Dead zones commonly happen in summer after winter snowmelt and heavy spring rains wash sediment and nitrogen and phosphorus from farm animal waste, fertilizers and urban street garbage into the bay’s tributaries, such as the Susquehanna and Potomac rivers. Six states — Maryland, Virginia, West Virginia, Delaware, Pennsylvania and New York, as well as the District — sit in the bay watershed.
Nutrient pollution feeds algae blooms that grow and die quickly, decomposing into a thick black goo. Microbes eat decomposed algae during summer and suck out oxygen in the process. Dead zones usually dissipate in September.
The benefits of the year’s sizzling heat pretty much end at the bay, the beach and your community swimming pool. Farmers are worried that record-high temperatures and dry conditions will ruin corn crops well before the harvest. Elderly residents on a limited budget struggle to stave off deadly heat with open windows and electric fans. In the west, dry trees and brush are kindling waiting for a lightning strike or a match.
But rain is a Jekyll-and-Hyde element for the Chesapeake Bay, adding huge amounts of fresh water but also motor oil that drips from cars onto streets, lawn fertilizer spread liberally by homeowners, and animal manure from household pets and livestock such as chickens and cows in one of the most populous watersheds in the nation.
At the end of June last year, a dead zone stretched about 83 miles from the Baltimore Harbor to the mid-channel region in the Potomac River, according to the DNR and the Virginia Institute of Marine Science.
In July, it crept beyond the Potomac into Virginia, threatening to become the worst dead zone ever.
“It potentially would’ve been the worst if Hurricane Irene had not come in and redistributed the water, mixing it up so you didn’t have stratification with good water on the top and everything below nine meters is devoid of oxygen,” Michael said.
Hurricane Irene struck last year in late August, followed by Tropical Storm Lee in early September.
But state monitoring that started last month amid record-breaking heat and abnormally dry conditions showed a dramatic improvement from this time last year — very few low-oxygen areas in the bay’s main stem so far, “better than average,” Michael said. About 12 percent of water volume in Maryland’s portion of the bay was low in oxygen in June, compared with more than 30 percent last year.
“We were kind of predicting this, because we had relatively low flows in winter and spring . . . not much runoff at all,” Michael said.