The oxygen isotopic composition of phosphate in Elkhorn Slough, California: A tracer for phosphate sources

Elkhorn Slough, a small seasonal estuary in central California, has been subjected to increased nutrient loading from agricultural and other non-point sources. However, because nutrients do not behave conservatively, tracing nutrient sources and cycling in ecosystems like Elkhorn Slough has been difficult to assess. This is particularly true of phosphorus (P), which has only one stable isotope and cannot be used as an isotopic tracer. However, isotopic fractionation of oxygen in phosphate at surface water temperatures only occurs as a result of enzyme-mediated, biochemical reactions. Thus, if phosphate demand is low relative to input and is not heavily cycled within the ecosystem, the delta O-18 of phosphate will reflect the isotopic composition of phosphate sources to the system. We utilized the delta O-18 of dissolved inorganic phosphate (DIP) within the main channel of the slough and nearby Moss Landing Harbor and the delta O-18 of reactive phosphate from sediment and soil samples collected within the watershed to understand phosphate sources and cycling within Elkhorn Slough. Trends in the delta O-18 of DIP were seasonally consistent with high values near the mouth reflecting oceanic phosphate (19.1 parts per thousand-20.3 parts per thousand), dropping to a minimum value near Hummingbird Island in the central slough (point source, 14.1 parts per thousand-14.4 parts per thousand), and increasing again near the head of the slough, reflecting fertilizer input (18.9 parts per thousand-19.3 parts per thousand). Reactive phosphate delta O-18 values extracted from sediments and soils in the watershed range from 10.6 parts per thousand, in a drainage ditch to 22.3 parts per thousand. in creek sediments near agriculture fields. The wide range in phosphate delta O-18 values reflects the variations in land use and application of different fertilizers in this agriculturally dominated landscape. These data suggest that phosphate delta O-18 can be an effective tool for identifying P sources and understanding phosphate dynamics in estuarine ecosystems. (c) 2006 Elsevier Ltd. All rights reserved.