Modeling the distribution of nitrogen species and isotopes in the water column of the Black Sea

Using new data for the concentrations and δ15N of the main nitrogen species of NO3−, NH4+, PON (particulate organic nitrogen) and N2 in the Black Sea, we studied the distributions of concentrations and δ15N using a coupled physical–biogeochemical model including known biogeochemical processes and nitrogen isotopic fractionation. In the deep anoxic water N2 is supersaturated. This supersaturation results from physical exchange with the Bosporus inflow that includes entrainment of N2 supersaturated water from above the suboxic zone and its injection into the deep water of the Black Sea. In the suboxic zone there is a N2 maximum and a δ15N2 minimum at the depth where anammox has been observed. We first used a version of the model that included both denitrification and anammox. Anammox in this case was fueled by the downward flux of NO3− and the upward flux of NH4+ from the deep water. This steady state model was able to successfully simulate the concentrations of all species and δ15N of NO3− and NH4+. Anammox was required to be much more important than denitrification in order to have the maximum of excess N2 be at the correct depth and to have a NO3− profile with the correct shape. This model can not simulate the depleted δ15N values of N2 observed in the suboxic zone in 2000, 2001 and 2003, unless we assume that PON leaving the suboxic zone is 15N enriched, as compared to observational data, or that there is a presently unaccounted process to preferably remove 15NH4+ from the suboxic zone. Our models support the hypothesis that 15N depleted PON, which could be made in the euphotic zone as the result of N2-fixation, may support production of 15N depleted N2 in the suboxic zone. This 15N depleted PON (with δ15N = − 2.0‰) sinks and is remineralized to produce 15N depleted NH4+, which is rapidly consumed by anammox in the suboxic zone to produce 15N depleted N2. The nitrogen system in the suboxic zone is variable and periods of high excess N2 correspond to periods with depleted δ15N2. We hypothesize that the system oscillates between two states. The first occurs when anammox is driven by remineralization of 15N depleted PON produced by N2-fixation. This results in 15N depleted N2. The second occurs when anammox is driven by the downward flux of NO3− and the upward flux of NH4+. This results in 15N enriched N2.