While last year’s “dead zone” in the Chesapeake Bay was the second smallest observed since 1985, this year’s assessment paints a more complex picture of the Bay’s health, according to data reported Nov. 30 by the Chesapeake Bay Program.
Throughout the year, researchers track Bay oxygen conditions using a variety of methods. Oxygen and nutrient levels are measured as part of the Chesapeake Bay Monitoring Program, a Bay-wide cooperative effort involving watershed jurisdictions, several federal agencies, 10 academic institutions and over 30 scientists.
Among these institutions, the Maryland Department of Natural Resources, Virginia Department of Environmental Quality and Old Dominion University conduct nine cruises between May and October to track summer hypoxia in the Bay. Scientists at the Virginia Institute of Marine Science (VIMS), in collaboration with Anchor QEA, use computer models combined with local weather information, to produce daily real-time estimates of dead-zone size throughout the summer.
Based on the nine monitoring cruises conducted in 2021, the Maryland Department of Natural Resources assessed that the dead zone was only slightly below average. On the other hand, the model simulations provided by the VIMS and Anchor QEA indicated that the dead zone was slightly more extensive than average, largely due to the duration of the dead zone in 2021. However, both estimates suggest a near-average-sized dead zone for this year.
“Monitoring of hypoxia and other Chesapeake Bay water-quality and habitat conditions provide valuable data to assess and improve the health of the Bay,” said Mark Trice, chief of the Water Quality Informatics Branch at Maryland’s Department of Natural Resources. “Maryland and its Chesapeake Bay Program partners are working towards achieving nutrient and sediment pollution reduction goals that will provide improved habitat for iconic species such as crabs, oysters and rockfish, while fostering a robust economy that those Bay resources support.”
This year’s assessment confirms the earlier forecast of a slightly smaller than average dead zone, due to reduced spring rainfall and less nutrient-rich runoff flowing into the Chesapeake from the watershed. Since 2007, a model developed by the University of Michigan has been used to forecast the volume of summer hypoxia for the mainstem of the Chesapeake based on the amount of nitrogen pollution flowing into the Bay from the Susquehanna River from the previous January-May. The model is informed by data provided by a variety of sources.
“The general agreement between the multiple methods used to assess the size of this summer’s dead zone is reassuring”, said Dr. Marjy Friedrichs, research professor at the VIMS. “The relatively average dead-zone size, despite warming summer temperatures, is a testament to the success of management actions that have reduced nutrients entering the Bay.”
Weather conditions play a large role in the size and duration of the annual dead zone. Below average river flows, due to less springtime rainfall bringing nutrients and sediment into the Bay, is believed to have played a role in the smaller than average dead zone noted during monitoring cruises conducted from May to July. However, due to calm winds, increased precipitation and warm temperatures throughout the late summer of 2021, conditions were perfect for the dead zone to grow larger than it had during this time than in previous years.
For example, temperatures in August and September were recorded as the fifth hottest on record for those months in Maryland, according to the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information. Despite the short-lived high winds and cool temperatures associated with the passage of Hurricane Ida remnants, the September dead zone remained above average in size throughout the month and well into October.
Overall, the 2021 dead zone lasted longer than 89 percent of those recorded over the past 36 years.
The primary way in which nutrients can enter the Bay is through its tributaries in the watershed. Higher river flows bring increased amounts of nutrient pollution into the Bay. Despite lower flows in spring 2021, the U.S. Geological Survey reported that the average for the water year (measured from Oct. 1, 2020 to Sept. 30, 2021) was higher than normal, with flows entering the Bay at an average of 84,880 cubic feet per second, which is above the long-term average of 79,000 cubic feet per second.
The dead zone is an area of little to no oxygen that forms in deep Bay waters when excess nutrients, including both nitrogen and phosphorus, enter the water through polluted runoff and feed naturally-occurring algae. This drives the growth of algae blooms, which eventually die and decompose, removing oxygen from the surrounding waters faster than it can be replenished.
This creates low-oxygen –or “hypoxic” – conditions at the bottom of the Bay. Plant and animal life are often unable to survive in this environment.