The Chesapeake Bay’s summer “dead zone” is continuing to track well below its historical average, according to Maryland Department of Natural Resources monitoring data released this week, extending a pattern that has held since dissolved-oxygen tracking began in May.
Hypoxic waters — those with less than 2 milligrams of oxygen per liter, too little for crabs, fish, oysters and other Bay species to comfortably survive — measured slightly below average in early July and dropped further in late July, when the mainstem hypoxic volume ranked 8th-smallest out of 41 years of late-July monitoring dating to 1985. Anoxic waters, an even more severe oxygen shortage of less than 0.2 milligrams per liter, measured 0.30 cubic miles in early July and 0.39 cubic miles in late July. The results continue a stretch of favorable readings all season: May recorded zero measured hypoxia, one of only 10 such years in 42 years of record-keeping, and early June posted the lowest hypoxic volume ever measured for that period, according to DNR and Old Dominion University data collected jointly with Virginia water-quality monitors.
Scientists attribute the mild conditions primarily to an ongoing regional drought. The U.S. Geological Survey has tracked below-average water flow into the Bay dating back to August 2025, which in turn has meant less nitrogen and phosphorus washing into the watershed from farms, wastewater systems and urban runoff. Those nutrients fuel the algal blooms that die off, decompose and strip oxygen from deeper Bay waters each summer. Federal and state estimates found that from January through April, river flow into the Bay ran 32% below its long-term average, while nitrogen loading came in about 39% lower than average — roughly 59 million pounds of nitrogen, by monitoring-station estimates.
This year’s conditions track closely with a seasonal forecast issued in May by scientists at William & Mary’s Batten School of Marine Science, the University of Maryland Center for Environmental Science and the consulting firm FlowWest, who projected the 2026 dead zone would rank among the smallest 10% of years since 1985 and run about 31% below average. FlowWest senior managing scientist Aaron Bever said at the time that the Bay could see “one of its milder dead zones in recent decades.” It isn’t unprecedented — a similarly dry 2022 produced a dead zone roughly 13% smaller than average, according to a joint assessment from DNR and the Virginia Institute of Marine Science that year — but 2026’s numbers so far are running even further below normal.
Scientists caution that the outlook could still shift. Forecasters have noted that record-high air temperatures in the watershed and the potential for late-summer rain events could work against the favorable trend: warmer water holds less oxygen, and a sudden pulse of freshwater can increase stratification, the layering effect that prevents oxygen-rich surface water from mixing into deeper layers where hypoxia forms.
DNR calculates Bay hypoxia volumes from water-quality data collected with the Virginia Department of Environmental Quality, funded jointly by both states and the Chesapeake Bay Program, a federal-state partnership overseeing Bay restoration efforts. Monitoring will continue on its regular schedule through October, with results and methodology posted on the state’s Eyes on the Bay website.
