The short version
Mashpee is using shellfish as part of its plan to cut nitrogen in Popponesset Bay and Waquoit Bay. Scientists from UMass Dartmouth’s School for Marine Science and Technology (SMAST) spent three summers measuring whether it works. Water flowing past oyster beds in the Mashpee River came out with less algae, less suspended material and clearer conditions on the bottom. Quahog seeding in Waquoit Bay’s Great and Little rivers also appeared to lower nitrogen locally. But the authors are clear that shellfish can only add to, not replace, cutting nitrogen at its source.
What the researchers did
Brian Howes and colleagues studied the Town of Mashpee’s oyster farm in the Mashpee River, where oysters grow in about 330 bottom trays below the marsh edge, and a reef the Mashpee Wampanoag Tribe began building in Shoestring Bay in 2016.
- From 2016 to 2018 they sampled water above, within and below the shellfish beds 23 times, collecting 221 samples, and placed instruments that logged oxygen, algae and light around the clock.
- They measured how much nitrogen oysters hold, and tested whether oyster waste boosts natural nitrogen removal in the sediments.
- In Waquoit Bay, they used long-term town monitoring stations to look for changes after about 10 million quahog seed were planted in the Great River, Little River, Hamblin Pond and Jehu Pond area from 2014 to 2017.
What they found
- Oysters filter the water. Water passing the beds lost suspended solids, algae and some nitrogen. The effect was clearest in the Mashpee River, where oysters were spread over a larger area, and there was more light at the bottom downstream of the beds.
- No measured boost in sediment nitrogen removal. Sediment cores showed no enhanced denitrification under the oysters, possibly because control cores were too close. Low oxygen, below 3 mg/L for 8% of the time from August to October, may also limit that process.
- Harvest removes nitrogen. A 100-gram (3-inch) oyster holds about 481 mg of nitrogen. Mashpee River harvests removed an estimated 201 kg of nitrogen in 2017 (416,600 oysters) and 73 kg in 2018.
- Scale matters. A sustained harvest of about 1.5 million oysters a year would cut the Mashpee River’s watershed nitrogen load by about 15%. Meeting the river’s full target through oysters alone would take about 29,000 oysters harvested every day.
- Quahogs in Waquoit Bay. Between the upper Great River and its mouth, the difference in total nitrogen grew from 8.6% in 2017 to 26% in 2018, which the authors say may reflect the seeded quahogs between those stations. They conclude seeding had a positive effect in the Little and Great rivers, though tidal studies would be needed to confirm it.
What it means for Waquoit Bay today
Shellfish are already part of Mashpee’s nitrogen plan for both bays, and this report gives the first local evidence of how much they help. The results are encouraging for the eastern arms of Waquoit Bay, where quahog seeding seems to be making a measurable local difference. They also show the limits. In the authors’ words, The role of aquaculture is only to supplement an overarching plan of reducing nitrogen
(Howes et al., 2020).
In practice that means sewers and better septic systems still have to do most of the work, with shellfish adding local water-clarity benefits and some nitrogen removal. For a broader look at how much nitrogen shellfish carry, see our post on nitrogen in Cape Cod oysters and quahogs.
Read the report
Howes, B. L., Samimy, R., Schlezinger, D., Benson, J., Labrie, M., Unruh, A., & Horvet, S. (2020). Water Quality Improvements Associated with Mashpee Shellfish Aquaculture: Effectiveness in Reducing Water Column Nutrient Concentrations in Popponesset and Waquoit Bays. Final technical report to the Town of Mashpee. Coastal Systems Program, School for Marine Science and Technology, University of Massachusetts Dartmouth. https://www.mashpeema.gov/1329/Shellfish-Division
License: A technical report prepared for the Town of Mashpee, not openly licensed. The Town posts it on its Shellfish Division page as a downloadable Word document, so we link there rather than hosting a copy.
Drafted with AI assistance from the linked source.
