How the Chesapeake Bay Dead Zone is looking in 2025

 

from VMIS

The “Dead Zone” of the Chesapeake Bay refers to a volume of hypoxic water that is characterized by dissolved oxygen concentrations less than 2 mg/L, which is too low for aquatic organisms such as fish and blue crabs to thrive. Within the hypoxic area life of the bay dies and a “Dead Zone” forms. The Chesapeake Bay experiences hypoxic conditions every year, with the severity varying from year to year, depending on nutrient and freshwater flows into the bay, wind, and temperature.

Due to logistical constraints, the yearly Chesapeake Bay Program partners’ seasonal hypoxia forecast was not calculated for 2025. Data collected by the Maryland Department of Natural Resources and Old Dominion University found an increasing volume of hypoxiain the Chesapeake Bay mainstem of Maryland and Virginia from early to late June. 

No hypoxia was observed in May, marking a later start to low dissolved oxygen conditions than in recent years. Hypoxia increased from below average levels in early June to above average in late June following heavy rains in May and hot temperatures during June. Historically, heavy rainfall early in the year can precede greater levels of hypoxia due to rain runoff carrying excess nutrients into the Bay, which can contribute to algal blooms and reduce water clarity. 

Dead Zone 2025 YTD from VIMS

Although different types of nutrients contribute to the annual dead zone, scientist say it is the amount of nitrogen that enters the Bay during spring that is a key driver in how hypoxic conditions can vary from year-to-year. During June freshwater flows into the Chesapeake Bay leading up to summer were approximately 20% above average carrying more nutrients to the bay. The above average freshwater  volumes in late June are probably the result of significant rainfall in Maryland and Pennsylvania during May. This combined with higher than average temperature in the region during June fueled algal blooms, their decomposition, and resultant oxygen consumption. Warmer waters also hold less oxygen

Each year the Maryland Department of Natural Resources measures the actual dissolved oxygen in the Maryland portion of the Chesapeake Bay main stem and the size of the Dead Zone. While the Virginia Institute of Marine Science (VIMS), Anchor QEA and collaborators at UMCES, operate a real-time three-dimensional hypoxia forecast model using measured inputs that predicts daily dissolved oxygen concentrations throughout the Bay (www.vims.edu/hypoxia) using the National Weather Service wind monitoring data.

The peak of oxygen depletion occurs in July or August when water temperatures are highest and the days are longest accelerating the growth of phytoplankton that ultimately consumes all the dissolved oxygen. The dead zone is typically gone by late fall. Cooler air temperatures at that time of year chill the surface waters, while the deeper water remains warm and allows more mixing of the layers during storms. Cooler water also will hold more oxygen. The size and shape of the dead zone is variable from month to month during the summer. The cooler front and big storms that rolled into part of the region seemed to be curtailing the growth of the dead zone.

Dead Zone 2024 from VIMS

At the end of the season the Virginia Institute of Marine Science (VIMS), Anchor QEA and collaborators at UMCES compile all the collected data to report the actual results. In the summer of 2024 the amount of hypoxia consistently increased to a moderate value over the month of May and was higher than in the past 4 years at the end of May. Modeled hypoxia then increased quickly in the beginning of June to a level not often seen in early June. Hypoxia remained relatively severe throughout June; both for the month of June and relative to the past 4 years. The CBEFS team is investigating what conditions may have caused the increase to a large amount of hypoxia in early June.