2025 Chesapeake Bay dead zone near long-term average

An algal bloom stretched from New Point Comfort (Matthews) north along the western shore of Chesapeake Bay. (Photo by Professor Wolfgang Vogelbein)

The following story originally appeared on the website for W&M's Batten School & VIMS. - Ed.

The Chesapeake Bay experienced hypoxia close to the long-term average in overall size and duration this year, according to the annual Chesapeake Bay Dead Zone Report Card produced by William & Mary's Batten School & VIMS in collaboration with Anchor QEA.

The analysis, based on daily outputs from the Chesapeake Bay Environmental Forecast System (CBEFS), shows that hypoxia generally remained within the normal range seen over the past four decades despite low-oxygen conditions being more persistent and higher than usual in July.

Similar to past years, the bay's dead zone expanded through June and peaked in July. However, it remained consistently high throughout July in comparison to past, more variable years. Elevated winds and cooler temperatures in early August helped reduce hypoxia in the bay before the passage of Hurricane Erin in mid-August triggered a rapid decline. Hypoxia lingered at low levels through September before dissipating as temperatures cooled in the fall.

"Even in what we classify as an average year, the bay can experience periods of prolonged stress," said Batten School of Coastal & Marine Sciences & VIMS Professor Marjorie Friedrichs. "July's persistent hypoxia illustrates how sensitive the ecosystem is to subtle shifts in wind, temperature and river flow. Those changes can have real implications for the distribution and behavior of fish, crabs and other species that rely on well-oxygenated habitat."

Each summer, dead zones, a term for water containing low levels of dissolved oxygen that can be harmful to many organisms, form when nutrient-driven algae blooms die and decompose, consuming oxygen that is required by marine life. The size and intensity of the bay's dead zone fluctuate each year depending on variables such as nutrient inputs, freshwater flow, wind and temperature.

The primary indicators used to assess the severity of the dead zone - the daily maximum hypoxic volume, the total annual hypoxic volume and the duration of hypoxia - pointed to an average year. The daily maximum reached 10.1 km³, close to the long-term median of 10.7 km³. The total hypoxic volume in the bay for 2025 was 462 km³ days, which is well within the historically normal range. Duration was somewhat shorter than average, with 60 days of sustained hypoxia compared to the average long-term span of 78-116 days.

Findings from the Batten School & VIMS forecast system are consistent with results from the Maryland Department of Natural Resources' analysis. Long-running water quality monitoring including biweekly research cruises confirmed elevated hypoxia throughout July, followed by a reduction after Hurricane Erin.

"This year's results show the power of combining high-resolution model results with long-standing monitoring programs," said Aaron Bever, senior managing scientist at Anchor QEA and coauthor of the report. "Persistent low-oxygen conditions in July followed by a rapid decline after a single storm event remind us that modeling tools like CBEFS are critical for parsing natural variability from long-term change."

Visit the Batten School & VIMS website to view the full report, learn more about dead zones and explore other CBEFS variables such as estimates of bay-wide salinity, temperature, sea nettles, harmful algal blooms and more.

John Wallace, W&M's Batten School & VIMS

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