Citation:

Oczkowski, A., C. Schmidt, E. Santos, K. Miller, Alana Hanson, D. Cobb, J. Krumholz, A. Pimenta, L. Heffner, S. Robinson, J. Chaves, AND Rick Mckinney. How the distribution of anthropogenic nitrogen has changed in Narragansett Bay (RI, USA) following major reductions in nutrient loads. Estuaries and Coasts. Estuarine Research Federation, Port Republic, MD, 41(8):2260-2276, (2018). https://doi.org/10.1007/s12237-018-0435-2

Impact/Purpose:

Here we assess the response of the Narragansett Bay (RI, USA) ecosystem to recent reductions in sewage nitrogen loads. With most of the sewage treatment plants in the Narragansett Bay watershed upgrading from secondary to tertiary treatment, the amount of nitrogen being released to the bay has been cut by more than half. We look at long-term records of water column nutrient concentrations, Secchi depth (a measure of water clarity), and stable isotopes of nitrogen and carbon to assess how the ecosystem has responded to the reductions. We also make the case that, given how rapidly the nitrogen loads declined and the nature of the Bay itself, Narragansett Bay provides an excellent test 'mesocosm' from which we can observe ecosystem responses to our efforts at improving water quality. Overall, our study suggests that, in the first five years after the nitrogen reduction target was met, sewage is still the dominant source to the Bay. Although, there are some early indications that water column productivity, particularly at the mouth of the Bay, is declining.

Description:

Over the past decade, nitrogen (N) loads to Narragansett Bay have decreased by more than 50%. These reductions were, in large part, the direct result of multiple wastewater treatment facility 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 shortly after the upgrades. While others have observed clear responses when data were considered annually, our seasonal and regional comparisons of pre- and post-tertiary treatment dissolved inorganic nitrogen (DIN) concentrations and Secchi depth data, from bay-wide surveys conducted periodically from the early 1970s through 2016, resulted in only a few subtle differences. Thus, we sought to use stable isotope data to assess how sewage N is incorporated into the ecology of the Bay and how its distribution may have changed after the upgrades. The nitrogen (δ15N) and carbon (δ13C) stable isotope measurements of particulate matter served as a proxy for phytoplankton, while macroalgae served as short-term integrators of water column bio-available N, and hard clams (Mercenaria mercenaria) as integrators of water column production. In contrast to other estuarine stable isotope studies that have observed an increased influence of isotopically lower marine N when sewage N is reduced, the opposite has occurred in Narragansett Bay. The tertiary treatment upgrades have increased the effluent δ15N values by at least 2‰. The plants and animals throughout Narragansett Bay have similarly increased by 1–2‰, on average. In contrast, the δ13C values measured in particulate matter and hard clams have declined by about the same amount. The δ15N results indicated that, even after the N reductions, sewage N still plays an important role in supporting primary and secondary production throughout the bay. However, the δ13C suggests that overall net production in Narragansett Bay has decreased. In the 5 years after the major wastewater treatment facilities came on-line for nutrient removal, oligotrophication has begun but sewage remains the dominant source of N to Narragansett Bay.

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