The short version
In the 2000s Rhode Island required many wastewater treatment plants around Narragansett Bay to upgrade so they would remove nitrogen. Scientists at the University of Rhode Island and the Narragansett Bay Estuary Program then checked whether the bay actually got better. Comparing 2013–2019 with 2005–2012, the bay’s summer nitrogen load fell 34%, the worst algae events declined by about a third, and low-oxygen events dropped to zero or nearly zero from 2014 on. The bay warmed during the same years, which took away some of the gain, and the authors warn that warmer, wetter summers could undo part of the progress.
What the researchers did
The team looked at June through September of each year from 2005 to 2019, the season when algae peak and oxygen runs low.
- They added up daily nitrogen loads from 18 sources: 11 treatment plants that discharge straight to the bay, six rivers (including the plants upstream on them), and land that drains directly to the bay.
- They used readings taken every 15 minutes by sensors at eight monitoring buoys spread across the bay.
- They scored each summer for algae (how far and how long chlorophyll stayed above set levels) and for low oxygen near the bottom (how far and how long it stayed below 2.9 and 1.4 mg per liter, levels set to protect young fish and shellfish).
- They tested whether river flow, layering of the water, wind, tides, sea level or temperature could explain the changes better than nitrogen could.
What they found
- The load came down. Average summer nitrogen load in 2013–2019 was 34% lower than in 2005–2012, a drop of roughly a million kilograms a year. That cut was about as large as the normal swings caused by wet and dry years.
- Fewer algae blooms. From about 2007 to 2019 the algae score fell steadily: about 33% for severe events and 16% for moderate ones.
- Low oxygen nearly disappeared. Severe low-oxygen events had dropped sharply by 2009, and from 2014 to 2019 both low-oxygen scores stayed at or near zero.
- Nitrogen, not weather, explains it. River flow drove year-to-year ups and downs, but the long-term decline in algae tracked the nitrogen cut. Wind, tides, sea level and temperature did not explain it.
- The bay did not respond instantly. In 2013, when one of the largest plants finished its upgrade, the load fell but algae did not drop right away, suggesting a lag of a year or more, possibly from nitrogen stored in bottom sediments.
- Warming and rain push the other way. Summer water warmed about 0.46 °C per decade, which lowers how much oxygen water can hold. The authors write:
Without warming, the managed load decrease would have curtailed hypoxia more effectively
(Codiga et al., 2022). A summer with river flow about 55% above average could add back as much nitrogen from other sources as the plant upgrades removed.
What it means for Waquoit Bay today
This is one of the clearest cases of a New England estuary improving after a deliberate nitrogen cut, and it continues the story in our summaries of the 1998 Narragansett Bay report card and Long Island Sound’s progress report. The main lesson for us is simple: when nitrogen goes down, algae and low oxygen follow. But Waquoit Bay differs in important ways. Narragansett Bay’s nitrogen came largely from treatment plants that could be upgraded in a few years. Most of Waquoit Bay’s comes from septic systems and moves through groundwater, so even after Falmouth and Mashpee cut it at the source, the bay will wait years for the change to arrive (see our summary of the legacy nitrogen model).
The warning about warming applies here too, perhaps more so, since Waquoit Bay is much shallower. Warmer water holds less oxygen, so nitrogen targets set years ago may need extra margin. The authors also note that such targets may need to be revisited and that long-term monitoring is what made this result visible.
Read the paper
Codiga, D. L., Stoffel, H. E., Oviatt, C. A., & Schmidt, C. E. (2022). Managed nitrogen load decrease reduces chlorophyll and hypoxia in warming temperate urban estuary. Frontiers in Marine Science, 9, 930347. https://doi.org/10.3389/fmars.2022.930347
License: This article is open access under a Creative Commons Attribution 4.0 (CC BY 4.0) license. The PDF below is the publisher’s version from Frontiers; CPWB has compressed its images so it downloads faster, and the text is unchanged. The card image is the article’s first page.
Drafted with AI assistance from the linked source.
