Cool August temperatures lead to a shorter-than-average dead zone in 2025

It’s long been proven that weather, particularly rainfall, can heavily impact ecosystems. In 2018 and 2019, heavy rains brought higher-than-normal amounts of fresh water flowing into the Chesapeake, causing blue crab populations, who prefer salty water, to migrate to the southern Bay. Heavier river flows also wash increased amounts of pollution into waterways from the land, which can smother underwater grass species, cause oyster-killing diseases to rise, stimulate algal blooms and ultimately impact the size and duration of dead zones. 

Much of the Chesapeake Bay watershed experienced similar weather conditions in 2025–an extremely wet May, followed by high temperatures and a drier-than-usual summer, leading to drought warnings across the region. These conditions were to blame for above average hypoxia noted in the Chesapeake Bay in late June through early August during the summer of 2025. 

Dr. Marjy Friedrichs from the Virginia Institute of Marine Science (VIMS), noted that “July’s persistent hypoxia illustrates how sensitive the ecosystem is to subtle shifts in wind, temperature and river flow. These changes can have real implications for the distribution and behavior of fish, crabs and other species that rely on well-oxygenated habitat.”

Hypoxia refers to low-oxygen conditions occurring in a body of water. These circumstances are often unable to support marine life, which is why these regions are referred to as “dead zones.” Hypoxic conditions form when nitrogen, phosphorus and sediment enter waterways in greater amounts than the ecosystem can handle. It causes naturally-occurring algae to grow and bloom in large quantities and then die off. When these algae blooms decompose, they remove oxygen from the surrounding water faster than it can be replenished, causing marine life to seek new habitat or die off.

Each year, the Maryland Department of Natural Resources (DNR) and Old Dominion University conduct water quality monitoring cruises from May–October to measure the amount of hypoxia in the Bay. Results from these cruises can be found on Maryland DNR’s Eyes on the Bay  and Virginia’s Estuarine & Coastal Observing System (VECOS) websites, respectively. 

Additionally, VIMS and Anchor QEA use a sophisticated computer model, combined with local weather information and estimates of nitrogen and phosphorus entering the Bay from its surrounding watershed to  produce daily, real-time estimates of the size of the dead zone throughout the summer. These forecasts, along with other daily estimates of environmental conditions throughout the Bay, can be viewed on the Chesapeake Bay Environmental Forecast System.

In 2025, temperatures in mid-to-late August were cooler than normal, and when Hurricane Erin passed offshore of the Chesapeake Bay in August, hypoxia levels began to dissipate. By mid-September, VIMS notes that it had ended for the year, although Maryland DNR cautions that several stations in Maryland’s lower Bay were unable to be sampled in September.

The duration of hypoxia noted in the Chesapeake Bay in summer 2025 was lower than the long-term average (1985-2024) at only 60 days. The daily amount of hypoxia noted in the Bay, along with the total annual average amount, fell within the normal range and was close to the long-term average. Maryland DNR estimated that 2025 had the 11th largest observed seasonal volume of hypoxia over the past 41 years. 

Actions taken throughout the watershed also play a significant role in the size and duration of the annual dead zone. Best management practices installed by farmers, nonprofits, local governments and state agencies can help manage the amount of nitrogen, phosphorus and sediment pollutants that enter the Bay and cause hypoxic conditions. 

Want to make sure that a smaller dead zone is observed in 2026? Check out ways that you can help protect the Chesapeake Bay.