The oxygen isotope ratio of dissolved inorganic phosphate (δ18Op) represents a novel and potentially powerful stable isotope tracer for biogeochemical research. Analysis of δ18Op may offer new insights into the relative importance of different sources of phosphorus within natural ecosystems. Due to the isotope fractionations that occur alongside metabolism of phosphorus-containing compounds, δ18Op could also be used to better understand the intracellular and extracellular reaction mechanisms that control phosphorus cycling. In this thesis, new methods were developed and tested for the extraction of dissolved inorganic phosphate (Pi) from freshwaters and its isolation from other oxygen-containing compounds, including nitrate, sulfate and dissolved organic matter. Excluding contaminant sources of oxygen during δ18Op analysis is a critical analytical challenge that has constrained δ18Op research in freshwaters to date. These new methods were evaluated against existing protocols for analysis of δ18Op. While the protocol developed in this thesis exhibited greater accuracy and precision for freshwater matrices compared to the traditional approach reported by McLaughlin et al. (2004), further development work is required to increase the accuracy of this protocol compared to that reported by Gooddy et al. (2016). Through the application of δ18Op within two exemplar freshwater ecosystems in this thesis, the in-stream fate of Pi derived from the effluent of a wastewater treatment plant and from groundwater discharge was examined. Within both ecosystems, δ18Op revealed the occurrence of metabolic processes that influenced the in-stream fate of P yet were masked in the hydrochemical data. In addition, the data reported here increase the worldwide groundwater δ18Op dataset nearly threefold. These groundwater data highlight the important potential differences in δ18Op due to bedrock geology, alongside the potential to use δ18Op to better understand the importance of groundwater-derived P following discharge to surface water ecosystems. -Gooddy, D.C., D.J. Lapworth, S.A. Bennett, T.H.E. Heaton, P.J. Williams & B.W.J. Surridge. 2016. A multi-stable isotope framework to understand eutrophication in aquatic ecosystems. Water Research. 88 , 623 -McLaughlin, K., S. Silva, C. Kendall, H. Stuart-Williams & A. Paytan. 2004. A precise method for the analysis of δ18O of dissolved inorganic phosphate in seawater. Limnology and Oceanography: Methods. 2 , 20
