Many urban estuaries worldwide suffer from excess phytoplankton and hypoxia (low oxygen) due to high nutrient loads. A common water quality management strategy is to require wastewater treatment facility upgrades. This case study examines Narragansett Bay, a warming temperate mid-latitude urban estuary with seasonal periodic hypoxia, during June through September from 2005 to 2019. Within this period, numerous facilities were upgraded to nitrogen removal over several years. The response of the bay is more consistent with “textbook” expectations for reduced chlorophyll and hypoxia than what was seen in many other systems—despite its complex coastline geometry, numerous river inputs, and widely-distributed treatment facilities. River flow drives inter-annual variability with increased load, density stratification, chlorophyll, and hypoxia in wet years. Mean 2013-2019 bay-wide total nitrogen load was 34% less than the 2005-2012 mean, a reduction of about 10 6 kg yr-1, comparable to the range of flow-driven inter-annual variations. Chlorophyll Index and Hypoxia Index event-based metrics applied to high-frequency time series observations at eight sites quantify exceedances of severe and moderate thresholds. Relatively steady 33% and 16% Chlorophyll Index declines, for severe and moderate thresholds, occurred from about 2007 to 2019. The Hypoxia Index declined markedly by 2009 and 2014 for severe and moderate thresholds, respectively, and remained at or near zero from 2014 to 2019. The load reduction explains chlorophyll and hypoxia declines better than physical processes including river flow, stratification, tidal variations, winds, sea level differences, and temperatures. River flow about 55% higher than the 2005-2019 mean would increase non-treatment facility loads by an amount comparable to the managed load decrease, so future wet summers could partially reverse the improvements. Long-term trends include warming of about 0.5°C decade -1 , which reduces oxygen saturation by 0.1 mg l -1 decade -1 . This rate is likely a lower bound for temperature-driven oxygen decreases, because warming can also accelerate phytoplankton growth and bacterial consumption. Without warming, the managed load decrease would have curtailed hypoxia more effectively. Climate trends should be at least as important to future eutrophication as the managed load decline because, in addition to warming influences, long-term increases in river flow would increase load and stratification.

Over the last 40 years, Narragansett Bay has experienced significant water quality improvements in response to reductions in nitrogen, heavy metal and organic contaminants, and bacterial pathogen discharge. The sources of these pollutants stem from the long history of population growth and a manufacturing-based economy. These reductions were documented by decades of persistent research and are the product of implementing the Clean Water Act by cooperation among all levels of government, utilities commissions, industry, nonprofits, universities, and advocacy groups. Wastewater treatment facilities remain a significant nitrogen source to the bay and continuing evaluation will determine if further reductions are needed. Metals/organic contaminants in sediment remain at or below thresholds for negative biological impacts. In response to pathogen reductions, over 3,000 acres have been reclassified as approved for shellfish growing between 2010 and 2017. The watershed still faces significant challenges in addressing stormwater runoff of both nitrogen and pathogens, and legacy methylmercury contamination in fish. Changes in population and land use and climate change will need to be addressed as well. The highlighted case studies showcase the abilities of public and private entities to collaboratively identify indicators, define problems, track changes, and respond to watershed-scale problems through ongoing adaptive management. Widely sharing successes and lessons learned in Narragansett Bay – from processes to research to management actions – can inform other estuarine collaborative management efforts to address complex and challenging environmental issues.This report has been peer-reviewed by 24 individuals from NBEP staff, EPA, NBEP's Science Advisory Committee, NBEP partners, and 6 anonymous reviewers from 2 academic journals.The views expressed in this report are those of the authors and do not necessarily represent the views or policies of the U.S. Environmental Protection Agency or the New England Interstate Water Pollution Control Commission. Any mention of trade names, products, or services does not imply an endorsement by the U.S. Government, the U.S. Environmental Protection Agency, or the New England Interstate Water Pollution Control Commission. The EPA and NEIWPCC do not endorse any commercial products, services, or enterprises.

We develop an integrated assessment model for spatially simulating water quality and social welfare from linked ecosystem services that extends prior modeling by incorporating a broader suite of pollutants than conventionally measured factors like phosphorus and nitrogen. Beyond demonstrating the feasibility of such a model, we provide guidance on the impact of omitting or holding constant relevant pollutants and their effect on estimates of water quality and willingness to pay. Applying the model to Narragansett Bay, we find that recent wastewater treatment upgrades and a legacy network of dams are providing millions in annual value to adjacent residents.

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 (δ 15 N) and carbon (δ 13 C) 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 δ 15 N values by at least 2‰. The plants and animals throughout Narragansett Bay have similarly increased by 1–2‰, on average. In contrast, the δ 13 C values measured in particulate matter and hard clams have declined by about the same amount. The δ 15 N 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 δ 13 C 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.

Due to nitrogen load reduction policies, wastewater treatment facilities (WWTFs) have upgraded to tertiary treatment — where denitrification reduces and removes nitrogen. Changes to the stable isotopic composition of nitrate inputs after upgrades or how it transfers to the estuary have not been assessed in Rhode Island. We investigate whether these upgrades impact the isotopic signature of nitrate inputs to Narragansett Bay. Samples from rivers and WWTFs discharging to Narragansett Bay characterize the anthropogenic source nitrate (NO3−) isotopic composition (δ15N–NO3− and δ18O–NO3−) and temporal variability. At one WWTF, tertiary treatment increased effluent nitrate δ15N–NO3− and δ18O–NO3− values by ~16‰. Riverine values increased by ~4‰, likely due to the combination of decreases in N and upgrades. Combined river and WWTF flux-weighted isotopic compositions showed enriched values and an amplitude reduction in monthly variability. When seasonal isotopic means are significantly different from other sources, δ15N–NO3− may be a useful tracer of inputs.