ANNAPOLIS, Md. –– Monitoring data from the Maryland Department of Natural Resources and Old Dominion University indicate above-average hypoxia volumes in the mainstem of the Chesapeake Bay during early July 2025, with levels nearly double the historical average, marking the sixth-highest early July volume in 40 years. Hypoxia, defined as waters with less than 2 milligrams per liter of dissolved oxygen, decreased slightly in late July but remained above average, while anoxia—waters with less than 0.2 milligrams per liter—was nearly double the average and limited to Maryland waters.
The early July cruise recorded a hypoxic volume of 2.29 cubic miles, compared to the 1985-2024 average of 1.59 cubic miles, ranking 35th out of 40 years analyzed, according to the Maryland Department of Natural Resources report. Anoxia volumes were also elevated during this period. By late July, hypoxia dropped to 1.74 cubic miles, closer to the average of 1.62 cubic miles and ranking 24th out of 40 years, but anoxia persisted at heightened levels exclusively in Maryland’s portion of the bay.
These conditions follow a pattern observed earlier in the season. In late June, hypoxia reached 1.76 cubic miles, above the average of 1.25 cubic miles and ranking 26th out of 35 years. Early June saw below-average hypoxia at 0.68 cubic miles, ranking 13th out of 37 years, while May recorded zero hypoxia, tying for the lowest in 41 years. Freshwater flows into the bay were about 20% above average leading into summer, contributing to the spike by carrying excess nutrients that fuel algal blooms and subsequent oxygen depletion.
Heavy rainfall in May played a key role, with Maryland receiving 7.81 inches, the highest in 131 years, and Pennsylvania logging 7.72 inches, the second-highest in that period, based on data from NOAA’s National Centers for Environmental Information. This was followed by elevated temperatures: June averaged 74.8 degrees in Maryland, the fourth-highest in 131 years, and July averaged 79.9 degrees, the second-highest. Warmer waters hold less oxygen, exacerbating hypoxia as decomposition of algal blooms consumes available dissolved oxygen.
Hypoxia poses risks to bay life, including crabs, fish and oysters, which require adequate dissolved oxygen to survive. In Southern Maryland, where counties like Calvert, Charles and St. Mary’s border the bay and rely on it for commercial fishing and recreation, these conditions can force species to shallower waters or lead to die-offs if prolonged. For instance, blue crabs may migrate to avoid low-oxygen zones, potentially affecting local harvests valued at millions annually in the region.
Historically, Chesapeake Bay hypoxia has been linked to nutrient pollution from agriculture, wastewater and urban runoff, with volumes expanding significantly from the 1950s to the 1980s as nutrient loads doubled. The dead zone, an area of hypoxia and anoxia, forms annually from excess nutrients stimulating algal growth, followed by bacterial decomposition that depletes oxygen. Climate factors like increased rainfall and warming temperatures intensify this, as seen in a nonlinear response where hypoxia volumes surged with rising nutrient inputs. In 2024, hypoxia was near average in size but shorter in duration, according to a Virginia Institute of Marine Science report, highlighting year-to-year variability driven by weather patterns.
Efforts to mitigate hypoxia focus on reducing nitrogen and phosphorus pollution through the Chesapeake Bay Program, a partnership involving Maryland, Virginia and other states. Initiatives target industrial sources, wastewater treatment upgrades, agricultural best practices like cover crops and urban stormwater management to curb runoff. These measures have shown progress; nitrogen reductions since the 1980s have decreased hypoxia duration and extent in some areas. In Maryland, the Department of Natural Resources oversees monitoring from May through October, computing volumes from water quality data collected with Virginia partners and funded by the Chesapeake Bay Program.
Monitoring cruises provide critical data for resource managers to assess impacts. In Southern Maryland, where the Patuxent and Potomac rivers feed into the bay, local hypoxia can affect oyster restoration projects and recreational fishing. A 2023 record low hypoxia volume of 0.58 cubic miles offered a benchmark for improvement, but 2025’s elevated levels underscore ongoing challenges amid wetter springs and hotter summers.
The Maryland Department of Natural Resources maintains detailed hypoxia data and calculation methods on its Eyes on the Bay website, including graphics for each cruise. Continued reporting through the summer will track whether hypoxia persists or declines as temperatures cool. As climate models predict more intense rainfall and warming, adaptive strategies like enhanced nutrient modeling aim to forecast and reduce future dead zones.
In Calvert County, for example, bay-dependent industries monitor these reports closely, as hypoxia can disrupt ecosystems supporting over 3,600 species. Statewide, the bay’s health influences economies in tourism and seafood, with Maryland’s portion generating billions in annual revenue. Resource managers emphasize that sustained pollution controls are essential to counteract weather-driven spikes in Chesapeake Bay hypoxia.
