Characterizing the Oxygen Isotopic Composition of Phosphate Sources to Aquatic Ecosystems YoungMegan B. McLaughlinKaren KendallCarol StringfellowWilliam RollogMark ElsburyKaty DonaldElizabeth PaytanAdina 2009 The oxygen isotopic composition of dissolved inorganic phosphate (δ<sup>18</sup>O<sub>p</sub>) in many aquatic ecosystems is not in isotopic equilibrium with ambient water and, therefore, may reflect the source δ<sup>18</sup>O<sub>p</sub>. Identification of phosphate sources to water bodies is critical for designing best management practices for phosphate load reduction to control eutrophication. In order for δ<sup>18</sup>O<sub>p</sub> to be a useful tool for source tracking, the δ<sup>18</sup>O<sub>p</sub> of phosphate sources must be distinguishable from one another; however, the δ<sup>18</sup>O<sub>p</sub> of potential sources has not been well characterized. We measured the δ<sup>18</sup>O<sub>p</sub> of a variety of known phosphate sources, including fertilizers, semiprocessed phosphorite ore, particulate aerosols, detergents, leachates of vegetation, soil, animal feces, and wastewater treatment plant effluent. We found a considerable range of δ<sup>18</sup>O<sub>p</sub> values (from +8.4 to +24.9‰) for the various sources, and statistically significant differences were found between several of the source types. δ<sup>18</sup>O<sub>p</sub> measured in three different fresh water systems was generally not in equilibrium with ambient water. Although there is overlap in δ<sup>18</sup>O<sub>p</sub> values among the groups of samples, our results indicate that some sources are isotopically distinct and δ<sup>18</sup>O<sub>p</sub> can be used for identifying phosphate sources to aquatic systems.