Nitrogen is an essential nutrient for living systems in the ocean; it is a component of proteins, chlorophyll, and nucleic acids. In the oceans, nitrogen exists as a dissolved gas (N2), dissolved organic matter, or as the dissolved inorganic ions nitrate (NO3–), nitrite (NO2–), and ammonium (NH4+). Nitrogen has a relatively short lifetime in the ocean and we are still striving to understand its sources and sinks. Transformations under oxygen-limited conditions are some of its main sinks. The Black Sea is an ideal location to study nitrogen reactions that occur under low-oxygen conditions because it has a well-defined low-oxygen, or suboxic, zone at the boundary between its oxic surface layer and sulfidic (anoxic) deep water.
Murray, J.W., C. Fuchsman, J. Kirkpatrick, B. Paul, and S.K. Konovalov. 2005. Species and δ15N signatures of nitrogen transformations in the suboxic zone of the Black Sea. Oceanography 18(2):36–47, https://doi.org/10.5670/oceanog.2005.40.