REDOX CONDITIONS AND ALKALINITY GENERATION IN A SEASONALLY ANOXIC LAKE (LAKE GREIFEN)

The redox conditions occurring in Lake Greifen during summer stagnation are examined on the basis of the redox species nitrate and ammonium, managanese (manganese(II) and manganese(III, IV) oxides) and iron (iron (II) and iron(III) oxides). The appearance of the reduced species over the course of the stagnation period corresponds qualitatively to the thermodynamic redox sequence, although no equilibrium is established between the various redox species and thus no single p-epsilon value can be defined. Occurrence of manganese(II) and the reduction of nitrate are predominant, while iron(II) and sulphide occur only at the last stage of stagnation. The concentration of manganese(II) near the sediment-water interface is limited by the solubility of MnCO3(s). The increase of alkalinity in the anoxic hypolimnion over stagnation time is related to the reduction of nitrate.