Chesapeake Bay Monitoring Shows Hypoxia Conditions Improved in Late August and September

Hypoxia volumes decreased in late summer from earlier highs

Data collected by the Maryland Department of Natural Resources and Old Dominion University showed an above average volume of hypoxia—waters with less than 2 mg/l of oxygen—in the Chesapeake Bay mainstem of Maryland and Virginia in early August, that then decreased to below average in late August and September.

Early August hypoxia remained consistent with late July with a volume of 1.85 cubic miles. and the tenth-highest early August volume in the past 38 years sampled. Anoxia – waters with less than 0.2 mg/l oxygen – was more than double the average at a volume of 0.54 cubic miles. 

Hypoxia volumes decreased in late August to just below average, and anoxia also decreased to 0.06 cubic miles, one-third of average observed volumes.

September saw a further decrease in hypoxia volumes to less than half the average volume, and no anoxia was present; however, mechanical issues during the Sept. 15-17 cruise resulted in some missing data in the mainstem Bay around the Maryland/Virginia state line.

Summary results from combined Maryland and Virginia mainstem Bay monitoring cruises are as follows, with volumes in cubic miles. Rank is in order of lowest (better conditions) to highest hypoxia volume (worse conditions) versus years analyzed.

Qualitatively, freshwater flows into the Chesapeake Bay leading up to summer were about 20% above average, indicating a greater than average hypoxic volume may be observed during this season due to more nutrients entering the Bay. 

Above average volumes in late June and into July could be the result of significant rainfall in Maryland and Pennsylvania during May, and high average air temperatures in the region during June and July, with nutrients from runoff fueling algal blooms, their decomposition, and resultant oxygen consumption. 

The summer of 2025 has presented a number of meteorological extremes for the region. According to NOAA’s National Centers for Environmental Information, Maryland received 7.81 inches of rainfall in May, the highest in the past 131 years. Pennsylvania’s precipitation was 7.72 inches, the second highest in that same time frame.This year’s rainy May was followed by a hot June and July. Maryland’s average temperatures in June was 74.8 degrees, fourth-highest in 131 years, and 79.9 degrees in July, second-highest in 131 years. The weather extremes changed from hot to cooler in August,, with Maryland’s average August temperatures the 25th lowest since 1895 at 71.8 degrees. Average temperatures this low had not been observed since 2004 and 1994. Precipitation in August was the third and seventh lowest in Pennsylvania and Maryland respectively.

Crabs, fish, oysters, and other creatures in the Chesapeake Bay require dissolved oxygen to survive. Scientists and natural resource managers study the volume and duration of Bay hypoxia to determine possible impacts to Bay life.

Ongoing efforts to reduce nitrogen and phosphorus pollution from industrial and wastewater sources, agricultural land, and cities and towns are aimed at reducing hypoxic conditions in the Bay. In the water, nitrogen and phosphorus fuel algal blooms that remove oxygen from the water when they die off and decompose.

Each year from May through October, the Department of Natural Resources computes hypoxia volumes from the water quality data managed by department staff and the Virginia Department of Environmental Quality. Data collection is funded by these states and the EPA Chesapeake Bay Program. Bay hypoxia monitoring and reporting will continue through the summer. Additional Maryland water quality data and information, including the Department of Natural Resources hypoxic volume calculation methods, can be found on the Eyes on the Bay website.