Tracing the sources of phosphate into the San Joaquin River using oxygen isotope signatures

Phosphorous (P) is an essential macronutrient in aquatic ecosystems and eutrophication due to anthropogenic inputs of phosphorous has been observed in many places throughout the world. Although P has only one stable isotope, advances in stable isotope mass spectrometry have made it possible to analyze the isotopic composition of the oxygen atoms bound to the P in both dissolved inorganic phosphate and in various organic phosphorus containing compounds including particulate organic matter. The oxygen isotopic composition of phosphate (δ18Op) can be used as a tracer of phosphate sources, and has been used successfully in recent studies to identify the presence of different phosphate sources in the coastal ocean and estuaries. In this study, we report the δ18Op of primary anthropogenic phosphate sources to aquatic ecosystems including fertilizers, manures, detergents, urban run-off, and sewage treatment effluents. We examine the δ18Op of both river water and potential phosphate sources throughout the San Joaquin River area, and combine this information with multi-isotope data from our other ongoing studies in order to gain a better understanding of nutrient sources and cycling within this watershed. The San Joaquin River watershed was chosen for this study due to the wide variety of potential nutrient sources in the area including waste water treatment plant discharge, multiple types of agricultural and livestock run-off containing fertilizers and manure waste, wetland drainage, groundwater discharge, urban run-off, and tributaries draining different soil types. The data from this study will be valuable for understanding the contributions of various P sources to the San Joaquin River area, and for identifying and distinguishing various natural and anthropogenic phosphate inputs to aquatic ecosystems in general.