The Chesapeake Bay’s 2023 dead zone was the smallest recorded since monitoring began in 1985.
Data was released by the Chesapeake Bay Program’s monitoring partners: The Maryland Department of Natural Resources (MD DNR), Old Dominion University and Virginia Institute of Marine Science (VIMS).
The summer 2023 dead zone is attributed to the combination of pollution reduction practices and below-average rainfall.
“These results illustrate that nutrient input reductions can produce a significant improvement for fish, crab and oyster habitats, and that we need to continue and advance our management efforts throughout the watershed,” said Mark Trice, program chief of water quality informatics with MD DNR’s Resource Assessment Service.
Dead zones are areas of low oxygen (less than 2 milligrams per liter oxygen) that form in deep Bay waters when nitrogen and phosphorus (nutrients) enter the water through polluted runoff and feed naturally-occurring algae. A dead zone drives the growth of algal blooms, which eventually die and decompose, removing oxygen from the surrounding waters faster than it can be replenished. An area of low-oxygen — or hypoxic — conditions is created at the bottom of the Bay that limit habitat for crabs, oysters, fish and other wildlife.
Based on water quality data the size of the dead zone from May to October 2023 was estimated using separate methods. The hypoxic volume within the Bay’s mainstem from that time period was then calculated, and the hypoxic water volume was found to be an averaged 0.52 cubic miles. VIMS reported a similarly low estimate of 0.58 cubic miles. Both estimates are the lowest on record and much lower than the historical average of 0.97 cubic miles taken from 1985-2022. The findings also align with the forecast released in June 2023, which predicted a 33 percent smaller than average dead zone.
Dissolved oxygen was better than average in May through August 2023, according to a MD DNR report, with early August having the lowest volume of hypoxia ever calculated during that time period. The dead zone was larger than average in September, but observations showed no hypoxic conditions in October. For the past four years, the summer dead zone has been below the long-term average size.
What does rainfall have to do with a dead zone?
Rainfall plays an important role in the development of dead zones. Rainfall washes nutrients from the land into the Bay. Precipitation was below-average for most of 2023 with estimates from the United States Geological Survey (USGS) show that freshwater flowing into the Bay was below average from October 1, 2022 to September 30, 2023, with 65,649 cubic feet of water per second compared to the long-term average of 79,000 cubic feet per second. In June, freshwater flows were in the historical lowest 25th percentile.
The 2023 dead zone would have been even smaller if not for the season’s above average temperatures and average wind speeds. Warmer air leads to warmer Bay waters, which in turn hold less oxygen and support higher rates of oxygen consumption by microorganisms. Faster wind speeds help mix oxygen into the deeper waters of the Bay, which can prevent hypoxic conditions.
“The low levels of hypoxia in 2023, despite the high temperatures, are truly surprising,” Dr. Marjy Friedrichs, research professor at the Virginia Institute of Marine Science, said. “This may finally be clear evidence that our nutrient reduction strategies are improving water quality and fish and shellfish habitats.”
What other factors contribute to a dead zone?
The size and duration of the Bay’s dead zone is affected by the amount of nutrients entering the Chesapeake from its surrounding watershed. Pollution-reducing practices put into place by Delaware, Maryland, New York, Pennsylvania, Virginia, West Virginia and the District of Columbia, help reduce the amount of nutrients that enter local waterways and the Chesapeake Bay from sources such as wastewater, agriculture and stormwater runoff. Data estimates that between 2009 and 2022 the six watershed states and D.C. have met 51 percent of the goal to reduce nitrogen and 60 percent of the goal to reduce phosphorus by 2025.
“This year’s Chesapeake Bay dissolved oxygen conditions are the best on record, and it is encouraging news,” Sec. Josh Kurtz of the MD DNR, who is also Chair of the Chesapeake Bay Program’s Principal Staff Committee, said. “These results show that the ongoing work to reduce pollution across the Bay’s watershed is making the Chesapeake Bay a better place for fish, crabs, oysters, and other marine life. As we focus our cleanup efforts during the next decade, we can accelerate and build on this progress.”
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Rebecca Barnabi
