Variation of manganese, dissolved oxygen and related chemical parameters in the bottom waters of Lake Mendota, Wisconsin

The increase of Mn concentration and the simultaneous depletion of dissolved oxygen in the bottom waters (hypolimnion) of Lake Mendota, Wisconsin were studied in situ. Considerably higher Mn concentrations were found in the hypolimnion compared to Fe, despite a sedimentary Fe/Mn weight ratio of 19 : 1. Laboratory equilibration studies under anoxic conditions indicated that pH was an important parameter involved in the release of Mn from the sediments. This was also implied by the Mn and pH data measured in situ. Possible mechanisms involved in the release of Mn from the sediments including desorption of Mn from oxide substrates, reduction of Mn oxides and dissolution of carbonates and sulfides are discussed.