Chesapeake Bay Monitoring Shows Hypoxia Increased in June After Rainy May

Dissolved oxygen conditions show mixed results in May and June 2025

Data collected by the Maryland Department of Natural Resources and Old Dominion University show an increasing volume of hypoxia—waters with less than 2 mg/l of oxygen—in the Chesapeake Bay mainstem of Maryland and Virginia from early to late June. 

No hypoxia was observed in May, marking a later start to low dissolved oxygen conditions than in recent years. This year was one of nine years with no monitored hypoxia in May during the past 41 years. 

Hypoxia increased from below average levels in early June to above average in late June following heavy rains in May and hot temperatures during June. Historically, heavy rainfall early in the year can precede greater levels of hypoxia–areas of water with low dissolved oxygen levels–due to rain runoff carrying excess nutrients into the Bay, which can contribute to algal blooms and reduce water clarity. 

Late June had the largest historical volume of hypoxia seen within Maryland for the time period of the monitoring program. Both June monitoring cruises showed a greater than average volume of anoxia – waters with less than 0.2 mg/l oxygen – with late June reaching triple the normal average. 

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).

Qualitatively, freshwater flows into the Chesapeake Bay leading up to summer were approximately 20% above average, indicating a greater than average hypoxic volume may be observed over this season due to more nutrients entering the bay. Above average volumes in late June could be the result of significant rainfall in Maryland and Pennsylvania during May, and high average temperatures in the region during June fueling algal blooms, their decomposition, and resultant oxygen consumption. Warmer waters also hold less oxygen.

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–Maryland’s average temperature was 74.8 degrees, fourth-highest in 131 years, with more than 10 days in June reaching above 90 degrees.

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.

On or around July 6, lower Potomac watermen were reporting dead crabs in their pots in waters as shallow as 7 feet. An analysis by DNR in consultation with a Virginia Institute of Marine Science (VIMS) consultant found that sustained west/southwest winds around July 1 to July 4 likely pushed surface waters eastward, allowing low dissolved oxygen bottom water to come closer to the surface on the western shore. Southeasterly winds around July 6th could have helped to further push the low dissolved oxygen water towards Maryland’s lower Potomac shoreline. VIMS daily forecast models of dissolved oxygen and salinity also support this theory.

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.

Due to logistical constraints, the yearly Chesapeake Bay Program partners’ seasonal hypoxia forecast was not calculated for 2025. A hindcast is anticipated to be available at the end of the 2025 hypoxia season to validate and improve the forecasting model.

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 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.