Chesapeake Bay Monitoring Found High Volume of Hypoxia in Maryland Prior to September Nor’easter

Virginia’s post-storm readings show reduced levels of hypoxia 

Data collected by the Maryland Department of Natural Resources and Old Dominion University in September 2026 showed that the volume of hypoxia —waters with less than 2 mg/l of oxygen— was larger than average that month. Out of 42 sampling years, only two yielded higher hypoxic volume in September. 

Observed hypoxia was confined to Maryland, but the timing of sampling notably impacted the results. Maryland waters were sampled before a passing Nor’easter. Virginia waters were sampled afterwards when waters were mixed and temperatures decreased. 

September had 0.30 cubic miles of anoxia—waters with less than 0.2 mg/l of oxygen, which was slightly more than was observed in August, and three and a half times the September historical 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).

The better-than-average dissolved oxygen conditions early in this season are in part attributable to the recent and persistent drought conditions experienced in the watershed. This has led to lower water volumes flowing into the Bay, according to the U.S. Geological Survey, which means less nutrient pollution from phosphorus and nitrogen that fuels algal blooms that cause hypoxia in deeper Bay waters. An increase later in the season may be in part attributable to higher temperatures. The 2026 average Maryland air temperature for August through September was tied for the fifth highest on record. 

The Chesapeake Bay Program partners’ seasonal dissolved oxygen forecast states “from January through April 2026, the amount of water entering the Bay from rivers was 32% below the long-term average, while the amount of nitrogen was 39% lower than average, totaling about 59 million pounds of nitrogen, according to estimates from monitoring stations.” 

Monitoring results thus far align with the seasonal forecast, which predicted Bay hypoxia volumes will be about 31% below the long-term average. 

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 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.