The short version
The Chesapeake Bay still has a large low-oxygen “dead zone” every summer, and in the very wet years of 2018 and 2019 it was unusually big. That made some people ask whether decades of nitrogen cuts had done any good. Researchers at the Virginia Institute of Marine Science and the Chesapeake Bay Program asked a sharper question: how much worse would the dead zone have been without those cuts? Their computer model says it would have been about 50–120% larger in normal years and 20–50% larger in wet years. Warming water has already wiped out an estimated 6–34% of that improvement.
What the researchers did
The team used two independent approaches and compared the answers.
- A statistical look at monitoring data from 1985 to 2019, comparing how much low-oxygen water formed for a given amount of river flow in 1985–1990 versus 2014–2019.
- A detailed 3-D computer model of the bay run for 2016–2019 (two average years and two very wet ones), first with real conditions and then with river nitrogen set back to 1985 levels.
- An extra model run with water temperatures set back to 1985, when the bay was about 1.4 °C (2.5 °F) cooler in summer, to isolate the effect of warming.
What they found
- A much bigger dead zone without the cuts. With 1985 nitrogen levels, the yearly volume of water below 3 mg/L of oxygen would have been about 50–120% greater in 2016–2017 and 20–50% greater in 2018–2019. For severely low oxygen (below 1 mg/L) the increases were larger still: about 80–280% and 30–100%.
- The data agree, more modestly. The statistical analysis also showed less low-oxygen water for the same river flow in recent years: without that improvement, the volume below 3 mg/L would have been 18–26% larger in 2016–2019.
- Warming explains most of the gap. The model run with 1985 temperatures showed warming made recent low-oxygen volumes about 7–13% larger, offsetting roughly 6–34% of the gain from nitrogen cuts. Warm water holds less oxygen and speeds up the bacteria that use it.
- The southern edge benefited most. The biggest gains were in the middle bay between the Patuxent and Rappahannock rivers, where the cuts spared roughly 50–90 days of low oxygen a year in places.
- More cuts are still needed. The authors conclude the bay is more resilient than it would have been, but that larger reductions are needed to keep pace with climate change.
What it means for Waquoit Bay today
The Chesapeake is much bigger and deeper than Waquoit Bay, with a watershed spanning six states and nitrogen coming largely from farms and cities, so its numbers don’t transfer directly. The lesson that does transfer is about how to judge progress. Looking only at this year’s water quality can make a cleanup look like a failure, especially after a wet year, when the real question is how much worse things would be without it. Our own bay’s monitoring results also swing from year to year with rain and temperature, not just with nitrogen.
The warming finding matches what researchers saw in Narragansett Bay: rising temperatures quietly eat into the benefit of nitrogen cuts. For Falmouth and Mashpee, that argues for meeting the Waquoit Bay nitrogen targets fully, and soon, rather than assuming a partial cut will be enough in a warmer future. For another Chesapeake study on the value of clean water, see our summary on water clarity and home prices.
Read the paper
Frankel, L. T., Friedrichs, M. A. M., St-Laurent, P., Bever, A. J., Lipcius, R. N., Bhatt, G., & Shenk, G. W. (2022). Nitrogen reductions have decreased hypoxia in the Chesapeake Bay: Evidence from empirical and numerical modeling. Science of the Total Environment, 814, 152722. https://doi.org/10.1016/j.scitotenv.2021.152722
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 (Elsevier), reproduced unmodified from the copy in the NOAA Institutional Repository. It was published online on December 31, 2021, in the journal’s 2022 volume. The card image is the article’s first page.
Drafted with AI assistance from the linked source.
