ANNAPOLIS — The Chesapeake Bay’s summer of unusually healthy oxygen levels ended on a sour note, with September monitoring finding one of the largest volumes of oxygen-starved water on record, all of it in Maryland’s portion of the Bay.
The Maryland Department of Natural Resources reported Oct. 19 that the Bay’s mainstem held 1.02 cubic miles of hypoxic water in September, nearly double the long-term September average of 0.53 cubic miles. Only two of 42 years on record had a larger September volume, according to the DNR report.
Hypoxic water has less than 2 milligrams of dissolved oxygen per liter, too little for crabs, fish and oysters to thrive. Anoxic water, with almost no oxygen at all, also ran high: 0.30 cubic miles in September, slightly more than in August and about three and a half times the September average.
For Southern Maryland, that matters because the Maryland mainstem runs along the Calvert and St. Mary’s shorelines, where the deep-water dead zone is a key factor in the health of crabs, oysters and the fish that local watermen and anglers depend on.
A storm split the reading
Timing shaped the September numbers. Maryland’s waters were sampled before a nor’easter passed through, while Virginia’s were sampled afterward, when the storm had mixed the water column and cooled temperatures, DNR said. As a result, the hypoxia recorded in September was confined to Maryland waters.
DNR said the late-season rise may partly reflect heat. Maryland’s average air temperature for August and September tied for the fifth-highest on record. Warm water holds less oxygen and can fuel the algae whose decay drains oxygen from deeper water.
From record-low to near record-high
September’s reading is a sharp turn from the rest of the season. The year began with no measurable hypoxia in May and the smallest early-June volume on record, results the Chronicle covered as they came in, from below-average readings in early summer to a July dead zone well below average and conditions that stayed below average through most of August.
Cruise 2026 hypoxic volume (cubic miles) 1985-2025 average 2026 rank (1 = smallest) May 0.0 0.18 1st (tied) of 42 Early June 0.076 0.85 1st of 38 Late June 0.97 1.27 14th of 36 Early July 1.48 1.61 21st of 41 Late July 1.08 1.60 8th of 41 Early August 0.67 1.30 5th of 39 Late August 1.12 1.06 28th of 42 September 1.02 0.53 40th of 42
Late August was the first cruise of the year to come in above average, and September followed with a much larger jump.
The strong start had a clear cause. Persistent drought in the watershed cut the flow of rivers into the Bay, according to the U.S. Geological Survey, and less water meant less nitrogen and phosphorus washing in to feed algae blooms. The Chesapeake Bay Program’s spring forecast, which predicted a smaller-than-average dead zone, was based on January-to-April river flow 32% below average and nitrogen loads 39% lower, about 59 million pounds. That forecast called for a season about 31% below the long-term average, and DNR said monitoring so far is consistent with it.
What comes next
DNR calculates hypoxic volumes from May through October using data collected by DNR and the Virginia Department of Environmental Quality, so one more monthly reading remains before the season’s final tally. Results and maps for each cruise are posted on DNR’s Eyes on the Bay site.
Over the long term, the state and its Bay partners are relying on cutting nitrogen and phosphorus from wastewater plants, farms, industry and urban runoff to shrink the dead zone. A late-season spike driven partly by heat is a reminder that rising temperatures can work against those gains even in a dry year.
