ANNAPOLIS — Data from the Maryland Department of Natural Resources and Old Dominion University indicate lower-than-average hypoxic volumes in the Chesapeake Bay mainstem during May through late June 2026.
Hypoxia refers to waters with less than 2 mg/l of oxygen. No hypoxia was observed in May — one of only ten years without monitored hypoxia in May over the past 42 years. Early June recorded the lowest hypoxic volume ever measured for that period, at less than a tenth of a cubic mile. Late June hypoxia increased to just under one cubic mile but remained below the historical average.
Anoxia — waters with less than 0.2 mg/l of oxygen — was absent in May and early June, with only 0.05 cubic miles observed in late June.
Summary results from combined Maryland and Virginia mainstem monitoring cruises (volumes in cubic miles):
| 2026 Cruise | Hypoxic Volume | Average Volume (1985-2025) | 2026 Rank |
|---|---|---|---|
| May | 0.0 | 0.18 | 1st (tied) / 42 |
| Early June | 0.076 | 0.85 | 1st / 38 |
| Late June | 0.97 | 1.27 | 14th / 36 |
The improved conditions are partly due to persistent drought in the watershed, leading to below-average water flow into the Bay since August 2025, according to the U.S. Geological Survey. Lower flows mean reduced nutrient pollution from nitrogen and phosphorus, which fuel algal blooms that deplete oxygen in deeper waters.
Chesapeake Bay Program partners’ seasonal forecast predicted hypoxia volumes about 31% below the long-term average, aligning with monitoring results so far. From January through April 2026, river flows were 32% below average and nitrogen loads 39% lower.
However, recent record-high air temperatures and expected seasonal rain storms could influence future conditions. Warmer waters hold less oxygen, and increased freshwater flow may enhance stratification, limiting oxygen mixing and introducing more nutrients.
Crabs, fish, oysters and other Bay creatures depend on adequate dissolved oxygen. Scientists monitor hypoxia volume and duration to assess impacts on aquatic life.
Ongoing efforts by Chesapeake Bay Program partners aim to reduce nitrogen and phosphorus from wastewater, agriculture, and urban sources. Each year from May through October, DNR computes hypoxia volumes from water quality data. Monitoring and reporting will continue through summer.
Additional Maryland water quality data and information are available on the Eyes on the Bay website.
