Dead Zone Decline

Every summer, death stalks the Cheseapeake Bay, a world-renown estuary near Washington, D.C. known for it blue crabs and biological wealth. Fed by pollutants, blooms of plankton help create oxygen-starved “dead zones” that spread across a huge chunk of the 200-mile long bay. A new analysis of 60 years worth of water quality data, however, suggests that efforts to control the nutrients are leading to smaller and shorter-lived dead zones.

“I was really excited by these results because they point to improvement in the health of the Chesapeake Bay,” said Rebecca R. Murphy, a doctoral student at Johns Hopkins University in Baltimore, Maryland and a lead author of the study, published in Estuaries and Coasts. “We now have evidence that cutting back on the nutrient pollutants pouring into the bay can make a difference. I think that’s really significant.”

The bay’s health has deteriorated during much of the 20th century, the authors note, contributing to a drop in the Chesapeake’s fish and shellfish populations. One big reason for the decline: The surge of nutrients that enters the bay each spring, from sources such as farm fertilizer, animal waste, water treatment discharge and atmospheric deposition. Heavy spring rains typically flush these chemicals, primarily nitrogen and phosphorus, into the Susquehanna River and other waterways that empty into the Chesapeake. There, the nutrients promote the prolific growth of algae. When the algae die, their remains sink to the bottom, where they are consumed by bacteria. The bacteria consume available oxygen, leading to hypoxia, or a depletion of oxygen. The resulting dead zones can kill anything that can’t swim away, and sometimes produces fish kills too.

To see if the bay’s dead zones were expanding or contracting, the researchers retrieved and analyzed water quality records from the past 60 years. They determined that the size of the dead zone in mid-to-late summer has decreased steadily since the late 1980s and that the duration — how long the dead zone persists each summer — is closely linked each year to the amount of nutrients entering the bay.

That timeline coincides with the launch of state and federal efforts to reduce the flow of nutrients into the bay. Water treatment plants, for example, began to remove more pollutants from their effluent, and air pollution control measures curbed the movement of nitrogen from the atmosphere into the bay. Farmers also were encouraged to plant natural barriers and take other steps to keep fertilizer out of waterways.

Another part of the study looked at a trend that has troubled some bay watchers. In recent years, Chesapeake researchers have seen an early summer spike in dead zones. They feared that keeping more nutrients out of the bay was not improving its health. But the new study found that the early summer jump in dead zones was influenced more by climate forces, not by polluted runoff. In a phenomenon called stratification, fresh water from the rivers entering the bay forms a layer on top of the more dense salt water, which comes from the ocean. The two layers don’t easily mix, so when air near the surface adds oxygen to the top layer, it doesn’t reach the deeper salt water. Without oxygen at these lower depths, marine animals cannot live, and a dead zone is formed.

“Rebecca discovered that the increase in these early summer dead zones is because of changes in climate forces like wind, sea levels and the salinity of the water,” said William P. Ball of Johns Hopkins, a co-author. “We believe that without those efforts to rein in the pollutants, the dead zone conditions in June and early July would have been even worse.”

“Regional efforts to limit nutrient pollution may be producing results,” says Don Boesch, president of the University of Maryland Center for Environmental Science. But continuing nutrient reduction, he says, “remains critically important for achieving bay restoration goals.” David Malakoff | November 7, 2011

Source: Murphy, R., Kemp, W., & Ball, W. (2011). Long-Term Trends in Chesapeake Bay Seasonal Hypoxia, Stratification, and Nutrient Loading. Estuaries and Coasts, 34 (6), 1293-1309 DOI: 10.1007/s12237-011-9413-7

Image © Feng Yu |