The Chesapeake Bay’s dead zone measured near average in size this past summer, coming close to scientists’ June prediction of a smaller than average hypoxic zone in the nation’s largest estuary.
Dead zones, or areas of little to no dissolved oxygen, form when nutrient-fueled algae blooms die. The bacteria that aid in algae bloom decomposition suck up oxygen from the surrounding waters. The resulting hypoxic or anoxic conditions can suffocate marine life, shrinking the habitat available for fish, crabs and other critters.
Each summer, the Maryland Department of Natural Resources (DNR) and the Virginia Department of Environmental Quality (DEQ) collect water samples to measure the hypoxic volume of the Bay. Since 1983, this number has ranged from 15.3 to 33.1 percent. In 2013, it measured 22.1 percent: 5.6 percent higher than the previous year and just above the 21.9 percent average.
Dead zones are impacting the distribution and abundance of fish that live and feed near the bottom of the Chesapeake Bay, according to new research from the Virginia Institute of Marine Science (VIMS).
Dead zones, or areas of little to no dissolved oxygen, form when nutrient-fed algae blooms die and decompose, and are most pronounced in the deep waters of the Bay’s mainstem during warm summer months. During a decade-long study of the bottom-feeding fish that inhabit this portion of the Bay’s water column, scientists noticed drastic declines in species richness, diversity and catch rate as dead zones restricted habitat and displaced the fish toward more hospitable waters.
So-called “demersal” fish—which include Atlantic croaker, white perch, spot, striped bass and summer flounder—avoid dead zones because a lack of oxygen can place stress on their respiratory and metabolic systems. While the fish often return to their former habitat when oxygen levels improve, dead zones can also wreak havoc on their forage grounds, stressing or killing the bottom-dwelling invertebrates the fish need for food.
“Once oxygen levels go up, we do see the average catch rate go up,” said Andre Buchheister, Ph.D. student and author of the VIMS study. “That’s a good sign. It indicates that once those waters are re-oxygenated, it’s possible for fish to move back in. But the availability of food is compromised, and studies have shown that the productivity of benthic biomass—or the critters that live in and on the bottom of the Bay—is stressed.”
The impact that demersal fish displacement could have on Bay fisheries is unclear, Buchheister said. Commercial fishermen who work outside of the mainstem might not be affected. But recreational anglers searching for striped bass could struggle if their forced move out of cool, deep waters is shown to contribute to poor health among the population.
In June, a forecast from researchers at the University of Maryland Center for Environmental Science (UMCES) and the University of Michigan predicted a smaller than average dead zone for the coming summer, thanks to lower than average nutrient loads that entered the Bay last spring. But to return the Bay’s mainstem to its former health, “one or two good summers won’t make that much of a difference,” said Buchheister. Instead, benthic habitat must be rebuilt, as long-term improvements boost Bay health from the bottom up.
Images courtesy Virginia Institute of Marine Science (VIMS)
Dead zones, or areas of little to no dissolved oxygen, form when nutrient-fueled algae blooms die. As bacteria help these blooms decompose, they suck up oxygen from the surrounding waters. The resulting hypoxic or anoxic conditions can suffocate marine life.
The Chesapeake Bay Program tracks dissolved oxygen as an indicator of water quality and Bay health.
The latest NOAA-funded forecast from researchers at the University of Maryland Center for Environmental Science (UMCES) and the University of Michigan predicts an average summer hypoxic zone of 1.108 cubic miles, lower than last year’s mid-summer hypoxic zone of 1.45 cubic miles.
This predicted improvement should result from the lower than average nutrient loads that entered the Bay this spring. According to the U.S. Geological Survey (USGS), 36,600 metric tons of nutrients entered the estuary from the Potomac and Susquehanna rivers, which is 30 percent lower than average.
The Bay’s dead zones are measured at regular intervals each year by the Maryland Department of Natural Resources (DNR) and the Virginia Department of Environmental Quality. While the final dead zone measurement will not take place until October, DNR biologists measured better than average dissolved oxygen on its June monitoring cruise, confirming the dead zone forecast.