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 watershed. To assess dominant sources contributing eutrophication Little Narragansett Bay estuary in New England, we carried out an annual study N Pawcatuck River. We conducted weekly monitoring nutrients and nitrate (NO3-) isotope ratios (15N / 14N, 18O 16O, 17O 16O) at mouth river larger two wastewater treatment facilities (WWTFs) along estuary, as well seasonal along-river surveys. Our observations reveal a direct relationship between magnitude discharge consequent seasonality into – rendering WWTFs industrial site more important lower flows during warmer months, comprising ? 23 % 18 loading, respectively. Riverine derived predominantly deeper groundwater point source upriver summer shallower surface flow colder months wherein NO3- associated with had higher 15N 14N than groundwater. Corresponding 16O were warm season, increased biological cycling in-river. Uncycled atmospheric NO3-, detected its unique mass-independent vs. fractionation, accounted for < 3 riverine even elevated discharge. Along-river, showed correspondence regional land use, increasing agricultural forested catchments urbanized watershed downriver. The evolution conformed notion nutrient spiraling, reflecting input catchment in-river nitrification coincident removal by consumption. These findings stress importance considering mitigate receiving estuaries. further advances conceptual framework that reconciles current theory cycling, which robustly interpret constrain partitioning systems.