Chesapeake Bay 2025 Hypoxia Slightly Above Average

The Chesapeake Bay experienced slightly above-average hypoxic conditions in 2025, with the seasonal “dead zone” volume ranking as the 11th largest in 41 years of records, according to data released by the Maryland Department of Natural Resources and Old Dominion University.

Hypoxia—defined as dissolved oxygen levels below 2 milligrams per liter—occurs when excess nutrients trigger algal blooms that die, sink and decompose, consuming oxygen needed by crabs, fish, oysters and other aquatic life. The 2025 mainstem Maryland-Virginia volume fell between the long-term average and the severe years that dominate the top of the historical rankings.

Seasonal patterns varied sharply. Hypoxic volumes remained below average in May and early June, then spiked above average from late June through early August before dropping below average again in late August and September. September measurements may underestimate actual hypoxia because several lower Bay stations in Maryland went unsampled. October volumes were low but still above the monthly average, possibly because sampling occurred during the first week of the month—earlier than typical.

Freshwater inflow into the Bay leading into summer 2025 ran roughly 20 percent above average, delivering additional nitrogen and phosphorus that fuel oxygen-depleting blooms. Heavy spring rains and record heat played key roles.

May 2025 brought extreme precipitation across the watershed. Maryland recorded 7.81 inches of rain—the highest May total in 131 years—while Pennsylvania saw 7.72 inches, the second-wettest May on record. Those storms flushed nutrients downstream just as summer warming began.

June and July followed with intense heat. Maryland’s average June temperature reached 74.8 degrees Fahrenheit, the fourth-highest in 131 years, while July averaged 79.9 degrees—the second-hottest July on record. The combination of nutrient-rich runoff and prolonged high temperatures accelerated algal growth and subsequent oxygen loss in deep Bay waters.

Conditions shifted dramatically in August. Maryland’s average temperature cooled to 71.8 degrees, the 25th-lowest August since 1895 and the coolest since 2004. August precipitation ranked among the driest on record in both Maryland and Pennsylvania, reducing additional nutrient loads and allowing oxygen levels to rebound.

The Maryland Department of Natural Resources calculates hypoxic volumes each year from May through October using water quality data collected on joint Maryland-Virginia monitoring cruises. Virginia Department of Environmental Quality and the EPA Chesapeake Bay Program provide funding support. Real-time and historical data, along with detailed calculation methods, are publicly available on the DNR’s Eyes on the Bay website.

A separate 2025 Dead Zone Report Card produced by scientists at William & Mary’s Batten School, the Virginia Institute of Marine Science and Anchor QEA evaluates both mainstem and tributary hypoxia duration in addition to total volume. That report, released concurrently, provides a broader picture of low-oxygen impacts across the estuary.

In Southern Maryland, where the lower Potomac River and Patuxent River empty into the Bay, residents and watermen closely track hypoxia because dead zones can force blue crabs and striped bass into shallower, warmer water where they become more vulnerable to harvest or stress. St. Mary’s, Calvert and Charles counties host numerous monitoring stations that feed into the statewide dataset.

Long-term efforts to meet Chesapeake Bay Total Maximum Daily Load goals continue targeting nitrogen and phosphorus from wastewater plants, farms and urban runoff. Maryland’s upgraded wastewater treatment plants, cover-crop programs and stormwater regulations have contributed to gradual water-quality improvements, though weather extremes like those in 2025 can temporarily reverse gains.

Bay-wide monitoring cruises will resume in May 2026, with monthly hypoxia updates posted throughout the summer on the Maryland DNR website.