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

N-2 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 (O-2 < 10 mu mol/L). During the high organic matter event (March 2005), an additional shallower N-2 gas and delta N-15-N-2 maxima occurred above the suboxic zone in the oxycline where oxygen concentrations were 30-50 p.mol/L. Examination of 16S rRNA indicated that anammox bacteria were not present in the oxycline. The delta N-15 of biologically produced N-2 in the oxycline in March 2005 was significantly enriched (+7 parts per thousand to +38 parts per thousand), 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 [delta N-15 = 11 parts per thousand] and half of N atoms originated from ambient nitrate [delta N-15 = 5 parts per thousand-7 parts per thousand], producing enriched delta N-15-N-2 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.