Phosphate oxygen isotope ratios as a tracer for sources and cycling of phosphate in North San Francisco Bay, California

composition of DIP (d 18 Op) will reflect the isotopic composition of the source of phosphate to the system. Such is the case for the North San Francisco Bay, an anthropogenically impacted estuary with high surface water phosphate concentrations. Variability in the d 18 Op in the bay is primarily controlled by mixing of water masses with different d 18 Op signatures. The d 18 Op values range from 11.4% at the Sacramento River to 20.1% at the Golden Gate. Deviations from the two-component mixing model for the North Bay reflect additional, local sources of phosphate to the estuary that vary seasonally. Most notably, deviations from the mixing model occur at the confluence of a major river into the bay during periods of high river discharge and near wastewater treatment outlets. These data suggest that d 18 Op can be an effective tool for identifying P point sources and understanding phosphate dynamics in estuarine systems.