Sulfide, iron, manganese, and phosphate in the deep water of the Chesapeake Bay during anoxia

Abstract Concentrations of dissolved oxygen and sulfide, and of dissolved and particulate iron, manganese, and phosphate were measured as functions of salinity at a station in the Chesapeake Bay during stratification and deep water anoxia in spring and summer, 1981. The observed concentration/salinity profiles showed that oxygen was transported in a direction opposite to that of salt, while dissolved sulfide was transported in the same direction as salt through the anoxic water to be oxidized in oxygen consumption zones located below the steepest parts of the halocline. Both oxygen and sulfide were transported conservatively on 18 June. Their fluxes were 1·2 and 2 mol m −2 d −1 , respectively. The oxygen flux was 30% of that stoichiometrically needed to oxidize the sulfide transported, suggesting that the oxygen consumption zone was advancing to shallower, less saline water, thus increasing the volume of anoxic water. Although oxygen was transported conservatively, sulfide was produced as it was transported through the anoxic water on 8 July. The anoxic water was supersaturated with respect to ferrous sulfide on 18 June, but most of the anoxic water was saturated on 8 July. Precipitation of ferrous sulfide had little effect on the sulfide flux on 18 June. The manganese(II) concentration/salinity profile exhibited a maximum in the oxygen consumption zone on 18 June. On 8 July the profile was independent of salinity at high salinities. Iron(II) and manganese(II) consumed little if any oxygen in the oxygen consumption zone. Soluble reactive phosphate was transported conservatively through the anoxic water on 18 June. It was produced as it was transported on 8 July. All of the phosphate was consumed in the oxygen consumption zones by sulfide oxidizing bacteria. On 18 June its flux, estimated to be 2·8 mmol m −2 d −1 , was less than 10% of that required for bacterial oxidation of the sulfide reaching the oxygen consumption zone. The rest was oxidized chemically. The growth and activity of the bacteria were limited by the rate at which soluble reactive phosphate was transported to the oxygen consumption zone. Little or none of the sulfide, iron(II), or phosphate originated in the bottom sediment at the station. The results indicate that they were transported into the water sampled from deeper more saline water downstream, suggesting that they originated in the deep trough that extends along the spine of the Bay. Manganese(II), however, resulted from the reduction and dissolution of oxidized manganese particles as they sank into the anoxic water.