Summertime on Cape Cod, Massachusetts, is usually bursting with swimmers and boaters. Falmouth is no exception, drawing tourists to its finger-shaped ponds that open to the Nantucket Sound. But in July 2012, residents of Falmouth noticed green cloudy water and a foul smell coming from Little Pond. Seventeen striped bass and other dead marine animals washed onto the shore. The likely cause? An abundance of nitrogen from septic systems spurring algal blooms that depleted the oxygen in the water.

For decades, construction of housing developments in Falmouth has outpaced wastewater management, leading to seriously impaired water in 14 estuaries. A year before the fish kill, the town formed the Water Quality Management Committee to weigh various solutions, including sewers, I/A (innovative/alternative septic systems), and household urine diversion. That’s when committee member Ron Zweig, an aquatic resources and fisheries management specialist, suggested that a lowly bivalve might help: the oyster.

Meet Eastern oysters

Whether they’re served fresh on the half shell, fried, stewed or baked, oysters are popular and packed with protein, minerals, and even Vitamin B12. The eastern oyster (Crassostrea virginica) thrives in warm estuaries — where fresh water meets salty —and can form dense colonies that are home to other creatures. Although this bivalve was once prevalent in natural reefs, today it’s more frequently farmed in large mesh bags that hang from horizontal lines. These bags protect the oysters from green crabs, starfish, and other predators.

Nitrogen harvesters

Oysters are full of surprises. Born male, baby oysters — or “seed” — attach to a surface and after about a year of growing, become female. During the warmer months of the year, a fully grown 3-inch oyster can filter about 50 gallons of water daily as it feeds on tiny floating plants (phytoplankton). “In the process,” explains Zweig, “they harvest nitrogen for flesh and shell growth because phytoplankton grow on dissolved nitrogen. That oyster growth thus removes nitrogen from the aquatic ecosystem.”

Oysters decrease nitrogen in at least one other way. Their feces, or “biodeposits” containing nitrogen, settle to the pond bottom where they eventually decompose. Through a series of microbial processes, the nitrogen is finally released to the atmosphere as gas. This nitrogen is harmless and makes up to 78 percent of the air we breathe.

Oysters grow well in saltwater bays, but before 2011 no one on Falmouth’s south shore was growing them in bags. “People wondered whether we could even grow oysters in estuaries,” admitted Zweig, “so we decided to find out.” They conducted a trial to test the concept in some of Falmouth’s poor-quality estuaries. Growers donated seed, which was put into donated bags, “and we had more than 95 percent survival,” says Zweig. “They grew beautifully. Then we were able to get the town of Falmouth to commit $250,000 to conduct a pilot project.”

That pilot project led to others, and by 2021, the town had enough confidence and experience to set up a shellfish aquaculture program. Three growers rented and farmed an acre and a half of oysters in an estuary degraded from septic effluent. They put 1.43 million oysters in mesh bags on 1.5 acres (.6 ha) of estuary and confirmed that a 3-inch adult can remove roughly 0.26 gram (.009 ounce) of nitrogen from the water. That might sound trivial, but after Falmouth’s 2023 harvest, the oysters had removed at least 882 pounds (400 kg) of nitrogen from the estuary. That’s roughly equivalent to the amount of nitrogen that 89 Cape Cod homes with leaky septic systems release to the groundwater each year (10 pounds—or 4.5 kg—per household). These results don’t even account for the denitrification happening in the muck beneath the bags. If that could be measured, it might add nearly 50% more nitrogen being removed.

Two Falmouth environmental advocates, Hilda Maingay and Earle Barnhart, estimate that 10 acres of oysters would reduce planned sewering by 800 homes, saving Falmouth more than $64 million. They cite other advantages, too: oyster farming doesn’t involve massive infrastructure disruptions (such as digging up streets and landscapes) required by the installation of sewers and innovative/alternative septic systems. Additionally, the construction and operation of those systems emits greenhouse gases, whereas oysters farming does not. In fact, oysters pull carbon dioxide from the water and sequester it in their shells, which is another plus.

Oyster investment pays off

Growers can sell one oyster for about $0.30 to $0.70 (sometimes even as high as $1.00), depending on the season and supply. The 1.43 million oysters that Falmouth and the oyster farmers grew had an estimated market value of $650,000-$750,000. Growers are eager for more acreage to farm, and other coastal towns have begun to see oyster farming as one of the more cost-effective solutions to nitrogen pollution.

These public-private partnerships in Falmouth now account for 15 acres of oyster farming in several estuaries. The town will probably continue to provide seed for multiple growers who farm the estuaries and pay back a percentage. Stephen Rafferty, chair of the Falmouth Water Quality Management Committee, says that in 2024, fees amounted to about $55,000, which the town put toward more seed, equipment, and other expenses. They’re working with the state on permits for up to 200 more acres that would be suitable for rent. “Oysters are absolutely part of the long-range plan for reducing nitrogen,” says Rafferty. That plan will feature land-based solutions like stormwater improvements, sewers, reducing fertilizers, and innovative septic systems, in addition to oyster farming.

What’s not to like?

If oyster farming is a win-win for the environment, growers, and municipalities, what’s the catch? First, oysters are productive in the summer months, but during Northeast winters they need to be transported in their bags to cold storage so they don’t freeze. This translates to extra handling and the reality of seasonal — not year-round — nitrogen capture. In addition, oyster bags must be flipped weekly to expose different portions of the shells to sunlight, a practice that dries out algal growth and barnacles and improves the oysters’ size and shape.

Oyster farms also affect nearby residents, some of whom don’t like the look of floating bags. Educating and engaging the community about the benefits is time-consuming and requires compromises. For example, residents who dig quahogs, a type of clam, need access to areas without oyster bags.

Finally, when an area’s shellfish industry is temporarily closed due to pollution, toxins, or storms, the harvest is disrupted. Some growers diversify by farming other shellfish, but none are as superb at removing excess nitrogen as the oyster. Challenges aside, the end result, Rafferty points out, speaks for itself: “The fact that we took the worst estuary in town, and now it doesn’t stink — people can go paddleboarding on it — shows that if you focus your efforts, it can be done.”