Phosphate-oxygen isotope ratios as a tracer for sources and cycling of phosphorus in the Illinois River Watershed

Excess phosphorus in streams causes eutrophication, which can potentially diminish usability of streams and reservoirs. Phosphorus concentrations and sources are a significant regional concern in the Upper Illinois River Watershed in northwestern Arkansas and northeastern Oklahoma. Recently developed isotopic methods enable comparison of oxygen isotopes in soluble dissolved inorganic phosphate (DIP), potentially allowing sources of phosphates in aquatic systems to be identified. For the phosphate isotope method, phosphate is chelated into a magnesium hydroxide precipitate, precipitated as cerium phosphate, and then dissolved and precipitated as silver phosphate. This method allows reduction of large sample volumes into manageable sizes. Silver phosphate is a pure, stable, non-hygroscopic compound that works well for isotopic analysis. Phosphate oxygen isotope ratios reflect the isotopic ratio of input sources. As organic phosphorus is oxidized, oxygen is derived largely from water and the delta18OP reflects the 5180 of local water. Isotopic fractionation of dissolved inorganic phosphate occurs only as a result of enzyme mediated biologic reactions. The expected equilibrium of delta 18OP has been empirically derived for phosphates produced by microbial cultures. These temperature dependent reactions may add insight to the amount of SRP cycling occurring in the river. If PO4 demand is low relative to input, PO4 oxygen fractionation during biologic processing is negligible, and the delta18OP will reflect the isotopic signatures of the input sources. Sources can be identified and phosphorus uptake rates downstream of sources can be studied. Input sources such as wastewater effluent (25.2‰), poultry litter extract (20.0‰) and commercial fertilizer extract (18.2‰) were sampled and analyzed. Water from selected Illinois River and tributary sites were sampled and analyzed for delta18OP. The method yielded distinguishable 18O signatures for samples from the watershed. This method has not been applied in the central United States, but may provide the opportunity to better understand the sources, transport, and cycling of phosphorus in the Illinois River and similarly impacted streams in the region.