Section 7
Chapter 6,634

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

Davison, W.; Woof, C.; Rigg, E.

Limnology and Oceanography 27(6): 987-1003


ISSN/ISBN: 0024-3590
DOI: 10.2307/2836170
Accession: 006633592

Sediment traps [20] were employed for a year in 5 arrays at 3 locations in a seasonal anoxic basin in Esthwaite Water, U.K. Accumulated Fe and Mn were measured fortnightly, as were vertical profiles of temperature, O2, total Fe and Mn, polarographically determined Fe(II) and Mn(II) and light attenuance. Most of the Fe and Mn 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 Fe retained with minimal loss. Although dissolved Fe 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 Fe loading to the lake. The overall cycling of Fe 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 Fe due to vertical transport of ferrous Fe by eddy diffusion, oxidation to Fe(III) by O2, sinking of the resultant ferric particles, and redissolution to ferrous Fe. The Mn cycle contrasts markedly with that of Fe. The Mn 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 Mn never reaches the sediment because it is reduced and accumulated in the anoxic hypolimnion. Relatively little Mn is supplied from the sediment during this time because the supply of particles is interrupted. Although most of the Mn which enters the lake is washed out again it will nearly all have undergone a redox cycle.

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