Data collected by the Maryland Department of Natural Resources (DNR) and Old Dominion University show that dissolved oxygen conditions in the Chesapeake Bay mainstem of Maryland and Virginia worsened in August following two better-than average months. The hypoxic water volume — areas with less than 2 mg/l oxygen — was 1.88 and 1.61 cubic miles during early and late August monitoring cruises, compared to historical early and late August averages (1985-2020) of 1.35 and 1.06 cubic miles.
Additionally, in early and late August, within the mainstem Bay’s hypoxic waters, 0.76 and 0.51 cubic miles were anoxic — zones with less than 0.2 mg/l oxygen.
According to the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information, this August was the third-warmest on record for Maryland, with lower-than-average wind speeds and greater-than-average precipitation. These factors likely contributed to worsening hypoxia as warmer waters hold less oxygen, less wind results in decreased mixing of oxygen into deeper waters, and higher freshwater flows into the Bay increase the stratification of the water column. This stratification, where less dense freshwater sits on top of denser saltier bottom waters, also inhibits mixing of oxygen into deeper water.
Each year from June through September, DNR estimates hypoxic volumes in the bay using monitoring data collected by the State of Maryland and Virginia’s Department of Environmental Quality. Data collection is funded by these states and the U.S. Environmental Protection Agency’s Chesapeake Bay Program.