Use of Phosphate Oxygen Isotopes for Identifying Atmospheric‑P Sources: A Case Study at Lake Kinneret Avner Gross,* ,† Ami Nishri, ‡ and Alon Angert † † The Institute of Earth Sciences, The Hebrew University of Jerusalem, Israel ‡ Kinneret Limnological Laboratory, KLL, Israel Oceanographic and Limnological Institute, IOLR, Migdal, Israel ABSTRACT: The input of phosphorus (P) through atmospheric deposition can be a major source of P to fresh water bodies and may strongly affect their biogeochemistry. In Lake Kinneret (LK), northern Israel, dust deposition provides a significant fraction of the bioavailable P input. Here, we demonstrate that the oxygen isotopic composition of resin-extractable inorganic phosphate (δ 18 O P ) in dust particles can be used to identify the phosphate source. Samples of soils with both natural vegetation and agricultural cover were collected upwind of LK and found to have distinct δ 18 O P value ranges (17.4-18.2‰ and 19.3-22.1‰, respectively). The δ 18 O P values for dust, collected continuously over LK during June 2011 to March 2012, were in the same range as agricultural soils. The dust concentration in the air decreased from the dry to the wet season and was correlated with a decrease in P concentration in air, yet no correlation was found between these parameters and dust δ 18 O P . Dust deposited during short-term desert dust events was characterized by a combination of high δ 18 O P values ranging from 22.2‰ to 22.7‰ and high concentrations of dust in the air. The data we present demonstrates a new application of δ 18 O P measurements for direct estimation of dust-P sources to lakes, as well as the potential for tracing dust-P on larger scales. 1. INTRODUCTION Studies of atmospheric phosphorus (P) provide evidence for the importance of airborne particles in delivering P to oceans, fresh water bodies, and terrestrial ecosystems. 1-3 These airborne particles (hereafter “dust”), derived from soils, consist mainly of fine particles. The significant role of dust particles in P transport is probably due to their highly specific surface area that contributes to their high P sorption capacity. 4 Dust-P inputs sustain marine ecosystem productivity in certain areas and may strongly impact terrestrial ecosystem biogeochemistry in other areas. 5,6 In fresh water bodies, excessive P input has the potential to accelerate eutrophication. Lake Kinneret (LK; the Sea of Galilee, Figure 1) in northern Israel is the only large body of fresh water in Israel and provides about one-quarter of Israel’s water supply. It also supplies water to the Hashemite Kingdom of Jordan. Maintaining water quality is therefore of prime importance; hence, nutrient loads in the Amud Stream and Jordan River that feed the lake and the concentrations in the lake itself have been monitored since 1969, and factors affecting the lake biological and chemical composition have been studied intensively. 7-9 However, the effects of dust deposition on the lake and its water properties have only been considered recently by Ganor and Foner 10 that showed that it comprises 40% of the lake annual P supply. Here, we developed an approach based on using phosphate-stable oxygen isotope (δ 18 O P ) to trace the sources of P in dust. This approach is based on comparing the δ 18 O P values in the dust particles collected at the lake shore to those values found in local soils upwind of the lake. The ability to use the δ 18 O P as a conservative tracer is based on the chemical stability of the P-O bond, which under most surface environmental conditions (T < 80 °C), and in the short time scales relevant to this study is broken only by enzyme- mediated reactions. These biotic reactions produce an exchange between oxygen in water and oxygen in phosphate, a process which is accompanied by isotopic fractionation. In contrast, abiotic processes bring about insignificant exchange and only weakly fractionate phosphate oxygen isotopes. 11-13 Hence, we can assume that no isotopic fractionation is associated with abiotic adsorption of phosphate to fine soil particles and therefore that the dust δ 18 O P value reflects the actual source soil value. The utilization of δ 18 O P as a tracer of various phosphate sources to lakes, rivers, estuaries, and oceans has been found useful when the source isotopic signature has not been erased by P biological cycling after reaching the sink reservoir. 14-17 To overcome this confounding factor of possible alteration of δ 18 O P values after the dust was deposited in LK, dust was collected prior to its deposition in the lake; thus, its δ 18 O P retained the isotopic signature of the source. The dust contains few P pools, which can be operationally defined by the extraction method (HCl-extractable, NaOH- extractable, anion-exchange-resin-extractable, etc.) For being useful as a tracer, the selected P pool should have different δ 18 O P values at the different possible dust sources. Here, we decided to focus on the resin-P pool, considered to be a form of labile inorganic P, which is lightly adsorbed to the outer Received: July 2, 2012 Revised: February 10, 2013 Accepted: February 18, 2013 Published: February 18, 2013 Article © 2013 American Chemical Society 2721 dx.doi.org/10.1021/es305306k | Environ. Sci. Technol. 2013, 47, 2721-2727