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

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 (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.