Abstract. 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 (NO3-) 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 NO3- associated with deeper groundwater had higher 15N / 14N ratios than shallower groundwater. Corresponding NO3- 18O / 16O ratios were lower during the warm season, due to increased biological cycling in-river. 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. 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. The evolution of 18O / 16O isotope ratios along-river conformed to the notion of nutrient spiraling, reflecting the input of NO3- 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 NO3- isotope ratios to constrain cycling and source partitioning in river systems.

No abstract

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

Please include reference here. We added a reference. L542: which limited light penetration. Fixed. L620: to primarily reflect Added "primarily." L640: there was little to no accumulated snow in March 2019 Fixed.L645: I think it would be good to state that no samples were taken from Kenyon Industries much earlier -when it is introduced as a potential point source. We added this information at line 161 in the Methods.

S1. Isotope composition of rainwater NO3 -DIN concentrations in rainwater samples ranged from 2.3 to 56.4 µM (Figure S2a). Corresponding isotope ratios ranged from -6.1 to 1.7 ‰ for δ 15 NNO3, from 57.8 to 75.7 ‰ for δ 18 ONO3, and from 19.7 to 27.2 ‰ for ∆ 17 ONO3 (Figure S2b). Both δ 18 ONO3 and ∆ 17 ONO3 values in local rainwater -measured only from early September through December 2018 -scaled directly with discharge at the Stillman Bridge (Fig. S2c), implicating an increase in both as a function of mean precipitation. In order to extrapolate these precipitation isotopic end-member values to the whole observation period, we imposed best-fit logarithmic function to rainwater δ 18 ONO3 and ∆ 17 ONO3 values vs. daily river discharge measurements at the Stillman Bridge, using the latter as a proxy for precipitation. The precipitation δ 18 ONO3 and ∆ 17 ONO3 end-member values thus derived were then used to (a) estimate the fraction of atmospheric NO3in river water (Fig. 2g) and (b) account for the influence of atmospheric NO3on riverine δ 18 ONO3 (Fig. 2h).