Ferrous Iron and Phototrophy as Alternative Sinks for Sulphide in the Anoxic Hypolimnia of Two Adjacent Lakes

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Abstract
(1) Ferrous iron and sulphide accumulated to similar concentrations in the anoxic hypolimnia of two adjacent lakes. Initial consideration of solubility products indicated that the waters were saturated with, and concentrations of these ions so presumably controlled by, particulate iron sulphide. (2) However, in the shallower lake (Priest Pot, <4 m), light penetrated to the hypolomnion and there supported photosynthetic bacteria that oxidized sulphide to sulphate. Iron sulphide therefore failed to form in the water column, but was formed in the sediment from the high concentrations of dissolved components in the pore water. (3) In the deeper lake (Esthwaite Water, 15 m), light was attenuated within the oxidized epilimnion. This precluded phototrophic oxidation of sulphide and allowed simple chemical formation and sedimentation of FeS. (4) These results illustrate a natural competition between geochemical and biological processes of sulphide removal in biologically productive freshwater lakes.