Influence of redox potential on phosphate-uptake by sediments in two sub-tropical eutrophic lakes

Biogeochemical reactions in shallow eutrophic lakes are affected by the changes in redox potential (Eh) as bottom sediments undergo temporal resuspension and settling. The stability of various sediment P fractions and kinetics of P-uptake were evaluated for two sub-tropical lakes (Lake Apopka and Lake Okeechobee, Florida) using sediment suspensions in closed systems maintained at various Eh levels ranging from -235 to 555 mV. Redox potential had minimal effect on the stability of NaOH-P (Fe-/Al-bound P plus moderately resistant organic P) and HCl-P (Ca-/Mg-P) fractions in Lake Apopka sediments. Increases in ortho-P and NH4Cl-P (loosely-bound P plus labile organic P) concentrations were observed in highly reduced (Eh = -225 mV) Apopka sediments. Phosphate solubility diagrams and mineral equilibria calculations suggest that P-uptake by Apopka bottom sediments at elevated P concentrations (ortho-P less than or equal to 110 mu M) was due to formation of Ca-P compounds and/or co-precipitation of P with CaCO3. In contrast, the ortho-P concentrations for Lake Okeechobee bottom sediments increased exponentially with decreasing Eh. The quantity of NaOH-P fractions for these sediments decreased with decreasing Eh, suggesting the release of Fe- and Mn-bound P into solution. Phosphate-uptake by Okeechobee bottom sediments (pH 7.5, ambient) followed first order kinetics, yielding a rate constant (k) of 0.51 +/- 0.05 h(-1). Unlike that of Apopka, the mud sediments in Lake Okeechobee have strong affinity for P in either aerobic or anaerobic conditions. Results suggest that even in calcareous systems, Fe and Al, when present in high concentrations (as in the case of Lake Okeechobee), are actively involved in regulating P-uptake and geochemistry.