Identifying Sources and Impacts of Precipitation-Derived Nitrogen in Narragansett Bay, RI

Anthropogenic changes to nitrogen cycling have dramatically altered ecosystems across the globe, particularly in coastal estuaries where nitrogen limits primary production. The impacts of nitrogen pollution are especially evident in Narragansett Bay, RI, where eutrophication in the urbanized Providence River Estuary persists despite a > 50% decrease in wastewater nitrogen loading over the past two decades. Precipitation-derived nitrogen tends to be understudied and may be underestimated in contemporary budgets. Here, the magnitude and ecological impacts of precipitation-derived nitrogen in the Providence River Estuary were assessed by the following: relationships between precipitation, river discharge, and chlorophyll abundance; inorganic nitrogen concentration and isotopic composition of precipitation, surface runoff, and stormwater; and changes in stable nitrogen (δ 15 N) and carbon (δ 13 C) isotopes of macroalgae ( Ulva spp.) after large precipitation events. Precipitation and chlorophyll concentration measured 5 days after the rain event exhibited a linear relationship. Furthermore, Ulva spp. experienced decreases in δ 15 N of 0.3–1‰ and increases in δ 13 C of 1–3‰ that peaked 5 days after precipitation events. These results are indicative of a direct impact of precipitation-derived nitrogen on nutrient dynamics, including summer phytoplankton blooms. However, the inorganic nitrogen concentrations of stormwater were approximately 240% higher than surface runoff, and the isotopic composition of stormwater discharge indicates that most precipitation-derived nitrate stems from terrestrial nitrogen and sewage rather than direct atmospheric deposition. The magnitude and spatial heterogeneity of precipitation-derived nitrogen remains uncertain, and further work is needed to understand how this nitrogen source will change in a wetter climate of the future.