Dissolved reactive manganese at pelagic redoxclines (part II): Hydrodynamic conditions for accumulation

Dissolved reactive Mn (dMn react ) has been determined at the redoxclines of two anoxic deeps from the Baltic Sea (Landsort Deep and Gotland Basin) and two seasonally anoxic freshwater lakes (Lake Dagow and Fuchskuhle, Germany). This dMn react fraction is rapidly oxidised under oxygen atmosphere and is assumed to consist predominantly of Mn(III). There is a distinct increase of dMn react from the outer regions towards the central part of the Landsort Deep. Although the presence of MnO x particles in the Gotland Basin is evidence of ongoing oxidation of reduced Mn species, almost no dMn react was detected. Since completely different processes of Mn oxidation appear rather unlikely, we suggest oceanographic properties are responsible. Lateral currents and intrusions in the Gotland Basin seem to prevent the formation of a stable suboxic zone, a prerequisite necessary for accumulation of dMn react . Such perturbations supply trace amounts of O 2 and H 2 S, causing either immediate oxidation/reduction of dMn react or deterioration of its stabilising ligands. dMn react has also been determined in Lake Dagow with values significantly exceeding the level of the Landsort Deep due to stable stratification of this lake. In contrast, H 2 S appearance throughout the entire water column and a pH react in Lake Fuchskuhle.