Dead zones have their genesis in heavy spring rains that wash pollution from lawn fertilizer, livestock manure, urban street garbage and other sources into rivers that flow into the bay.
The zones start to form in early summer and grow in the middle to late summer as nutrient pollutants such as nitrogen and phosphorus feed huge algae blooms that die and decompose. As summer progresses, microbes feed on the dead algae and use up the dissolved oxygen in the water.
The bay experienced an unusually large dead zone this past summer because of heavy flows of polluted water from the Susquehanna River earlier in the year. The bay was further fouled in September by sediment pollution whipped up by rains from Tropical Storm Lee. And heavy rains from Lee and Hurricane Irene caused major sewage overflows into several creeks and rivers that empty into the bay.
Estuaries are bodies of water where salt and fresh water mix. The 200-mile-long bay supports more than 3,500 plant and animal species. Bivalves, such as oysters, cannot survive in a dead zone’s depleted oxygen. Fish can swim to safer waters, but many die nevertheless.
Researchers studied water-quality data for the Chesapeake from 1949 to 2009 for the study, released Thursday and published in the November issue of the journal Estuaries and Coasts.
“We now have evidence that cutting back on the nutrient pollutants pouring into the bay can make a difference,” said Rebecca R. Murphy, a doctoral student in the Department of Geography and Environmental Engineering at Johns Hopkins and the study’s lead author.
The study was praised by environmental groups such as the Chesapeake Bay Foundation. The foundation said the study demonstrates that early pollution-reduction measures and an aggressive pollution diet imposed by the Environmental Protection Agency this year have yielded positive results.
Republicans in Congress and interest groups such as the American Farm Bureau Federation and the Home Builders Association have criticized the EPA’s effort as overly aggressive.
By curbing the influx of pollutants from the bay’s 64,000-square-mile watershed, the EPA hopes 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. House legislation has sought to roll back the agency’s plan.
Murphy said in an interview that the improvement was “very slight” and that although the results are encouraging, “with further nutrient reduction we should see even more improvements.”
The EPA’s pollution diet requires the District, Virginia, Maryland, Pennsylvania, West Virginia, Delaware and New York to implement billion-dollar upgrades to sewage-treatment facilities, with much of the cost passed on to ratepayers. The states are also required to implement expensive conservation measures on farmland, with some of the cost to be borne by farmers.
“This study shows that our regional efforts to limit nutrient pollution may be producing results,” said Don Boesch, president of U-Md.’s Center for Environmental Science. “Continuing nutrient reduction remains critically important for achieving bay restoration goals.”