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NCCOS Project

Hypoxia Response to Nutrient Management in Narragansett Bay

This project began in January 2010 and was completed in December 2015

We are developing observational and modeling tools and analyses to help management agencies in Narragansett Bay, Rhode Island assess the efficacy of nutrient management controls on hypoxia (low dissolved oxygen) in the bay.

Why We Care
Narragansett Bay is a mid-size (370 km2/230 mi2) temperate (mid-latitude) estuary subject to low oxygen (hypoxic) events that persist from days to weeks several times every year during the summer. Prior to the late 1990s, the bay was considered to have low vulnerability to hypoxia—despite nitrogen loading (nonpoint source runoff and point source discharges) that is among the highest in the northeast—but now the bay experiences periodic hypoxia. If such nutrient loading remains unchecked, estuaries can experience hypoxia that persists throughout the summer. Ultimately, these estuaries may reach a “tipping point” beyond which damages to ecosystems are likely to be permanent and not recover, even in response to complete alleviation of excessive nutrients.

In 2003, low oxygen in Narragansett Bay’s Greenwich Bay resulted in a fish kill event that galvanized public opinion in support of taking action to improve Narragansett Bay’s water quality. In response, the state passed a law in 2004 that imposes nitrogen limits on the area’s largest wastewater treatment facilities (WWTFs). The Rhode Island Department of Environmental Management (RI DEM) is implementing nutrient load reductions through the permitting process to all Rhode Island WWTFs discharging to Narragansett Bay with a target of 50 percent reduction in overall summertime loads to Narragansett Bay relative to 1995–1996 levels. Reductions in nutrient loads from bordering Massachusetts WWTFs are also being pursued. By 2006 the summertime loads were reduced by 35 percent overall relative to 1995–1996. Implementation of further reductions is underway and summertime loads are expected to continue decreasing and reach the target in 2013–2014.

What We Are Doing
We are documenting the effects of this transition to more efficient WWTFs by:

  1. Assessing the extent to which Narragansett Bay hypoxia changes in response to nutrient reductions,
  2. Characterizing the nature of the changes, and
  3. Applying models to test their capability to predict these changes and use them in a forecasting/alternate management scenario analysis. We are using a simplified ecosystem model linked to a 3-D hydrodynamic model to produce science-based predictive tools that simulate ecological responses and predict low oxygen events related to nitrogen inputs.

The project includes an integrated suite of observations with multiple modeling approaches. RI DEM installed a network of fixed-site stations to record water properties (oxygen, salinity, temperature, chlorophyll fluorescence). Also, RI DEM will conduct complementary spatial surveys of conductivity, temperature, and dissolved oxygen to characterize the geographic extent and spatial patterns of hypoxia. The surveys will also document the prevalence of macroalgae, of concern for its role in oxygen depletion and potentially the nutrient budget. The team will calculate an updated nutrient budget. To monitor water volume transport in the main channel of the bay that controls flushing, the team installed a current profiler on a Narragansett Bay ferry. The current profiler provides cost-effective and real-time data to improve hypoxia forecasts.

The project is led by the University of Rhode Island’s Graduate School of Oceanography, with partners from the Narragansett Bay National Estuary Program, the Rhode Island Department of Environmental Management, the College of William and Mary, the University of Connecticut, Brown University, and others from the University of Rhode Island.

Benefits of Our Work
The project team is working closely with resource management agencies to develop a set of models usable by anyone involved in Narragansett Bay nutrient management activities. Annual workshops allow project researchers and the staff from local management and regulatory agencies to report on their recent activities and exchange information. We have held these annual workshops since 2006 (http://www.gso.uri.edu/merl/merl.html). We also welcome participation by representatives from consulting firms with projects active in Narragansett Bay, environmental advocacy groups, and regulatory agencies from the broader northeast region.

A Management Advisory Group helps guide our research, ensures its applicability to management issues, and strengthens the interactions between researchers and management agency staff. The Management Advisory Group is composed of representatives from the Rhode Island Department of Environmental Management, the Narragansett Bay Commission, the Rhode Island Coastal Resource Management Council, the U.S. EPA’s Atlantic Ecology Division, and the Narragansett Bay National Estuarine Research Reserve.

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