Influence of redox potential on phosphate-uptake by sediments in two sub-tropical eutrophic lakes |
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Authors: | Olila O G Reddy K R |
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Institution: | (1) Soil and Water Science Department, University of Florida, 106 Newell Hall, P.O. Box 110510, Gainesville, FL 32611-0510, USA |
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Abstract: | Biogeochemical reactions in shallow eutrophic lakes areaffected bythe changes in redox potential (Eh) as bottom sedimentsundergotemporal resuspension and settling. The stability of varioussediment P fractions and kinetics of P-uptake were evaluatedfortwo sub-tropical lakes (Lake Apopka and Lake Okeechobee,Florida)using sediment suspensions in closed systems maintained atvariousEh levels ranging from –235 to 555 mV. Redox potential hadminimal effect on the stability of NaOH-P (Fe-/Al-bound P plusmoderately resistant organic P) and HCl-P (Ca-/Mg-P) fractionsinLake Apopka sediments. Increases in ortho-P and NH4Cl-P(loosely-bound P plus labile organic P) concentrations wereobserved in highly reduced (Eh=–225 mV) Apopkasediments.Phosphate solubility diagrams and mineral equilibriacalculationssuggest that P-uptake by Apopka bottom sediments at elevated Pconcentrations (ortho-P110 M) was due toformationof Ca-P compounds and/or co-precipitation of P withCaCO3. Incontrast, the ortho-P concentrations for Lake Okeechobeebottomsediments increased exponentially with decreasing Eh. Thequantityof NaOH-P fractions for these sediments decreased withdecreasingEh, suggesting the release of Fe- and Mn-bound P intosolution.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 mudsediments in Lake Okeechobee have strong affinity for P ineitheraerobic or anaerobic conditions. Results suggest that even incalcareous systems, Fe and Al, when present in highconcentrations(as in the case of Lake Okeechobee), are actively involved inregulating P-uptake and geochemistry. |
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Keywords: | P sorphon kinetics redox potential (Eh) lake sediments P fractions mineral P equilibria eutrophic lakes |
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