Concentration and natural stable isotope profiles of nitrogen species in theBlack Sea

Abstract

We measured natural abundance profiles of N 2, nitrate, ammonium, suspended particulate organic nitrogen (PON) and total organic nitrogen (TON) concentrations and nitrogen stable isotopes in the Black Sea during five research cruises: R/V Bilim September 1999, R/V Bilim September 2000, Voyage 162, Leg 17 of the R/V Knorr 2001, Voyage 172, Leg 7 of R/V Knorr 2003 and Cruise 403 of R/V Endeavor 2005. Excess N 2 of biogeochemical origin had maximum values that were centered at the lower part of the suboxic zone. The δ 15 N of this excess N 2 was highly depleted in 2000 and 2001 (− 30‰ to − 40‰) and was much less depleted in 1999, 2003 and 2005 (− 1‰). The variabilities of N 2/Ar ratios, excess N 2 and δ 15 N of excess N 2 cannot be explained by variability in the profiles of nitrate and ammonium. Using a nitrogen mass balance of NO 3 −, NH 4 + and N 2 we determined that a missing N flux was required to complete the nitrogen mass balance in some years, especially 2000 and 2001. We suggest that this missing flux is most likely from remineralization of sinking PON. The δ 15 N of the missing nitrogen flux was also highly depleted in 2001 and 2000. The concentration profiles of nitrate, nitrite and ammonium concentrations approached zero, indicating total consumption to form nitrogen gas. However, in 2000 and 2001 the δ 15 N of excess N 2 increased with increasingly excess N 2, which is consistent with partial consumption. We propose that there are different states for the nitrogen system in the Black Sea and the system varies between them due to climate oscillations and variations in the major nitrogen budget components. When the system is at a steady state, total consumption of nitrate and ammonium occurs in the suboxic zone and partial denitrification is of minor importance. This results in higher values of δ 15 N 2 and lower values of excess N 2 in the suboxic zone. The balance of nitrate and ammonium fluxes into the suboxic zone with N 2 gas fluxes out of the suboxic zone in 1999, 2003 and 2005 imply that the system allowed total consumption at steady state in those years. Remineralization of depleted organic matter, presumably from nitrogen fixation in the euphotic zone, is necessary to support N 2 production of − 1‰ during complete consumption of nitrate, nitrite and ammonium. Near surface (oxic region) values of δ 15 N–N 2 in 2000 and 2001 (0.9‰) are more enriched than for N 2 in equilibrium with the atmosphere (0.68‰). These enriched values can be explained by the consumption of N 2 during nitrogen fixation with a fractionation effect between − 1.3 to − 2.6‰. When the system is in a transitory state, like that observed in 2000 and 2001, sinking organic matter from a phytoplankton bloom may stimulate both partial denitrification and the anammox reaction which can produce depleted N 2 and support higher values of excess N 2 in the suboxic zone.