THE CORRELATION BETWEEN IRON SULFIDE PRECIPITATION AND HYPOLIMNETIC PHOSPHORUS ACCUMULATION DURING ONE SUMMER IN A SOFTWATER LAKE

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 (pK(sp)) at the bottom of the hypolimnion was 3.25, which is consistent with other lak e and laboratory studies and is in the range of both amorphous FeS and mackinawite. The precipitation of iron sulfide removed Fe2+ and HS- f rom the water column, and acid volatile sulfide (AVS) accumulated in t he 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 sedi ment was correlated with an increased accumulation of phosphorus in th e water column. The rate of accumulation of phosphorus in the hypolimn ion was greatest when iron was lost to the sediment as AVS. The statis tically significant link between iron storage in the sediment and phos phorus accumulation in the overlying water suggests that reoxidation o f ferrous iron to ferric (hydr)oxides is decreased, and therefore prec ipitation of ferric phosphate is limited.