Use of Phosphate-Oxygen Isotope Ratios as a Tracer for Sources and Cycling of Phosphorus in the Illinois River in AR and OK
Excess phosphorus in streams causes eutrophication, which diminishes an aquatic system's capacity for supporting a healthy and normal ecosystem and diverse aquatic communities, water supply needs, and aesthetic and recreational value. Phosphorus concentrations and sources are a significant regional concern at the Upper Illinois River Watershed in northwestern Arkansas and northeastern Oklahoma, as well as for streams across the Nation. Recently developed isotopic methods enable determination of oxygen isotope composition of soluble reactive phosphate (SRP), potentially allowing sources of phosphates in aquatic systems to be identified. For this method, phosphate is chelated into a magnesium hydroxide precipitate, reprecipitated as cerium phosphate, and then dissolved and precipitated as silver phosphate which works well for isotopic analysis. We are interested in phosphate isotopic composition because oxygen isotopic ratios reflect those of input sources. As organic phosphorus is oxidized, oxygen is derived largely from water, and d18OP reflects the d18O of local water. Isotopic fractionation of dissolved inorganic phosphate can occur, but only as a result of enzyme-mediated biologic reactions. The expected equilibrium of d18OP has been empirically derived for phosphates produced by microbial cultures and the temperature-dependent fractionation may add insight to the amount of SRP cycling occurring in the river. If PO4 demand is low relative to input, the d18OP will reflect the isotopic signatures of the input sources, allowing sources to be identified and transport of PO4 to be characterized. Input sources such as wastewater effluent (29‰), poultry litter extract (20‰) and commercial fertilizer extract (18‰) have been sampled and analyzed. Water samples from the Illinois River are now being sampled. This method has not been not previously been applied in the central United States, but we hope results will give us better understanding of the sources, transport, and cycling of phosphorus in the Illinois River and similarly impacted streams in the region.