Rates of oxidation of Mn(II) were measured by an in situ incubation technique in the water column of Toolik Lake, Alaska. Measured rates were lower than those observed in other aquatic systems but were sufficient to oxidize all Mn(II) in the lake within a 3-month period. Measured rates compared favorably with rates estimated from a previous study of the geochemical cycling of Mn in Toolik Lake. The Mn(II) oxidation was largely microbially mediated, as indicated by inhibition of oxidation rates by sodium azide. Azide had been previously demonstrated to be a suitable microbial poison for studying Mn(II) oxidation in seawater. This study demonstrates that azide is also a suitable poison for freshwaters and that it inhibits microbial but not abiotic oxidation of Mn(II). Manganese(II) oxidation rates were similar during cold, under-ice conditions in early spring and during warmer summer conditions. This observation suggests that Mn(II) concentration, rather than temperature or oxygen concentration, is the most important factor regulating Mn(II) oxidation rates in Toolik Lake.
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