Annapolis, Md., July 18, 2025 – Chesapeake Bay Hypoxia, characterized by waters with less than 2 mg/l of oxygen, increased significantly in the Chesapeake Bay mainstem from early to late June, according to data from the Maryland Department of Natural Resources (DNR) and Old Dominion University. This rise followed heavy May rainfall and high June temperatures, exacerbating low dissolved oxygen levels critical to aquatic life.
No Chesapeake Bay Hypoxia was recorded in May, a rare occurrence seen in only nine of the past 41 years, marking a delayed onset of low oxygen conditions. However, hypoxia surged from 0.68 cubic miles in early June (13th lowest of 37 years) to 1.76 cubic miles in late June (26th highest of 35 years), compared to historical averages of 0.85 and 1.25 cubic miles, respectively. Late June also saw triple the average anoxia volume—waters with less than 0.2 mg/l oxygen—making it Maryland’s highest for that period since monitoring began in 1985.
Heavy rainfall in May, with Maryland receiving 7.81 inches and Pennsylvania 7.72 inches, drove nutrient-rich runoff into the Bay, fueling algal blooms that deplete oxygen upon decomposition. June’s average temperature of 74.8 degrees in Maryland, the fourth-highest in 131 years, with over 10 days above 90 degrees, further reduced oxygen retention in warmer waters. Freshwater flows, 20% above average, contributed to elevated Chesapeake Bay Hypoxia levels by late June, as reported by NOAA’s National Centers for Environmental Information.
Hypoxia poses severe risks to crabs, fish, and oysters, which require adequate dissolved oxygen to survive. On July 6, lower Potomac watermen reported dead crabs in pots at depths as shallow as 7 feet. DNR, in consultation with the Virginia Institute of Marine Science (VIMS), attributed this to west/southwest winds from July 1 to 4 pushing low-oxygen bottom water toward the surface, worsened by southeasterly winds on July 6. VIMS daily forecast models of dissolved oxygen and salinity corroborated these findings.
Efforts to Mitigate Hypoxia Ongoing initiatives target nitrogen and phosphorus pollution from industrial, agricultural, and urban sources to curb Chesapeake Bay Hypoxia. These nutrients drive algal blooms, reducing oxygen as they decompose. The Chesapeake Bay Program, alongside Maryland and Virginia, funds monitoring from May to October. Due to logistical issues, no 2025 seasonal hypoxia forecast was issued, but a hindcast is expected post-season to refine models. Additional data is available at DNR’s Eyes on the Bay website.
DNR and the Virginia Department of Environmental Quality compute Chesapeake Bay Hypoxia volumes monthly, tracking impacts on aquatic ecosystems. Continued monitoring through October will assess whether hypoxia persists, with historical data suggesting it typically dissipates after September.
