Seasonality of nitrogen sources, cycling, and loading in a New England river discerned from nitrate isotope ratios
<p>Coastal waters globally are increasingly impacted due to the anthropogenic loading of nitrogen (N) from the watershed. To assess dominant sources contributing to the eutrophication of the Little Narragansett Bay estuary in New England, we carried out an annual study of N loading from the Pa...
Main Authors: | , , , , , , , , , , |
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Format: | Article |
Language: | English |
Published: |
Copernicus Publications
2021-06-01
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Series: | Biogeosciences |
Online Access: | https://bg.copernicus.org/articles/18/3421/2021/bg-18-3421-2021.pdf |
Summary: | <p>Coastal waters globally are increasingly impacted due to the anthropogenic
loading of nitrogen (N) from the watershed. To assess dominant sources
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
(NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="9pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="5c4cefaf8b78d41c1ce2f2ef151f712f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3421-2021-ie00001.svg" width="9pt" height="16pt" src="bg-18-3421-2021-ie00001.png"/></svg:svg></span></span>) isotope ratios (<span class="inline-formula"><sup>15</sup></span>N <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="e653eaf840568ee76bb20ba3bf368ae0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3421-2021-ie00002.svg" width="8pt" height="14pt" src="bg-18-3421-2021-ie00002.png"/></svg:svg></span></span> <span class="inline-formula"><sup>14</sup></span>N, <span class="inline-formula"><sup>18</sup></span>O <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="073414a2b77546d8d5847ae97897d626"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3421-2021-ie00003.svg" width="8pt" height="14pt" src="bg-18-3421-2021-ie00003.png"/></svg:svg></span></span> <span class="inline-formula"><sup>16</sup></span>O, and
<span class="inline-formula"><sup>17</sup></span>O <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="880d1b22cfae9b4167ff115d05c6894c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3421-2021-ie00004.svg" width="8pt" height="14pt" src="bg-18-3421-2021-ie00004.png"/></svg:svg></span></span> <span class="inline-formula"><sup>16</sup></span>O) at the mouth of the river and from the larger of two
wastewater 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 more important at lower river flows during
warmer months, comprising <span class="inline-formula">∼</span> 23 % and <span class="inline-formula">∼</span> 18 % of N loading,
respectively. Riverine nutrients derived predominantly from deeper
groundwater and the industrial point source upriver in summer and from
shallower groundwater and surface flow during colder months – wherein
NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="9pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="a192f22c747584054322d55d69a940ca"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3421-2021-ie00005.svg" width="9pt" height="16pt" src="bg-18-3421-2021-ie00005.png"/></svg:svg></span></span> associated with deeper groundwater had higher
<span class="inline-formula"><sup>15</sup></span>N <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="7572a9d7afeaa92ba0e8bb6f686362bd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3421-2021-ie00006.svg" width="8pt" height="14pt" src="bg-18-3421-2021-ie00006.png"/></svg:svg></span></span> <span class="inline-formula"><sup>14</sup></span>N ratios than shallower groundwater. Corresponding
NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M17" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="9pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="5a2143864edd3f7cf8f1639018917994"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3421-2021-ie00007.svg" width="9pt" height="16pt" src="bg-18-3421-2021-ie00007.png"/></svg:svg></span></span> <span class="inline-formula"><sup>18</sup></span>O <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M19" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="2cb9305d3133b5ddd344bed6f97e59c4"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3421-2021-ie00008.svg" width="8pt" height="14pt" src="bg-18-3421-2021-ie00008.png"/></svg:svg></span></span> <span class="inline-formula"><sup>16</sup></span>O ratios were lower during the warm season,
due to increased biological cycling in-river. Uncycled atmospheric
NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M21" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="9pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="1eb1de86984802614b8a8a62ef6ea2cd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3421-2021-ie00009.svg" width="9pt" height="16pt" src="bg-18-3421-2021-ie00009.png"/></svg:svg></span></span>, detected from its unique mass-independent NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M22" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="9pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="f1e19cb84feec81dda2dcfcce1dcb451"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3421-2021-ie00010.svg" width="9pt" height="16pt" src="bg-18-3421-2021-ie00010.png"/></svg:svg></span></span>
<span class="inline-formula"><sup>17</sup></span>O <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M24" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="cdb097d754d0791f99b194a2a037445d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3421-2021-ie00011.svg" width="8pt" height="14pt" src="bg-18-3421-2021-ie00011.png"/></svg:svg></span></span> <span class="inline-formula"><sup>16</sup></span>O vs. <span class="inline-formula"><sup>18</sup></span>O <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M27" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="69c0fe112c920c825e30a2abce4ab1e1"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3421-2021-ie00012.svg" width="8pt" height="14pt" src="bg-18-3421-2021-ie00012.png"/></svg:svg></span></span> <span class="inline-formula"><sup>16</sup></span>O fractionation, accounted for
<span class="inline-formula"><i><</i></span> 3 % of riverine NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M30" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="9pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="1081eea6660e9679f4c0def2b37c02eb"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3421-2021-ie00013.svg" width="9pt" height="16pt" src="bg-18-3421-2021-ie00013.png"/></svg:svg></span></span>, even at elevated discharge.
Along-river, NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M31" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="9pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="4f176b53dff692338dc475668c182da9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3421-2021-ie00014.svg" width="9pt" height="16pt" src="bg-18-3421-2021-ie00014.png"/></svg:svg></span></span> <span class="inline-formula"><sup>15</sup></span>N <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M33" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="1f30da269e2118f293c445361b5afb08"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3421-2021-ie00015.svg" width="8pt" height="14pt" src="bg-18-3421-2021-ie00015.png"/></svg:svg></span></span> <span class="inline-formula"><sup>14</sup></span>N ratios showed a correspondence
to regional land use, increasing from agricultural and forested catchments
to the more urbanized watershed downriver. The evolution of
<span class="inline-formula"><sup>18</sup></span>O <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M36" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="dd98d8cd8f59b38727bca8c16691d936"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3421-2021-ie00016.svg" width="8pt" height="14pt" src="bg-18-3421-2021-ie00016.png"/></svg:svg></span></span> <span class="inline-formula"><sup>16</sup></span>O isotope ratios along-river conformed to the notion of
nutrient spiraling, reflecting the input of NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M38" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="9pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="d7017b563c10191bef4344397740f8ab"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3421-2021-ie00017.svg" width="9pt" height="16pt" src="bg-18-3421-2021-ie00017.png"/></svg:svg></span></span> from the
catchment and from in-river nitrification and its coincident removal by
biological consumption. These findings stress the importance of considering
seasonality of riverine N sources and loading to mitigate eutrophication in
receiving estuaries. Our study further advances a conceptual framework that
reconciles with the current theory of riverine nutrient cycling, from which
to robustly interpret NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M39" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="9pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="d4ab4f4bbbf3853ed4e712bcab2aae0c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-3421-2021-ie00018.svg" width="9pt" height="16pt" src="bg-18-3421-2021-ie00018.png"/></svg:svg></span></span> isotope ratios to constrain cycling and
source partitioning in river systems.</p> |
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ISSN: | 1726-4170 1726-4189 |