title: Tracing the sources and cycling of phosphorus in river sediments using oxygen isotopes: Methodological adaptations and first results from a case study in France.
abstract: An essential aspect of eutrophication studies is to trace the ultimate origin of phosphate ions (P-PO4) associated with the solid phase of river sediments, as certain processes can make these ions available for algae. However, this is not a straightforward task because of the diversity of allochthonous and autochthonous sources that can supply P-PO4to river sediments as well as the existence of in-stream processes that can change the speciation of these inputs and obscure the original sources. Here, we present the results of a study designed to explore the potentials, limitations and conditions for the use of the oxygen isotope composition of phosphate (δ18Op) extracted from river sediments for this type of tracing. We first tested if the method commonly applied to soils to purify P-PO4and to measure their δ18Op concentrations could be adapted to sediments. We then applied this method to a set of sediments collected in a river along a gradient of anthropogenic pressure and compared their isotopic signatures with those from samples that are representative of the potential P-PO4inputs to the river system (soils and riverbank material). The results showed that following some adaptations, the purification method could be successfully transposed to river sediments with a high level of P-PO4purification (>97%) and high δ18Op measurement repeatability and accuracy (<0.4‰). The values for the potential allochthonous sources varied from 11.8 to 18.3‰, while the δ18Op value for the river sediments ranged from 12.2 to 15.8‰. Moreover, a sharp increase (>3‰) in the sediment δ18Op value immediately downstream from the discharge point revealed the strong impact of municipal wastewater. The calculation of the theoretical equilibrium δ18Opvalues using the river water temperature and δ18Owshowed that the downstream sediments were in equilibrium, which was not the case for the upstream sediments. This difference could be related to the contrast between the short residence time of the transfer system in the catchment head, which can preserve the isotopic variability of the source materials, and the longer residence times and higher P bioavailability in the lower catchment, possibly fostering the recycling of P-PO4by the biota and the equilibration of the oxygen isotope signature in P-PO4. These results demonstrate the potential of the isotopic approach to assess the sources and in-stream turnover of sedimentary P in river systems.
authors.affiliation: Group of Plant Nutrition, Inst. of Agricultural Sciences, Swiss Federal Institute of Technology (ETH Zürich), Switzerland. Electronic address: Chiarapi@ethz.ch.
authors.forename: Chiara
authors.initials: C
authors.lastname: Pistocchi
authors.affiliation: Group of Plant Nutrition, Inst. of Agricultural Sciences, Swiss Federal Institute of Technology (ETH Zürich), Switzerland.
authors.forename: Federica
authors.initials: F
authors.lastname: Tamburini
authors.affiliation: CNRS-Université de Rennes I, UMR Géosciences Rennes, France.
authors.forename: Gerard
authors.initials: G
authors.lastname: Gruau
authors.affiliation: UPMC - Université Pierre et Marie Curie, Paris 6, France.
authors.forename: André
authors.initials: A
authors.lastname: Ferhi
authors.affiliation: INRA, UMR Carrtel, France.
authors.forename: Dominique
authors.initials: D
authors.lastname: Trevisan
authors.affiliation: INRA, UMR Carrtel, France.
authors.forename: Jean-Marcel
authors.initials: JM
authors.lastname: Dorioz
mesh_terms: D004784:Environmental Monitoring; D005068:Eutrophication; D019015:Geologic Sediments; D010103:Oxygen Isotopes; D010758:Phosphorus; D045483:Rivers; D012987:Soil
chemical_list: D010103:Oxygen Isotopes; D012987:Soil; D010758:Phosphorus
keywords: Agriculture; Eutrophication; In-stream processes; Phosphate oxygen isotopes; Phosphorus transfer