A crab boat, Half Bushel, makes its way out of Fishing Creek to work on the Chesapeake Bay in June. The weirdly mild winter, the dry and toasty spring, and the hottest summer heat wave on record apparently had at least one upside: a cleaner Chesapeake Bay. (Ray K. Saunders/WASHINGTON POST)

The weirdly mild winter, the dry and toasty spring, and the hottest summer heat wave on record apparently had at least one upside: a cleaner Chesapeake Bay.

Last year around this time, the bay was smothered by one of its largest dead zones — low-oxygen water caused by pollution where fish and plants cannot survive. This year, with so little rain to move pollution from farms and city streets into waterways, the zone “absolutely is much smaller,” said Bruce Michael, director of the resource assessment service for the Maryland Department of Natural Resources.

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.

This year’s dryness in a warming climate has complemented a joint federal and state effort to clean the Chesapeake, the nation’s largest estuary, biologists said. The 200-mile-long bay supports more than 3,500 plant and animal species. Many of the watershed’s fisheries play an important role in the Atlantic Ocean’s food chain.

A study of the bay’s water quality last year showed that a major pollution-reduction effort is improving its health. Johns Hopkins University and the University of Maryland Center for Environmental Science found that the size of dead zones have generally fallen since pollution limits were implemented in the 1980s.

Researchers studied water-quality data for the bay for 50 years starting in 1949 and said the findings are evidence that aggressive efforts by the Environmental Protection Agency and states in the watershed are making a difference.

As part of a cleanup that costs bay states hundreds of millions of dollars to upgrade waste-
water-treatment plants and better regulate building projects, among other things, the EPA hopes by 2025 to reduce the daily flow of nitrogen into the bay by 25 percent, to 186 million pounds; of phosphorus by 24 percent, to 12.5 million pounds; and of sediment by 20 percent, to 6.45 billion pounds.

“The rate of the decline tracks with the rate of reduction of nitrogen and other pollution coming into the bay,” said Donald Boesch, president of the U-Md. Center for Environmental Science and an expert on dead zones.

“The bay is responding and should respond the way our models tell us,” he said, adding that results take time. “We have to recognize that there is lag between what we do on land and what happens in the bay.”

House Republicans have proposed rolling back the EPA’s plan. Interest groups such as the American Farm Bureau Federation have said the cleanup plan is too aggressive, forcing farmers to pay for pollution-reduction measures that cut into their meager income.

The EPA plan requires farmers and builders of housing subdivisions to pay for costly conservation measures to curb nutrient and sediment pollution that has gone virtually unchecked for decades, environmentalists say. The Home Builders Association joined the farm federation in challenging the EPA’s authority to implement its “pollution diet.” The federal lawsuit is pending.

And because nature’s course is unpredictable, the dry summer’s helping hand in decreasing pollution could easily become a fist that worsens other problems.

A lack of rain and freshwater often lead to an increase in the bay’s salinity level. Two diseases that plague oysters, Dermo and MSX, thrive in saline. Biologists call the diseases a nightmare because they kill oysters by the bushel just as they become adults.

“It’s a double-edged sword,” Boesch said of the climate. So many thousands of oysters have died from the diseases and pollution that Maryland and Virginia created huge sanctuaries against fishing to protect them from being wiped out.

But Michael said he is not worried — yet.

“We’re certainly in the range we’ve had historically with salinity,” Michael said. But the peak usually comes in August and September. “Although we’re seeing higher levels, it’s certainly not unprecedented. In drought conditions, it can get really high, so we’ll wait and see.”