First Measurements on Triple Oxygen Isotopes of Dissolved Inorganic Phosphate in the Hydrosphere

Although dissolved inorganic phosphate (DIP) is an important nutrient in the hydrosphere, it is difficult to quantitatively clarify the dynamics of DIP in the hydrosphere using the δ¹⁸O value of DIP as a tracer. In this study, we quantified the triple oxygen isotopic compositions (Δ'¹⁷O) of DIP relative to VSMOW with the reference line with a slope of 0.528 as an additional tracer to clarify the sources and dynamics of DIP in the hydrosphere. We found significant variation in the Δ'¹⁷O values of riverine DIP in urban areas, ranging from -107 × 10^-6 to +3 × 10^-6, while those of DIP in the effluents from wastewater treatment plants (WWTP) and DIP extracted from the chemical fertilizers showed -56 ± 5 × 10^-6 (1SD) and -98 ± 5 × 10^-6, respectively. We conclude that both the DIP supplied directly from the artificial loads (the WWTP effluent and chemical fertilizers) showing ¹⁷O-depleted Δ'¹⁷O values and the DIP turned over via the aquatic biosphere showing ¹⁷O-enriched Δ'¹⁷O values similar to ambient H₂O were the major sources of riverine DIP. High-precision determination of the Δ'¹⁷O value of DIP can contribute to quantitative clarification of the dynamics of DIP in the hydrosphere.