GLOUCESTER POINT, Va. — The Chesapeake Bay has the third largest “dead zone” or oxygen-starved area of water in the United States, typically spreading across 1,500 square kilometers of water, said Robert Diaz, a top scientist at the Virginia Institute of Marine Science.
In a very bad year, that number can shoot up to 2,000 square kilometers, or more than one-tenth of the entire estuary. Only the Gulf of Mexico and Lake Erie have larger dead zones.
Early predictions for this year by researchers at the University of Maryland Center for Environmental Science suggest the Chesapeake’s water quality could be better this summer. The UMCES scientists this week said they were forecasting one of the smaller dead zones in Chesapeake for the last 25 years.
They credited positive climate conditions as well as effective agricultural policies, but cautioned that their predictions could change as the summer progresses. Last year, the bay’s dead zone turned out to be bigger than forecast.
Diaz, in an interview Thursday, said he was waiting to see predictions from Donald Scavia, a researcher formerly of NOAA and now with the University of Michigan.
Predicting and measuring the Chesapeake’s dead zone has become an annual ritual because oxygen levels in its waters are considered a critical factor in analyzing the health of the estuary’s ecosystem.
Dead zones are caused by an overly heavy influx of nutrients like nitrogen and phosphorous from farm run-off, storm run-off and sediment flushing into the Bay from its river tributaries. The nutrients feed algae, which proliferate, die and then suck oxygen out of the water. Sediment further clogs the system.
Typically the Chesapeake’s dead zone covers roughly 15 to 20 percent of the bay, Diaz said. In a bad year, it can reach close to 2,000 square kilometers, and in a good year can shrink to 800-900 square kilometers.
The culprit is a “combination of land modification, agriculture, expanded population,” Diaz said. “Until we bring these runoff nutrients under control, nothing will happen.” The result of a dead zone is typically a muddy bottom where little beyond microbes and bacteria survive. Fish and seafood flee the area and congregate in the shallows.
Reducing agricultural nutrient run-off is a central element of eliminating the dead zone. “These things can be reversed with proper management,” Diaz said.
The public has largely been unaware of the Chesapeake’s dead zones simply because they are not visible. “You could be boating over a dead zone and not even know it, because it’s a phenomena that affects bottom waters, and not surface water,” Diaz explained.
But underneath the surface, fish, crabs and shrimp are evacuating large areas where they can no longer breathe and escaping to shallower waters. Sometimes the dead zones will shift to these waters as well, forcing the panicking fish to physically jump out of the waters – and sometimes into fishermens’ nets, an event ironically known as a “jubilee.”
–by Sharon Behn