Modeling the influence of oxygenated inflows on the biogeochemical structure of the Gotland Sea, central Baltic Sea: Changes in the distribution of manganese

A coupled hydrodynamic-biogeochemical one-dimensional O–N–S–P–Mn–Fe-model based on the RedOx Layer Model, (ROLM) and the General Ocean Turbulence Model (GOTM) is used to simulate basic changes in the biogeochemical water column structure in transition phases between oxic and anoxic conditions in a marginal sea. Organic matter (OM) formation and decay, the reduction and oxidation of species of nitrogen, sulfur, manganese, iron, and the transformation of phosphorus species are parameterized in the model. The influence of oxygenated intrusions on the vertical biogeochemical structure of the central Gotland Basin of the Baltic Sea is modeled. The model-produced simulations demonstrate that a complete ventilation of the Gotland Deep bottom water caused by massive inflows of oxygenated North Sea water has led to substantial changes in the vertical biogeochemical structure of the basin. During the inflow events large amounts of iron and manganese precipitate and vanish from the water column. In addition redox reactions are accelerated and bacterial growth leads to an increase of particulate matter content and consequent sedimentation of particles. During reestablishment of anoxic conditions, the structure of the water column is unbalanced. This is partly due to the absence of Mn species that play a dominant role in the oxidation–reduction reactions at the pelagic redox interfaces. This unbalanced structure can serve as a biotope for the development of untypical microbial redox-cline reactions (i.e. anammox). According to our model simulations, the reestablishment of steady state biogeochemical conditions following a complete flushing takes ∼1.5 years.