Manganese eutrophic lake

Few examples of studies on cycling of trace elements (other than iron and manganese) at oxic-anoxic interfaces are found in the literature (11-17). Trace element cycling in the water column of a eutrophic lake (Figure 1) is affected by a number of processes related to the redox conditions. 

This chapter discusses the chemical mechanisms influencing the fate of trace elements (arsenic, chromium, and zinc) in a small eutrophic lake with a seasonally anoxic hypolimnion (Lake Greifen). Arsenic and chromium are redox-sensitive trace elements that may be directly involved in redox cycles, whereas zinc is indirectly influenced by the redox conditions. We will illustrate how the seasonal cycles and the variations between oxic and anoxic conditions affect the concentrations and speciation of iron, manganese, arsenic, chromium, and zinc in the water column. The redox processes occurring in the anoxic hypolimnion are discussed in detail. Interactions between major redox species and trace elements are demonstrated. 

Friedl, G., Wehrli, B., and Manceau, A., Solid phases in the cycling of manganese in eutrophic lakes New insights from EXAFS spectroscopy, Geochim. Cosmochim. Acta, 61, 275, 1997. 

The upper portion of the thermocline in a nutrient rich (eutrophic) lake contains some oxygen, but the lower portion of the zone may contain little or no oxygen. Thus, the thermocline can represent a transition zone in which the manganese would be in a relatively unstable state. Mn02(s), and Mn(II) in the free and sorbed state, are found in this zone (Fig. 5.11). 

During thermal stratification of a eutrophic lake, the hypolimnion would be essentially anoxic and manganese would occur primarily as Mn(II)), as found in Lake Mendota (U.S.A.) by Delfino and Lee (1968) and Nordbytjer-net (Norway) by Hongve (1974). Microbial activity certainly is responsible for reduction of Mn(IV) to Mn(II) under such conditions (Dubinina et al., 1974). The presence of dissolved sulphide and carbonate in the hypolimnion may result in the precipitation of MnS and MnCOs if conditions are suitable (Morgan and Stumm, 1965b Delfino and Lee, 1968 Hongve, 1974). 

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