The short version
The Quashnet River estuary, whose upper basin above the Meadow Neck Road bridge is known locally as the Moonakis River, is one of the most nitrogen-polluted corners of Waquoit Bay. Falmouth and Mashpee asked scientists at UMass Dartmouth’s School for Marine Science and Technology (SMAST) to test two ideas: that shoaling or the bridge was trapping polluted water, and that farmed oysters could help clean it. Their 2016 measurements showed the tide moves in and out almost exactly as it did in the early 2000s, so poor flushing is not what’s getting worse. Instead, more nitrogen was coming down the Quashnet River, and the estuary was as bad as ever. Oysters grew well wherever the water was salty enough.
What the researchers did
This follow-up to the Massachusetts Estuaries Project (MEP) analysis was done for Falmouth’s Water Quality Management Committee and Mashpee’s Sewer Commission, with most fieldwork in summer 2016.
- Set five tide gauges (four in the estuary, one in the main bay) from June to October 2016 and redid the depth survey first made for the MEP in 2002.
- Measured water and nitrogen moving under the Meadow Neck Road bridge over two full tide cycles (July 19 and August 18, 2016).
- Sampled the Quashnet River weekly at the USGS stream gauge above Route 28 (July to December 2016) and compared the results with 2003.
- Measured how much nitrogen the bottom mud releases at 12 sites, moored oxygen and chlorophyll sensors at three stations, and grew test oysters in floating bags and bottom trays at three spots.
What they found
- The tide isn’t blocked. High tides reached the same level at every gauge. Differences between the bay and the lower estuary were usually 0–5 cm (at most 19 cm), and almost all of that came from higher low tides inside, which the authors attribute to the shape of the basin rather than a restriction. The bridge channel is deep and scoured, not a bottleneck.
- Flushing is unchanged since the MEP. The estuary’s volume was within 1.7% of the earlier survey, the water exchanged each tide (about 2.8 million cubic feet) differed by only 0.2%, and water still stays about 1.6–1.7 days on average. The shoal at the inlet causes only a minor reduction; if any dredging is done, the memo says it should be limited to that shoal.
- More nitrogen is coming down the river. With almost identical stream flows in both periods, July–December nitrate loads were 31% higher in 2016 than in 2003 and total nitrogen loads 9% higher (15.8 versus 14.5 kg, or about 35 versus 32 lb, per day). Phosphate fell by about half. The authors point to more nitrogen from the watershed, a breached berm that once separated cranberry bogs from the river, or both, but say the cause is not yet clear.
- The upper estuary sends nitrogen to the bay. On both tide cycles measured, more nitrogen flowed out of the Moonakis on the ebb than came in on the flood: a net 8.6 kg in July and 17.1 kg in August.
- Still severely impaired. From July to October 2016, bottom-water oxygen was below 4 mg/L for 26% of the time at the upper station, 16% mid-estuary and 21% in the lower basin. Average chlorophyll was 46 and 59 µg/L in the upper and middle basins (above 10 signals over-enrichment), with blooms peaking at 150–200 µg/L. Nitrogen release from the bottom mud was about the same as in 2001.
- Oysters can grow here, where it’s salty enough. In floating bags at the upper station, surface water averaged only about 1.5 ppt salt; nearly half those oysters died (48.7%) and the rest didn’t grow. Everywhere else, including bottom trays at the same upper spot, losses were under 4% and oysters grew about 7–9 mm a month. The estuary is currently closed to shellfishing.
What it means for Waquoit Bay today
This memo is why dredging or bridge work isn’t the main answer for the Moonakis. The tide already reaches the head of the estuary; the problem is how much nitrogen arrives with the river and groundwater. The authors write that this sub-estuary has not improved over the past 10 years
(Howes et al., 2017), and they frame the Quashnet’s nitrogen as part of the load that keeps the whole bay above its limit. The MEP target for this sub-estuary is 0.50 mg/L of total nitrogen, explained in our summary of the 2013 Waquoit Bay threshold report.
The oyster results are encouraging but limited: one season, a few hundred oysters per bag, and no estimate of how much nitrogen a full-scale farm would remove. The memo recommends testing any combined plan with the MEP’s bay-wide model. For what has been proposed for the river since, see our summary of the Moonakis River watershed restoration plan; for how the estuary has fared in later years, see Mashpee water quality monitoring, 2010–2020. Keep in mind that these data are from 2016 and the memo is marked as a draft.
Read the memo
Howes, B., Samimy, R., Schlezinger, D., Labrie, M., & Sampieri, S. (2017, October 19). Technical memorandum (draft): Detailed assessment of water and nutrient exchange within the Quashnet River / Moonakis River sub-embayment to Waquoit Bay. Coastal Systems Program, School for Marine Science and Technology, University of Massachusetts Dartmouth. Prepared for the Town of Falmouth Water Quality Management Committee and the Town of Mashpee Sewer Commission. https://www.mashpeema.gov/sites/g/files/vyhlif3426/f/pages/quashnet-moonakis_tech_memo_draft_final_2017.pdf
License: This technical memorandum was prepared for two public bodies, Falmouth’s Water Quality Management Committee and Mashpee’s Sewer Commission, and is marked DRAFT. It carries no open license, so we link to the copy the Town of Mashpee posts on its website instead of hosting it here. The card image is the memo’s first page.
Drafted with AI assistance from the linked source.
