Estimation of denitrification in Baltic Sea deep water from gas tension measurements

Vertical profiles of dissolved N2 concentrations were calculated from measurements of gas tension, O2, CO2, and H2S concentrations, water temperature, and salinity at the Gotland Basin (Baltic Sea) central station in June 2008. A gas tension device (GTD) with a flow-through plenum was combined with a pump–CTD for continuous sampling. The maximum uncertainty of the N2 concentration measurement was 0.40%. This was upgraded to 0.20% in anoxic deep-water layers. The data were used to determine the N excess with respect to saturation values. The results were attributed to the amount of total denitrification that had occurred in the water mass since its last contact with the atmosphere and interpreted in relation to the history of the water mass. In the water layer between 75 and 150 m depth, which is occasionally influenced by lateral inputs of oxygenated water, the denitrification signal was 27 μmol N L−1. At a depth below 150 m, the N excess in the water mass was 41 μmol N L−1, which is considered to be the upper limit. Based on the results of earlier investigations of the nitrogen budget of the same water mass, our findings indicated that about half of the N2 excess was derived from denitrification in water masses located outside the Gotland Basin and subsequently transported to the basin with saltwater inflows. A comparison between N release due to mineralization processes (N2 excess and dissolved inorganic nitrogen) and the corresponding release of total CO2 yielded a mean C:N ratio of 7.7, which agrees reasonably with the elemental composition of particulate organic matter in the surface water.

Research highlights
► Total denitrification in deep waters was determined by gas tension measurements.
► In anoxic deep water of the Gotland Basin (Baltic Sea) it was about 41 µmol L−1.
► 55% of that value had occurred already during transport processes of the water mass.
► The release of N and total CO2 due to mineralization yielded a C:N ratio of 7.7.