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Biogeochemistry

, Volume 115, Issue 1–3, pp 287–298 | Cite as

Variations of the nitrate isotopic composition in the St. Lawrence River caused by seasonal changes in atmospheric nitrogen inputs

  • Benoit ThibodeauEmail author
  • Jean-François Hélie
  • Moritz F. Lehmann
Article

Abstract

We present 42 dual-isotope nitrate analyses of fresh water samples collected in the St. Lawrence River between June 2006 and July 2008. Measured δ15N–NO3 and δ18O–NO3 values correlate negatively, while δ18O–NO3 displays no negative correlation with nitrate concentration. This suggests that nitrate uptake and/or elimination by denitrification is not the main driver of observed variations in nitrate concentration and isotopic signature in the St. Lawrence River. In addition, δ18O–NO3 is negatively correlated with the seasonally variable δ18O of ambient water, indicating that the variation in the isotopic signature of nitrate is barely modulated by in-stream nitrate regeneration (nitrification). It rather is constrained by along-river changes in the external sources of nitrate. Given the distinct nitrogen (N) and oxygen (O) isotopic signature of atmospheric nitrate, we argue that observed seasonal variations of δ15N–NO3 and δ18O–NO3 in the St. Lawrence River are due to variable contributions of snowmelt-derived water. Based on a N and O isotope mass balance, we show that total nitrate loading in the St. Lawrence River is dominated by a N input from the Great Lakes (47 ± 28 %) and from nitrate regeneration of both internal and external N (48 ± 22 %). While temporal nitrate N and O isotope dynamics in the St. Lawrence River are mainly influenced by the atmospheric N input fluctuations, with an increase in atmospheric loading during spring, atmospheric N plays overall a rather insignificant role with regards to the N budget (5 ± 4 %).

Keywords

St. Lawrence Hydrology Time series Nitrogen Isotope Eutrophication 

Notes

Acknowledgments

This work was funded by Natural Sciences and Engineering Research Council of Canada (NSERC) through Allocation and Discovery Grants to MFL and Discovery Grants to C. Hillaire-Marcel. BT acknowledges Fonds Québécois de Recherche Nature et Technologie (FQRNT), GEOTOP Research Center and the Japan Society for the Promotion of Science (JSPS) for financial support. We thank S. Xiu Phuong for technical assistance during laboratory measurements, E. Rosa for the sampling, B. Williams and T. Miyajima for their comments on a earlier version of the manuscript. We are grateful to two anonymous reviewers for their valuable comments.

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Copyright information

© Springer Science+Business Media Dordrecht 2013

Authors and Affiliations

  • Benoit Thibodeau
    • 1
    Email author
  • Jean-François Hélie
    • 2
  • Moritz F. Lehmann
    • 3
  1. 1.Atmosphere and Ocean Research Institute (AORI)University of TokyoKashiwaJapan
  2. 2.Geochemistry and Geodynamics Research Center (GEOTOP)Université du Québec à MontréalMontrealCanada
  3. 3.Institute for Environmental GeosciencesUniversity of BaselBaselSwitzerland

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