The Chesapeake Bay’s “dead zones” — oxygen-depleted areas unsuitable for underwater life — were a little worse in 2021 due to warmer temperatures in late summer, environmental groups said Tuesday.
It comes a year after the bay’s dead zone was the second smallest since the 1980s. That was a significant step for aquatic life rebounding from record rainfall in 2019, which caused high amounts of nutrients from fertilizers and other pollutants to flow into the bay, stimulating the overgrowth of oxygen-sucking algae.
The story of this year’s dead zone is a “tale of two halves,” according to the Maryland Department of Natural Resources.
From May through July of 2021, the zone appeared smaller than usual, perhaps because there was less springtime rain, meaning that fewer nutrients and pollutants were flowing from rivers into the bay. But in August and September, calm winds and persistently warm temperatures allowed the dead zone to grow larger and linger longer, according to DNR.
Warmer water holds less dissolved oxygen, and, all the while, estuarine animals require more oxygen to stay alive. Average temperatures in Maryland in August and September were the fifth hottest since 1901 this year, according to the National Centers for Environmental Information.
As a result, this year’s dead zone lasted longer than 89% of the dead zones recorded over the past 36 years, according to the Chesapeake Bay Program. In October, the bay had the third-highest volume of hypoxic waters — areas with less than 2 milligrams per liter of dissolved oxygen — of any October since 1985, according to DNR.
DNR calculates the dead zone volume during its monthly monitoring cruises alongside researchers from Old Dominion University. The trips are more frequent during the summertime because of the increased prevalence of hypoxic areas.
Over the full summer season, the volume of hypoxic waters was slightly below the historical average, according to DNR. Simulations done by the Virginia Institute of Marine Science found that the dead zone was slightly more extensive than average, largely because of its duration.