The ecological and economic health of Duxbury Bay is being increasingly threatened by severe nitrogen pollution, primarily from septic systems and fertilizers, according to the comprehensive State of the Bay 2025 report presented to the Duxbury Selectboard Monday evening, Nov. 17.
Peter Quigley of the Duxbury Bay Management Commission delivered the sobering findings, drawn from a year-long volunteer effort analyzing over 15 years of water quality data. The report, which updates the 2009 assessment, highlights declining conditions in an estuary that supports a thriving 12 million oyster harvest annually and is the centerpiece of the town’s recreational and commercial life.
The Nitrogen Threat: A Canary in the Coal Mine
Quigley warned the board, “This is a canary in the coal mine type scenario,” explaining that nitrogen from human sources is driving excessive plant growth (eutrophication) that depletes oxygen and endangers marine life. The most critical finding is that most of Duxbury Bay north of a line from the Harbormaster’s dock to High Pines now has nitrogen levels at or above concerning ecological thresholds, indicating impaired water quality.
The data confirms that the pollution is worsening. Charts presented to the board showed nitrogen levels in the Bluefish River, below Powder Point Bridge, and at the Harbormaster’s dock are all trending upward over time. Measurements in the Bluefish River, in particular, are significantly above draft thresholds established in a 2017 Massachusetts Department of Environmental Protection (MassDEP) study.
The report identifies septic systems as the primary source of nitrogen pollution, with fertilizer use as a secondary contributor. Conventional Title 5 septic systems do not remove nitrogen before wastewater enters groundwater and eventually flows into the bay. The shared septic system located at Snug Harbor, directly above the Bluefish River, was cited as a particular concern because it lacks nitrogen removal technology.
Visibly, the problem is manifesting as a green coloration on tidal flats at low tide, a symptom of excess plant growth. Quigley noted that this extensive green coverage on the flats was not present as recently as five years ago.
The Stakes: Mandatory Restrictions Loom
While the MassDEP draft thresholds are not yet officially enforced by the EPA, Quigley cautioned that enforcement is likely coming. He pointed to the regulatory fate of Cape Cod, where all 15 towns are now subject to nitrogen-sensitive area designations requiring nitrogen removal in all septic systems, with no grandfathering.
Selectboard member Mike McGee stressed the need to act proactively, emphasizing that the septic issue is tightly intertwined with the PFAS contamination issues the town has already been addressing. The cost of mandated septic system upgrades could fall heavily on individual homeowners if the town does not intervene.
Broader Ecological Threats
The crisis extends beyond nitrogen levels. The report documented several other concerning trends:
1. Eelgrass Die-off: The bay has suffered a massive eelgrass die-off of up to 75 percent over the past decade. Eelgrass provides critical habitat, stabilizes sediments, and enhances water clarity. Loss is driven by nutrient-driven turbidity, rising water temperatures, and physical disturbance.
2. Temperature Stress: Duxbury Bay is experiencing increasingly frequent episodes of extremely warm water, defined as temperatures above 77 degrees Fahrenheit (25°C), which stress aquatic life and contribute to low oxygen events.
3. Invasive Species: The proliferation of invasive species poses a growing threat. These include European green crabs, tunicates (sea squirts), and bryozoans (moss animals). Cris Luttazi of the Duxbury Beach Reservation described an underwater video survey showing only green crabs, resembling “a horror movie,” which preys on juvenile shellfish and damages eelgrass.
The Bay’s Strengths and Solutions
Despite these severe stressors, the bay remains productive. Water quality for pathogens is generally very good, supporting swimming beaches and the commercial oyster industry, except following heavy rain events.
The burgeoning oyster industry provides a measurable environmental benefit: the 12 million oysters harvested in 2024 remove the equivalent of nitrogen waste from about 1,000 residents annually. Eelgrass restoration efforts are also underway through collaboration between the North and South River Watershed Association, DVMS, Mass Bays, and Island Creek Oysters.
Recommended Actions and Next Steps
Quigley recommended several immediate management actions:
1. Nitrogen Source Assessment: Conduct a comprehensive assessment to identify the sources and estimated volumes of nitrogen contamination, focusing on the Bluefish and Back River watersheds. The Board of Health and Conservation Commission are positioned to lead this effort.
2. Fertilizer Reduction: Implement short-term solutions such as seasonal fertilizer bans and voluntary landscaper pledges that follow best practices. Selectboard member Fernando Guitart suggested surveying fertilizer usage by town entities and the Duxbury Yacht Club.
3. Habitat Restoration: Develop and execute pilot projects for eelgrass and marsh grass, and investigate creating shellfish reefs for natural nitrogen removal.
The Selectboard endorsed holding quarterly meetings with the Planning Board to support town goals, and Selectboard members agreed to share the findings of the report with the Board of Health and Conservation Commission. The Bay Management Commission will present the findings to the Conservation Commission on Dec. 2 and the Board of Health soon thereafter to develop a nitrogen source assessment plan. The window for voluntary action, the report concludes, is rapidly closing.