ABSTRACT
Manganese enters rivers and lakes from a variety of sources. Manganese oxyhydroxides, as well as nitrate, iron oxyhydroxides, and sulphate, can act as alternative electron acceptors to oxygen in anaerobic decomposition processes. The nearly constant vertical profile of Mn in early summer is due to the dual supply from the sediment and from reduction of sinking manganese particles occurring within the water column. Although manganese particles are mainly present in freshwater as oxyhydroxides associated with bacteria, other particles often contain minor amounts of manganese. Distinct mineral-forms of manganese have mainly been identified as components of iron and manganese-rich nodules or crusts. The elemental composition of manganese oxide material collected directly from a soft water lake and obtained by oxidation of the anoxic lake water was similar but variable. The role of manganese particles in the cycling of trace components in lakes is only just beginning to emerge.
