IRON AND MANGANESE IN LAKES

The role of redox processes in determining the chemistry of iron and m anganese is considered systematically. Both metals have soluble reduce d forms and insoluble oxyhydroxides which are readily interconverted i n the vicinity of a redox boundary. Although the oxyhydroxides are dom inant in well-oxygenated waters, measureable concentrations of Fe(II) and Mn(II) can be observed, especially where photochemical reduction o ccurs. Differences in behaviour are largely attributed to Fe(II) oxidi zing more rapidly than Mn(II) and the relative ease of reduction of ma nganese oxyhydroxides. Mechanisms of reduction and oxidation, includin g the role of micro-organisms, are considered. Particulate and colloid al forms, which are the products of oxidation, are seen to have divers e morphometries and compositions and are as yet poorly characterized. They appear to play a smaller role in the transport of trace metals an d phosphorus than was once believed. Various lacustrine environments, sediments, the sediment-water interface and anoxic and oxygenated wate rs, are considered within a unifying context of the processes occurrin g at a redox boundary. Markedly elevated concentrations of Fe(II) and Mn(II) in anoxic bottom waters have encouraged detailed field studies of their production, transport and oxidation. Results from diverse sit es are brought together to provide an overall understanding of the red ox cycling of these two important elements in lakes.