Modelling nitrogen dynamics in the Black Sea

Abstract

The Black Sea is considered as the largest anoxic basin in the world, resulting in rapid variations of oxidation-reduction potential in the water column. These variations lead to the appearance of reactions such as denitrification and anaerobic oxidation of ammonium (anammox), which modify the content of fixed nitrogen (mainly ammonium and nitrate) in the Black Sea. The losses of nitrogen following these reactions are probably partially compensated by the inputs of rivers, but another source (nitrogen fixation by so-called ”diazotrophic” microorganisms - cyanobacteria or heterotrophic bacteria) has been demonstrated several times. However, it is difficult to quantify. The addition of diazotrophs (photo autotrophic cyanobacteria) in an existing model has shown that these microorganisms have a role to play in the nitrogen cycle of the Black Sea and that, even if the rates calculated by the model are lower than those measures/calculated so far, they probably compensate for some of the fixed nitrogen losses. It also seems important to continue the process by adding a second group of diazotrophs in the form of heterotrophic bacteria.