How the Distribution of Anthropogenic Nitrogen Has Changed in Narragansett Bay (RI, USA) Following Major Reductions in Nutrient Loads
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.
KeywordsStable isotope δ15N Eutrophication Sewage treatment plant Wastewater treatment facility Tertiary treatment Ecosystem response Oligotrophication
The late coastal ecologist Scott Nixon was fond of saying that the wise studied lakes, the less-wise studied rivers, where water flowed in one direction, and only the most foolish studied estuaries where water moved in all directions, changing over the course of a day. This has never been truer when trying to assess the impacts of oligotrophication. We would like to thank the Rhode Island Department of Environmental Management for providing hard clams to us for more than a decade. We would also like to thank Mike Charpentier for creating Fig. 1, Steve Granger for the assistance in the field during those first collections in the mid-2000s, and to Scott Nixon and Candace Oviatt for all of the valuable data that they have collected, and information and wisdom that they have provided, over the years. Thanks to Lindsey Fields for measuring the DIN concentrations and Rebecca Robinson for the support to C. Schmidt. We are grateful to Carol Thornber and Lindsay Green of the University of Rhode Island for their help with identifying macroalgae. This manuscript has been improved by input from Roxanne Johnson, Rose Martin, and Brenda Rashleigh. This is ORD tracking number ORD-023673. The views expressed in this article are those of the authors and do not necessarily represent the views or polices of the US Environmental Protection Agency, NEIWPCC, nor of the McLaughlin Research Corporation. Any mention of trade names, products, or services does not imply an endorsement by the US government or the US Environmental Protection Agency. The EPA does not endorse any commercial products, services, or enterprises.
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