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${}_{\mathrm{3}}^{-}$) isotope ratios (¹⁵N $/$ ¹⁴N, ¹⁸O $/$ ¹⁶O, and ¹⁷O $/$ ¹⁶O) 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 ∼ 23 % and ∼ 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${}_{\mathrm{3}}^{-}$ associated with deeper groundwater had higher ¹⁵N $/$ ¹⁴N ratios than shallower groundwater. Corresponding NO${}_{\mathrm{3}}^{-}$ ¹⁸O $/$ ¹⁶O ratios were lower during the warm season, due to increased biological cycling in-river. Uncycled atmospheric NO${}_{\mathrm{3}}^{-}$, detected from its unique mass-independent NO${}_{\mathrm{3}}^{-}$ ¹⁷O $/$ ¹⁶O vs. ¹⁸O $/$ ¹⁶O fractionation, accounted for < 3 % of riverine NO${}_{\mathrm{3}}^{-}$, even at elevated discharge. Along-river, NO${}_{\mathrm{3}}^{-}$ ¹⁵N $/$ ¹⁴N ratios showed a correspondence to regional land use, increasing from agricultural and forested catchments to the more urbanized watershed downriver. The evolution of ¹⁸O $/$ ¹⁶O isotope ratios along-river conformed to the notion of nutrient spiraling, reflecting the input of NO${}_{\mathrm{3}}^{-}$ 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${}_{\mathrm{3}}^{-}$ isotope ratios to constrain cycling and source partitioning in river systems.

中文翻译：

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从硝酸盐同位素比率识别出的新英格兰河流中氮源、循环和负荷的季节性

由于来自流域的人为氮 (N) 负荷，全球沿海水域受到的影响越来越大。为了评估导致新英格兰小纳拉甘西特湾河口富营养化的主要来源，我们对波卡塔克河的 N 负荷进行了年度研究。我们每周对营养素和硝酸盐（NO${}_{\mathrm{3}}^{——}$) 同位素比率 ( ¹⁵ N $/$ ¹⁴ N, ¹⁸ O $/$ ¹⁶ O 和 ¹⁷ O $/$ ¹⁶ O) 位于河口和沿河口的两个污水处理设施 (WWTF) 中较大的一个，以及季节性沿河调查。我们的观察揭示了 N 负荷与河流流量大小之间的直接关系，以及随之而来的 N 负荷进入河口的季节性 - 使得来自 WWTF 和工业场地的负荷在较温暖的月份在较低的河流流量中更为重要，包括~  23 %和~  18% 的 N 负载，分别。河流养分主要来自较深的地下水和夏季上游的工业点源，以及较冷的月份来自较浅的地下水和地表流——其中 NO${}_{\mathrm{3}}^{——}$与较深的地下水相关，具有较高的 ¹⁵ N $/$ ¹⁴ N 比率比较浅的地下水。对应NO${}_{\mathrm{3}}^{——}$ ¹⁸欧 $/$ 由于河内生物循环增加，暖季期间¹⁶ O 比率较低。未循环大气 NO${}_{\mathrm{3}}^{——}$，从其独特的与质量无关的 NO 检测到${}_{\mathrm{3}}^{——}$ ¹⁷奥 $/$ ¹⁶ O 对¹⁸ O $/$ ¹⁶ O 分馏，占  河流 NO 的< 3 %${}_{\mathrm{3}}^{——}$，即使在高放电。沿河，NO${}_{\mathrm{3}}^{——}$ ¹⁵牛顿 $/$ ¹⁴ N 比率显示出与区域土地利用的对应关系，从农业和森林集水区到下游城市化程度更高的流域。¹⁸ O 的演变 $/$ ¹⁶ O 同位素比值沿河符合养分螺旋的概念，反映了 NO 的输入${}_{\mathrm{3}}^{——}$来自集水区和河内硝化作用及其通过生物消耗同时去除。这些发现强调了考虑河流氮源和负荷的季节性以减轻接收河口富营养化的重要性。我们的研究进一步提出了一个概念框架，该框架与当前的河流养分循环理论相一致，从中可以有力地解释 NO${}_{\mathrm{3}}^{——}$ 同位素比限制河流系统中的循环和源分配。