The short version
Nitrogen from homes and fertilizer kept rising around Buzzards Bay, while nitrogen falling from the air began to decline in the mid-1990s. This study asked which trend won out and whether the harbors showed it. The team modeled nitrogen loads for eleven watershed-estuary systems on the bay's eastern and northern shores (1985 to 2013) and compared them with summer water samples collected by the Buzzards Bay Coalition (1992 to 2018). Watersheds held back 80% to 87% of the nitrogen reaching them, with more retention where there was more forest. After 1996, the drop in atmospheric nitrogen outweighed rising septic and fertilizer inputs at most sites, and estuary loads fell. Nitrogen concentrations in the water turned downward several years later, which matters for judging when a cleanup is working. This summary is based on the full text of the authors' manuscript.
What the researchers did
The authors combined a nitrogen-loading model with a long record of water samples from small, shallow harbors fed mainly by groundwater.
- Chose eleven watershed-estuary systems on the eastern and northern shores of Buzzards Bay, including Wareham River, Buttermilk Bay, Megansett Harbor, Wild Harbor, West Falmouth Harbor, Great Sippewissett Marsh and Quissett Harbor. The estuaries are 0.3 to 5.5 square kilometers and about 2 meters deep on average.
- Ran an established nitrogen-loading model year by year from 1985 to 2013. It tracks nitrogen from septic systems, fertilizer and atmospheric deposition (nitrogen that settles out of the air), and how much is lost on the way through each watershed.
- Added nitrogen falling directly onto each estuary, including tidal salt marsh, to get the load each estuary received.
- Compared loads before and after 1996, the year regional atmospheric nitrogen deposition turned from variable to declining.
- Matched the loads against Buzzards Bay Coalition monitoring data (1992 to 2018) on nitrate, ammonium, dissolved organic nitrogen, chlorophyll and oxygen: 64 stations and more than 7,000 observations per variable, mostly collected in summer.
- Treated two systems separately: West Falmouth Harbor and Wareham River receive sewage treatment plant discharge, while the other nine rely on septic systems.
What they found
- Watersheds kept most nitrogen out. Across 1985 to 2013, 80% to 87% of the nitrogen entering each watershed never reached the estuary, and retention rose with the share of forest (Lloret et al., 2022). Only about 16% of the nitrogen input was discharged to estuaries, and the authors call that small share the source of the harbors' eutrophication (nutrient overload).
- Septic nitrogen is the least well retained. Retention averaged about 73% to 74% for septic nitrogen, versus roughly 90% for fertilizer and atmospheric nitrogen. Septic nitrogen exceeded fertilizer in seven of the nine systems without a treatment plant. The two exceptions, Buttermilk Bay and Red Brook Harbor, have more cranberry bogs and other agriculture.
- Rising, then falling loads. Septic nitrogen rose in most watersheds, by 15% to 68% from 1985 to 2013, and fertilizer nitrogen rose by 8% to 139%. Even so, atmospheric deposition fell enough after 1996 to turn the totals around. For the nine septic-only estuaries, loads rose a weighted mean of 19.2% in 1985 to 1996 and fell 12.1% afterward. The authors estimate loads could have risen another 11% by 2013 had atmospheric nitrogen kept its earlier trend.
- Treatment plants can swamp everything else. At West Falmouth Harbor, discharge from the Falmouth sewage plant (via infiltration beds into the aquifer) drove the nitrogen load sharply upward from the early 1990s to early this century, then leveled off after plant improvements. The Wareham River plant discharges directly to the estuary, and its nitrogen fell as the plant was upgraded. The authors read the contrast as showing that plant discharges can dominate a system but can be managed.
- Higher loads, higher concentrations. Estuaries with larger loads had more nitrate, ammonium and total dissolved nitrogen in the water. Dissolved organic nitrogen, the largest pool, barely responded, and the authors note that about 70% of it may be refractory (hard for organisms to use). Concentrations of nitrate and total dissolved nitrogen were higher in the fresher, upper reaches, especially in estuaries with large loads.
- Water quality lagged by years. Nitrogen concentrations peaked and then began to decline at different times: ammonium in 2001, nitrate in 2003, total dissolved nitrogen in 2005 and dissolved organic nitrogen in 2006. Chlorophyll (a measure of algae) peaked in 2009. The authors suggest long groundwater travel times and nitrogen stored in aquifers, sediments and marshes as causes. They say a plan that cuts the nitrogen load to an estuary by 12% to 34% may take 4 to 10 years to show up in the water.
- Chlorophyll and oxygen were poor yardsticks. Summer chlorophyll scattered widely and did not track nitrogen loads. Oxygen readings scattered too, and only about 4% fell below 4 mg per liter, the level below which harm to marine life is expected. The authors conclude that these single measurements are not reliable early indicators of eutrophication in these harbors.
What it means for Waquoit Bay today
The study sites are on Buzzards Bay's eastern and northern shores, including West Falmouth Harbor, Wild Harbor, Megansett Harbor and Quissett Harbor, next to but outside the Waquoit Bay watershed. The setting will look familiar: small, shallow, groundwater-fed estuaries ringed by homes on septic systems. Three points carry over. Forest in the watershed keeps a large share of nitrogen from ever reaching the water, which supports protecting land (see how a Falmouth plan counts on sewers and other tools). Septic nitrogen is the part least well held back, and it was still growing. And a treatment plant's discharge can dominate a harbor's nitrogen budget, as it did in West Falmouth (see the harbor's slow wait for the old plume to clear).
The lag finding matters for anyone watching Waquoit Bay's recovery. Water-quality gains showed up four to ten years after loads changed, so a few flat years of monitoring after a sewering or treatment project do not mean it failed. Outside Cape Cod, a similar message comes from Long Island Sound's cleanup progress report. The study measured nitrogen mostly in summer and modeled loads rather than measuring them, and it did not test Waquoit Bay itself.
Read the paper
Lloret, J., Valva, C., Valiela, I., Rheuban, J., Jakuba, R. W., Hanacek, D., Chenoweth, K., & Elmstrom, E. (2022). Decadal trajectories of land-sea couplings: Nitrogen loads and interception in New England watersheds, discharges to estuaries, and water quality effects. *Estuarine, Coastal and Shelf Science*, 108057. https://doi.org/10.1016/j.ecss.2022.108057
License: The copy we were given is the authors' manuscript, and it carries no license statement. A link check suggested a public-domain status, but the manuscript does not say so, so we link to the publisher's copy through the DOI and do not host the PDF. This summary is based on the full text of that manuscript. The card image is the first page of the manuscript.
Drafted with AI assistance from the linked source.
