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How the distribution of anthropogenic nitrogen has changed in Narragansett Bay (RI, USA) following major reductions in nutrient loads
Citation:
Oczkowski, A., C. Schmidt, Alana Hanson, D. Cobb, J. Krumholz, AND Rick Mckinney. How the distribution of anthropogenic nitrogen has changed in Narragansett Bay (RI, USA) following major reductions in nutrient loads. Coastal & Estuarine Research Federation 24th Biennial Conference, Providence, Rhode Island, November 05 - 09, 2017.
Impact/Purpose:
Given the Agency's interest in better understanding how sensitive coastal ecosystems respond to the effective implementation of nutrient reduction plans, this work sought to quantify the ecological recovery of the Narragansett Bay ecosystem after large scale nutrient reductions. Here we pulled together pre-existing data for nutrient concentrations, water clarity, and nitrogen stable isotopes and compared them to data that we collected as part of SSWR 4.02B.
Description:
Over the past decade, nitrogen (N) loads to Narragansett Bay have decreased by about 65%. These reductions were the direct result of multiple sewage treatment plant upgrades to tertiary treatment, a process which employs N removal. Here we document ecosystem response to the N reductions and assess how the distribution of sewage N in Narragansett Bay has changed from before, during, and after the upgrades. Comparisons of dissolved inorganic nitrogen (DIN) concentrations from bay-wide surveys conducted in the early 1980s and 2014-2016 reflect lower water column DIN concentrations in the more recent samples. Secchi depth data collected during these same time periods illustrate a clearer water column. To look at spatial and temporal changes in bio-available N distribution between 2000 and 2016, we used nitrogen (δ15N) and carbon (δ13C) stable isotope measurements of particulate matter as a proxy for phytoplankton, macroalgae as short-term integrators of water column bio-available N, and hard clams (Mercenaria mercenaria) as long-term primary producer integrators. In contrast to stable isotope studies from other estuaries that have observed an increased influence of marine N when sewage N is reduced, the upgrades to tertiary treatment have unexpectedly increased the δ15N values measured in plants and animals throughout Narragansett Bay by 1-2‰, on average. Furthermore, the δ13C values measured in particulate matter and hard clams have declined by about the same amount. While δ15N results indicated that sewage N still plays an important role in supporting primary and secondary production, the δ13C and secchi data suggest that overall net production in Narragansett Bay has decreased.