ANNAPOLIS, Md. – Overall oxygen conditions in the Chesapeake Bay during 2025 were near the average observed in recent decades. But as is often the case, “average” obscures conditions that at times were both notably better and worse than normal.
In Maryland, the area affected by low oxygen was the 11th largest out of 41 years of monitoring, which was classified in the average range, according to the Maryland Department of Natural Resources.
A computer analysis of Baywide conditions by the Virginia Institute of Marine Science found that low oxygen, or hypoxic, conditions in mid-summer were severe at their peak but didn’t last as long as they typically do.
Both analyses show that the summer started out with a smaller than normal “dead zone” through early June. Then conditions began to worsen. In early July, DNR monitoring noted that nearly 20% of the mainstem Bay within Maryland had too little oxygen to support most aquatic life. That was the fifth worst report for early July in 40 years of monitoring.
The situation improved a bit in late July and early August but remained notably worse than the long-term average. By late August, about 10% of Maryland’s portion of the Bay was hypoxic — roughly normal for that time of year. Increased winds and cooler air temperatures in August likely resulted in a decrease from the July peak, DNR reported.
Hypoxia is defined as water with less than 2 milligrams of oxygen per liter of water. Most aquatic life will not survive for long with that little oxygen.
Hypoxic conditions exist naturally in deep areas of the Bay, especially during years of high freshwater flows. That results in strong stratification in the water, which prevents oxygen-rich surface water from mixing with low-oxygen bottom water.
In the last century, areas impacted by hypoxia have expanded as increased amounts of nutrients spurred more algae growth. When the water contains more algae than fish and other aquatic life can consume, the excess sinks to the bottom and decomposes in a process that consumes oxygen.
DNR reported that freshwater flows during the first several months of the year were about 20% larger than normal, washing more nutrients into the Chesapeake and increasing the likelihood of large dead zones.
Then came an extremely wet May, which saw the highest precipitation on record for Maryland and the second highest for Pennsylvania. DNR scientists said the rain washed even more nutrients downstream and likely set up the spike in hypoxia seen in late June and early July. Hotter than normal mid-summer weather likely exacerbated the problem, they said, because hot spells can reinforce stratification in the water and prevent mixing.
The weather turned cooler and drier later in the summer and winds picked up, which facilitated the mixing of water and relieved hypoxic conditions, DNR said.
A similar picture was painted by the Chesapeake Bay Environmental Forecast System developed by VIMS and Anchor QEA, an environmental engineering and consulting firm. The system uses computer models that use monitoring and other data to estimate Baywide hypoxia.
For the entire Bay, it showed conditions worsening through June, peaking in July with a relatively large dead zone that impacted about 12% of the Chesapeake. The zone shrank in August to better than normal conditions when temperatures cooled and winds from Hurricane Erin, which passed offshore, helped facilitate the mixing of water.
“Even in what we classify as an average year, the Bay can experience periods of prolonged stress,” said Marjorie Friedrichs, a VIMS professor. “July’s persistent hypoxia illustrates how sensitive the ecosystem is to subtle shifts in wind, temperature and river flow. Those changes can have real implications for the distribution and behavior of fish, crabs and other species that rely on well oxygenated habitat.”
The model also found that hypoxic conditions generally started later and ended earlier than in other years, with 60 days of sustained low oxygen, below the typical range of 78-116 days.
Taken as a whole, the model concluded that 2025 was generally an average year for the Bay’s dead zone. The total Baywide amount of hypoxia estimated by the model ranked 30th out of 40 years, and VIMS scientists said that generally fell into the normal range.