Dissolved oxygen conditions increased in early July but decreased later in the month
Annapolis, MD - Data collected by the Maryland Department of Natural Resources and Old Dominion University show 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 and late July.
Early July saw an increase in hypoxia from late June, with volumes almost double the historical average, and the sixth-highest early July volume in the past 40 years. Anoxia – waters with less than 0.2 mg/l oxygen – was also above average.
Hypoxia volumes decreased in late July and were closer to average, but anoxia was near double the historical average and confined solely to Maryland waters.
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. Warmer temperatures can also contribute to hypoxia as warmer waters hold less oxygen.
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.
2025 Cruise | Hypoxic Volume | Average Volume (1985-2024) | 2025 Rank | Graphic
May | 0.0 | 0.19 | First (tied) / 41 | Chart or Map
Early June | 0.68 | 0.85 | 13th / 37 | Chart or Map
Late June | 1.76 | 1.25 | 26th / 35 | Chart or Map
Early July | 2.29 | 1.59 | 35th / 40 | Chart or Map
Late July | 1.74 | 1.62 | 24th / 40 | Chart or Map
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 over 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.
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.
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.
