Seasonality of nitrogen sources, cycling, and loading in a New England river discerned from nitrate isotope ratios

Rollinson, Veronica R.; Granger, Julie; Clark, Sydney C.; Blanusa, Mackenzie L.; Koerting, Claudia P.; Vaudrey, Jamie M. P.; Treibergs, Lija A.; Westbrook, Holly C.; Matassa, Catherine M.; Hastings, Meredith G.; Tobias, Craig R.

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 (NOinline-formula M1inlinescrollmathml normal 3 - 9pt16ptsvg-formulamathimg5c4cefaf8b78d41c1ce2f2ef151f712f bg-18-3421-2021-ie00001.svg9pt16ptbg-18-3421-2021-ie00001.png ) isotope ratios (inline-formula15N inline-formula M3inlinescrollmathml / 8pt14ptsvg-formulamathimge653eaf840568ee76bb20ba3bf368ae0 bg-18-3421-2021-ie00002.svg8pt14ptbg-18-3421-2021-ie00002.png inline-formula14N, inline-formula18O inline-formula M6inlinescrollmathml / 8pt14ptsvg-formulamathimg073414a2b77546d8d5847ae97897d626 bg-18-3421-2021-ie00003.svg8pt14ptbg-18-3421-2021-ie00003.png inline-formula16O, and inline-formula17O inline-formula M9inlinescrollmathml / 8pt14ptsvg-formulamathimg880d1b22cfae9b4167ff115d05c6894c bg-18-3421-2021-ie00004.svg8pt14ptbg-18-3421-2021-ie00004.png inline-formula16O) 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 inline-formula∼ 23 % and inline-formula∼ 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 NOinline-formula M13inlinescrollmathml normal 3 - 9pt16ptsvg-formulamathimga192f22c747584054322d55d69a940ca bg-18-3421-2021-ie00005.svg9pt16ptbg-18-3421-2021-ie00005.png associated with deeper groundwater had higher inline-formula15N inline-formula M15inlinescrollmathml / 8pt14ptsvg-formulamathimg7572a9d7afeaa92ba0e8bb6f686362bd bg-18-3421-2021-ie00006.svg8pt14ptbg-18-3421-2021-ie00006.png inline-formula14N ratios than shallower groundwater. Corresponding NOinline-formula M17inlinescrollmathml normal 3 - 9pt16ptsvg-formulamathimg5a2143864edd3f7cf8f1639018917994 bg-18-3421-2021-ie00007.svg9pt16ptbg-18-3421-2021-ie00007.png inline-formula18O inline-formula M19inlinescrollmathml / 8pt14ptsvg-formulamathimg2cb9305d3133b5ddd344bed6f97e59c4 bg-18-3421-2021-ie00008.svg8pt14ptbg-18-3421-2021-ie00008.png inline-formula16O ratios were lower during the warm season, due to increased biological cycling in-river. Uncycled atmospheric NOinline-formula M21inlinescrollmathml normal 3 - 9pt16ptsvg-formulamathimg1eb1de86984802614b8a8a62ef6ea2cd bg-18-3421-2021-ie00009.svg9pt16ptbg-18-3421-2021-ie00009.png , detected from its unique mass-independent NOinline-formula M22inlinescrollmathml normal 3 - 9pt16ptsvg-formulamathimgf1e19cb84feec81dda2dcfcce1dcb451 bg-18-3421-2021-ie00010.svg9pt16ptbg-18-3421-2021-ie00010.png inline-formula17O inline-formula M24inlinescrollmathml / 8pt14ptsvg-formulamathimgcdb097d754d0791f99b194a2a037445d bg-18-3421-2021-ie00011.svg8pt14ptbg-18-3421-2021-ie00011.png inline-formula16O vs. inline-formula18O inline-formula M27inlinescrollmathml / 8pt14ptsvg-formulamathimg69c0fe112c920c825e30a2abce4ab1e1 bg-18-3421-2021-ie00012.svg8pt14ptbg-18-3421-2021-ie00012.png inline-formula16O fractionation, accounted for inline-formula< 3 % of riverine NOinline-formula M30inlinescrollmathml normal 3 - 9pt16ptsvg-formulamathimg1081eea6660e9679f4c0def2b37c02eb bg-18-3421-2021-ie00013.svg9pt16ptbg-18-3421-2021-ie00013.png , even at elevated discharge. Along-river, NOinline-formula M31inlinescrollmathml normal 3 - 9pt16ptsvg-formulamathimg4f176b53dff692338dc475668c182da9 bg-18-3421-2021-ie00014.svg9pt16ptbg-18-3421-2021-ie00014.png inline-formula15N inline-formula M33inlinescrollmathml / 8pt14ptsvg-formulamathimg1f30da269e2118f293c445361b5afb08 bg-18-3421-2021-ie00015.svg8pt14ptbg-18-3421-2021-ie00015.png inline-formula14N ratios showed a correspondence to regional land use, increasing from agricultural and forested catchments to the more urbanized watershed downriver. The evolution of inline-formula18O inline-formula M36inlinescrollmathml / 8pt14ptsvg-formulamathimgdd98d8cd8f59b38727bca8c16691d936 bg-18-3421-2021-ie00016.svg8pt14ptbg-18-3421-2021-ie00016.png inline-formula16O isotope ratios along-river conformed to the notion of nutrient spiraling, reflecting the input of NOinline-formula M38inlinescrollmathml normal 3 - 9pt16ptsvg-formulamathimgd7017b563c10191bef4344397740f8ab bg-18-3421-2021-ie00017.svg9pt16ptbg-18-3421-2021-ie00017.png 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 NOinline-formula M39inlinescrollmathml normal 3 - 9pt16ptsvg-formulamathimgd4ab4f4bbbf3853ed4e712bcab2aae0c bg-18-3421-2021-ie00018.svg9pt16ptbg-18-3421-2021-ie00018.png isotope ratios to constrain cycling and source partitioning in river systems.

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Rollinson, Veronica R. / Granger, Julie / Clark, Sydney C. / et al: Seasonality of nitrogen sources, cycling, and loading in a New England river discerned from nitrate isotope ratios. 2021. Copernicus Publications.

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