Concentrations of brown Eh-sensitive ferric iron by Moessbauer spectroscopy, in surficial silty clay sediments from several locations in Lakes Erie and Ontario. Concentrations of EhFe (super 3+) are well correlated with non-apatite inorganic phosphorus (NAIP) and with 0.1 M NaOH-extractable P. A relationship is derived, [NAIP] = 0.03 + 0.08[EhFe (super 3+) ], which demonstrates the fractionation of NAIP into EhFe (super 3+) -adsorbed P and into residual P. EhFe (super 3+) -P is the main source of available P under anoxic conditions; it also represents a readily exchangeable source of phosphate ion for utilization by algae and bacteria. Redox potentials at the sediment-water interface play a dominant role in determining EhFe (super 3+) values.--Modified journal abstract.

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