We demonstrate the utility of phosphate oxygen isotope measurements for tracking anthropogenic phosphorus loadings with measurements on sewage treatment plant effluent, the Connecticut River, and an aquifer on Cape Cod. Microbial activity acts to equilibrate the oxygen isotope composition of PO₄ with that of ambient water as a function of temperature. In surface and groundwater systems where dissolved PO₄ loadings are high, the capacity of a microbial community to cycle an ambient dissolved PO₄ reservoir may be exceeded. In such cases, disequilibrated oxygen isotope compositions for dissolved PO₄ may prevail and provide a diagnostic source signal. A simple model is used to show how the approach of d¹⁸O values towards equilibrium along a flow path can constrain the rate of microbial phosphate cycling.