, during two organic matter regimes: suspended particulate organic matter concentrations were high in the oxycline after the spring bloom in March 2005 compared to lower organic matter concentrations in June 2005, May and October 2007, July 2008, and May 2001. For all cruises, N2 gas had a maximum in the suboxic zone (O2 < 10 μmol/L). During the high organic matter event (March 2005), an additional shallower N2 gas and δ15N‐N2 maxima occurred above the suboxic zone in the oxycline where oxygen concentrations were 30–50 μmol/L. Examination of 16S rRNA indicated that anammox bacteria were not present in the oxycline. The δ15N of biologically produced N2 in the oxycline in March 2005 was significantly enriched (+7‰ to +38‰), not depleted, as would be expected from water column fractionation. A simple diffusion calculation indicated that ammonium produced from remineralization inside particles could be oxidized to nitrate and then completely consumed by denitrification inside the particle. In this calculation, half of denitrified N atoms originated from organic N [δ15N = 11‰] and half of N atoms originated from ambient nitrate [δ15N = 5‰–7‰], producing enriched δ15N‐N2 values. We suggest that denitrifiers were active in microzones inside particulates in hypoxic waters above the suboxic zone of the Black Sea. Denitrification in particles may also explain previous data from the oxycline above ocean oxygen deficient zones.

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

“…For instance, maxima of N 2 excess and b bp coincide around isopycnal 16.11 ± 0.11 kg m −3 ( Fig. 3; Konovalov et al, 2005;Fuchsman et al, 2008Fuchsman et al, , 2019. At this isopycnal, the mean concentration of NO − 3 is 1.19 ± 0.53 µM.…”

“…The Argo Program is part of the Global Ocean Observing System. Data on N 2 : Ar ratios are freely available at https://doi.org/10.1029/2018GB006032 (Fuchsman et al, 2019).…”

“…This depth was used only as a reference to highlight the periods when surface-derived small particles were clearly injected into the oxygen-poor zone. Published data on N 2 : Ar ratios and NO − 3 collected in the southwest of the Black Sea in March 2005 (Fuchsman et al, 2008(Fuchsman et al, , 2019 were exploited to complement discussion of our results. N 2 produced by anaerobic microbial communities (N 2 excess, µM) was estimated from N 2 : Ar ratios and argon concentrations at atmospheric saturation (Hamme and Emerson, 2004).…”

“…Optical proxies of tiny particles can be applied as an alternative approach to assess the vertical distribution of N 2 -yielding microbial communities in oxygen-poor water masses (Naqvi et al, 1993). For instance, nitrate-reducing SAR11 and anammox, denitrifying, and sulfur-oxidizing bacteria are found as free-living bacteria (0.2-2 µm) and can be associated with small suspended (> 2-30 µm) and large sinking (> 30 µm) particles (Fuchsman et al, 2011(Fuchsman et al, , 2012a(Fuchsman et al, , 2017Ganesh et al, 2014Ganesh et al, , 2015. Therefore, particle backscattering (b bp ), a proxy for particles in the ∼ 0.2-20 µm size range (Stramski et al, 1999(Stramski et al, , 2004Organelli et al, 2018), can serve to detect the presence of these free-living bacteria and those associated with small suspended particles.…”

The suspended small-particles layer in the suboxic Black Sea: a proxy for delineating the effective N&lt;sub&gt;2&lt;/sub&gt;-yielding section

“…This is because the vertical profiles of small particles and of N2 excess are qualitatively similar, and both profiles are clearly related to the rate at which NO3is removed from the SOD-A (Figures 3-4). For instance, maxima of N2 excess and bbp coincide around the isopycnal 16.11 ± 0.11 kg m -3 ( Figure 3, Konovalov et al, 2005;Fuchsman et al, 2008Fuchsman et al, , 2019. At this isopycnal, the mean concentration of NO3is 1.19 ± 0.53 µM.…”