Role of Eh and pH on Phosphorus Geochemistry in Sediments of Lake Okeechobee, Florida

Increases in total P levels in Lake Okeechobee, Florida, have given rise to concern over eutrophication. The objective of this study was to evaluate the effects of redox potential and pH on the solubility of P in lake sediments. Bulk sediment samples were obtained from the mud zone of Lake Okeecbobee and were equilibrated under controlled conditions at fixed Eh and pH levels. The pH levels evaluated were 5.5, 6.5, 7.5, and 8.5; the Eh levels studied were 500, 250, 0, and -250 mV. Redox reactions were very important in the regulation of P in Lake Okeechobee sediments. Under oxidized conditions, soluble reactive P (SRP) concentrations were low (≈ 0.1 mg P L^-1 ), whereas under reducing conditions SRP increased to over 1 mg P L^-1 . Soluble reactive P was extremely high (18 mg P L^-1 ) under acidic (pH 5.5), reducing (Eh < 0 mV) conditions. Water soluble Fe was highly correlated to water soluble P, implicating it as a possible agent governing P behavior. Sodium hydroxide-extractable P (Fe and Al bound) increased with increases in Eh, which indicated Fe phosphate precipitation or adsorption of P by Fe oxides or hydroxides. This was supported by mineral equilibria calculations, which showed porewaters were supersaturated with respect to strengite under oxidized conditions. Calcium-bound P was higher under reducing conditions. The results suggest that Fe phosphate precipitation controls the behavior of P in Lake Okeechobee sediments under oxidizing conditions, whereas Ca phosphate mineral precipitation governs P solubility under reducing conditions. These results also suggest that large fluxes of P from the sediment could occur if the lake water column were to experience low dissolved O₂ levels, due to the reduction and subsequent solubilization of ferric phosphate minerals in surficial sediments. Measurements of P fluxes from intact sediment cores and porewater SRP profiles taken in situ supported this hypothesis.