You are likely to find shellfish on the menu in more than a few Cape Cod restaurants but the small mollusks are also working beneath the surface of local waters to filter out nitrogen that is damaging ecosystems. And new data show they are having an impact.
The use of shellfish is one part of Mashpee’s large-scale plan to improve its water quality. The town has been targeting areas of Little River, Great River, Jehu Pond and Hamblin pond with quahogs (a type of hard clam) for the last several years and is now reaching a point where one can observe a positive impact in the data.
Under the Clean Water Act, towns across the Cape are required to implement comprehensive plans to improve their water quality. Mashpee needs to reduce nitrogen levels in its waters to 0.38 milligrams per liter. Between 2016 and 2017, the town achieved 30 percent of the reduction needed to reach this goal in the areas it targeted with quahogs, according to a report.
“We now have water quality monitoring results that show that the shellfish are... improving water quality,” said Richard York, director of the Mashpee Department of Natural Resources. The level of total nitrogen in the quahog-targeted waters is hovering above 0.5 milligrams of nitrogen per liter, a graph shows.
For years, Mashpee seeded approximately 1 million quahogs around town. Since 2013, DNR has stepped up these numbers in the target area, seeding more than 7 million last year and more than 4 million in 2016. The impact is not immediate, but now that it has been a few years, harvests are starting to register an impact, Mr. York explained.
Additionally, the town has only ever spent up to $150,000 per year on quahog seeds, but the Mashpee Watershed Nitrogen Management Plan calls for spending $1.5 million. “So, by dollar value we’re only at 10 percent of implementation,” Mr. York said.
The main way shellfish, including oysters and quahogs, help decrease pollution is by taking in nitrogen from the water and incorporating it into their shells and tissues. The shellfish are then harvested from the water.
Too much nitrogen in a body of water spurs algae growth, which can choke oxygen out of the water, eliminating habitats and killing fish. In a recent study, the Woods Hole Sea Grant program and Cape Cod Cooperative Extension partnered with Mashpee DNR to ascertain just how much nitrogen the shellfish harvested from Cape Cod waters could remove.
“We did this project specifically as a service to local municipalities to get them accurate data to utilize, if they’re going to go with the approach of seeding and growing shellfish as part of their water quality management plans,” said Joshua Reitsma, the lead author of the paper, in an article on the Woods Hole Oceanographic Institution’s website.
In Mashpee, staff from the DNR purchase quahog seed from a hatchery and grow them to a certain size before planting them in the waters.
The positive news stands in contrast to a presentation that Brain L. Howes, a researcher from the University of Massachusetts Dartmouth, made to the board of selectmen last month in which he painted a largely bleak picture of the town’s progress toward improved water quality. “These are very highly impaired conditions,” Dr. Howes told the board. “It’s a very sad story.”
Dr. Howes explained that between 2009 and 2015, the Waqouit and Pompanesset bays showed signs of improvement but conditions declined again in 2017, registering some of the worst numbers in 20 years of data. The Quashnet and Mashpee rivers showed notably higher nitrogen levels, due in part to a spike in rainfall after a years-long drought.
Shellfish will not be able to solve all of Mashpee wastewater problems, Mr. York said. Sewer programs will be a major part of the cleanup, especially in the most impaired areas, but the improvement in quahog-targeted areas is positive, he said. Members of the town’s shellfish commission and the Environmental Oversight Committee were pleased to see these numbers, particularly in light of Dr. Howe’s assessment.
“If this wasn’t working, we’d just go to sewering the whole town,” Mr. York said of the quahog-targeted areas. “I’m confident we can clean up this area with quahogs.”
The town has also been targeting the Mashpee River with oysters since 2004, when it began an effort to restore the oyster population lost in the 1980s. The harvest quantities are not yet enough to have a real numerical effect, but Mr. York said there have been no new fish kills since they took on this project. A fish kill is when a localized population of fish dies off.
Additionally, the Mashpee Wampanoag Tribe has a Healthy Communities Grant from the Environmental Protection Agency to restore oyster beds in Shoestring Bay, although it is also too early for that project to show an impact, Mr. York said. The DNR works closely with the tribe and its natural resources department on oyster restoration. “We’re working together because it’s more effective when we do,” Mr. York said.
There is an official partnership between the town, the selectmen, the tribe and the UMass Dartmouth School for Marine Science & Technology to conduct the water quality monitoring that provides data all parties need to guide their efforts.