The dynamics of iron and manganese in a seasonally anoxic lake; direct measurement of fluxes using sediment traps

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Abstract
Twenty sediment traps were deployed for a year in five arrays at three locations in a seasonal anoxic basin in Esthwaite Water, U.K. Accumulated iron and manganese were measured fortnightly, as were vertical profiles of temperature, oxygen, total iron and manganese, polarographically determined Fe(II) and Mn(II), and light attenuance.Most of the iron and manganese flows into the lake in winter and is nearly all caught in sediment traps in deep hypolimnetic water, indicating that both elements are transported to the sediment. Accumulation rates in the sediments show that the iron is retained with minimal loss. Although dissolved iron increases to very high concentrations in the hypolimnion during the period of summer anoxia, this accumulation only accounts for a small fraction of the annual iron loading to the lake. The overall cycling of iron in the lake is consistent with a simple conception of sediment‐water interactions. When a redox boundary exists in the water column there is a separate cycle of iron due to vertical transport of ferrous iron by eddy diffusion, oxidation to Fe(III) by oxygen, sinking of the resultant ferric particles, and redissolution to ferrous iron. The manganese cycle contrasts markedly with that of iron. The manganese which reaches the sediment in winter is rapidly reduced and released to the overlying oxic water so that <10% is permanently accumulated in the sediment. During summer the manganese never reaches the sediment because it is reduced and accumulated in the anoxic hypolimnion. Relatively little manganese is supplied from the sediment during this time because the supply of particles is interrupted. Although most of the manganese which enters the lake is washed out again it will nearly all have undergone a redox cycle.