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Identifying Sources and Impacts of Precipitation-Derived Nitrogen in Narragansett Bay, RI
Citation:
Balint, S., E. Joyce, S. Pennino, A. Oczkowski, Rick McKinney, AND M. Hastings. Identifying Sources and Impacts of Precipitation-Derived Nitrogen in Narragansett Bay, RI. Joint Aquatic Sciences Meeting, Grand Rapids, MI, May 14 - 20, 2022.
Impact/Purpose:
Here we look at the contributions of precipitation-derived nitrogen (e.g., nitrogen in rain) to the nitrogen budget of the Providence-Seekonk River Estuary and upper Narragansett Bay. With sewage treatment plants reducing the amount of nitrogen in their effluent, these other nitrogen sources may become proportionally more important.
Description:
Interpreting the effectiveness of point-source nutrient reductions in urbanized estuaries is often made more challenging by diffuse sources of nutrients such as atmospheric deposition and stormwater runoff. Such is the case in Narragansett Bay, RI where water quality in the urbanized Providence River Estuary has been slow to improve despite a >50% decrease in wastewater nitrogen loading over the past two decades. Precipitation-derived nitrogen is understudied in this system but predicted increases in the amount of precipitation due to climate change has elevated the importance of this nitrogen source. We assessed the magnitude and ecological impacts of precipitation-derived nitrogen using 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 and carbon isotopes of macroalgae (Ulva spp.) after large precipitation events. Chlorophyll concentration was found to be linearly correlated with precipitation amount on sub-weekly scales, and Ulva spp. experienced decreases in δ15N of 0.3–1‰ and increases in δ13C of 1–3‰ that peaked 5 days after precipitation events. These results indicate a direct impact of precipitation-derived nitrogen on nutrient dynamics, including summer phytoplankton blooms. However, the concentration and isotopic composition of stormwater discharge indicates that most precipitation-derived nitrate stems from terrestrial nitrogen and sewage rather than direct atmospheric deposition in this system. Further work is required to understand the magnitude and spatial heterogeneity of precipitation-derived nitrogen and how this nitrogen source will change in a wetter climate of the future.
