The correlation between iron sulfide precipitation and hypolimnetic phosphorus accumulation during one summer in a softwater lake

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Abstract
During late summer, 1992, the water immediately overlying the sediment in the anoxic hypolimnion of Crystal Lake, Conn., was saturated with ferrous sulfide (FeS). The average solubility product (pKsp) at the bottom of the hypolimnion was 3.25, which is consistent with other lake and laboratory studies and is in the range of both amorphous FeS and mackinawite. The precipitation of iron sulfide removed Fe2+ and HS− from the water column, and acid volatile sulfide (AVS) accumulated in the top 6–8 cm of the sediment. Pyrite was formed at deeper depths and did not increase during the study. The accumulation of AVS in the sediment was correlated with an increased accumulation of phosphorus in the water column. The rate of accumulation of phosphorus in the hypolimnion was greatest when iron was lost to the sediment as AVS. The statistically significant link between iron storage in the sediment and phosphorus accumulation in the overlying water suggests that reoxidation of ferrous iron to ferric (hydr)oxides is decreased, and therefore precipitation of ferric phosphate is limited.