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https://doi.org/10.5194/bg-2020-390
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-390
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  04 Nov 2020

04 Nov 2020

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This preprint is currently under review for the journal BG.

Sources and cycling of nitrogen in a New England river discerned from nitrate isotope ratios

Veronica R. Rollinson1, Julie Granger1, Sydney C. Clark2, Mackenzie L. Blanusa1, Claudia P. Koerting1, Jamie M. P. Vaudrey1, Lija A. Treibergs1,4, Holly C. Westbrook1,3, Catherine M. Matassa1, Meredith K. Hastings2, and Craig R. Tobias1 Veronica R. Rollinson et al.
  • 1Department of Marine Sciences, University of Connecticut, Groton, 06340, USA
  • 2Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, 02912, USA
  • 3School or the Earth, Ocean and Environment, University of South Carolina, Columbia, 29208, USA
  • 4Adirondack Watershed Institute, Paul Smith’s College, Paul Smith’s, 12970, USA

Abstract. Coastal waters globally are increasingly impacted due to the anthropogenic loading of nitrogen (N) from the watershed. In order to assess dominant sources of N contributing to the eutrophication of the Little Narragansett Bay estuary in New England, we carried out an annual study of N loading from the Pawcatuck River. We conducted weekly monitoring of nutrients and nitrate (NO3) isotope ratios (15N / 14N, 18O / 16O and 17O / 16O) at the mouth of the river and from the larger of two Waste Water Treatment Facilities (WWTFs) along the estuary, as well as seasonal along-river surveys. Our observations reveal a direct relationship between N loading and the magnitude of river discharge, and a consequent seasonality to N loading into the estuary – rendering loading from the WWTFs and from an industrial site upriver more important at lower river flows during warmer months, comprising ~23 % and ~18 % of N loading, respectively. Riverine nutrients derived predominantly from deeper groundwater and the industrial point source upriver during low base flow in summer, and from shallower groundwater and surface flow at higher river flows during colder months. Loading of dissolved organic nitrogen appeared to increase with river discharge, ostensibly delivered by surface water. The NO3 associated with deeper groundwater had higher 15N / 14N ratios than shallower groundwater, consistent with the expectation fractionation due to partial denitrification. Along-river, NO3 15N / 14N ratios showed a correspondence to regional land use, increasing from agricultural and forested catchments to the more urbanized watershed downriver, with the agricultural and urbanized portions of the watershed contributing disproportionately to total N loading. Corresponding NO3 18O / 16O ratios were lower during the warm season, a dynamic that we ascribe to increased biological cycling in-river. The 18O / 16O isotope ratios along-river were consistent with the notion of nutrient spiraling, reflecting NO3 input from the watershed and in-river nitrification and its coincident removal by biological consumption. Uncycled atmospheric NO3, detected from its unique mass-independent NO3 17O / 16O vs. 18O / 16O fractionation, accounted for < 3 % of riverine NO3, even at elevated discharge. We explore the implications of our findings for the mitigation of eutrophication in Little Narragansett Bay.

Veronica R. Rollinson et al.

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Short summary
We measured nutrients and the naturally occurring nitrogen (N) and oxygen (O) stable isotope ratios of nitrate discharged from a New England river over an annual cycle, to monitor N loading and identify dominant sources from the watershed. We uncovered a seasonality to loading and sources of N from the watershed. Seasonality in the nitrate isotope ratios also informed on N cycling, conforming to theoretical expectations of riverine nutrient cycling.
We measured nutrients and the naturally occurring nitrogen (N) and oxygen (O) stable isotope...
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