Rain of nutrients spells renewed trouble for Bay water quality

(July 30, 2003) During last year's drought, many scientists warned that a return to wetter conditions would offset some of the observed gains in Chesapeake Bay water quality, as renewed runoff flushed accumulated nutrients into Bay waters.

Now that the rains have returned , data collected by Virginia Institute of Marine Science (VIMS) researchers is helping to confirm the anticipated decline in water quality and highlight its effect on Bay organisms. Their data show that a 250-square-mile area of deep water in the upper Bay contains almost no oxygen or fish. Waters with normal oxygen levels produce up to 2,000 fish in a single 20-minute trawl sample. In contrast, the VIMS researchers collected less than one fish per tow sample from areas where the deep water had little or no oxygen. "We saw tow after tow return without any fish across the affected area," says ChesMMAP project leader Chris Bonzek.

ChesMMAP is the Chesapeake Bay Multispecies Monitoring and Assessment Program, a Bay-wide bimonthly survey funded by the NOAA Chesapeake Bay Office. This survey provides data to help managers as they move towards an ecosystem-based approach to fisheries management. "What's truly noteworthy is that these findings graphically illustrate that ecosystem conditions--like low dissolved oxygen--impact fish abundance and distribution," says Bob Wood of NOAA's Chesapeake Bay Office. "In other words, in addition to recreational fishing and commercial harvest, human activities on land affect fish as well."

"We found a huge pool of hypoxic water below about 20 feet, from just north of the Potomac River to north of Annapolis," says Bonzek. "These findings were startling, though not completely unexpected." Waters are termed hypoxic when their dissolved oxygen concentration falls below 2 milligrams per liter, generally the lowest level needed to support healthy bottom communities. Most organisms living above the bottom, such as fish, need 4 milligrams per liter or more.

Bonzek says that the low oxygen levels are likely associated with large nutrient inputs. Nutrients feed microscopic Bay plants, causing their numbers to increase dramatically. When these plants die, they sink to the bottom of the Bay floor and are decomposed by bacteria and other marine life. This process requires oxygen and therefore results in low oxygen in deeper areas of the Bay.

The excess nutrients were likely carried into the Bay by runoff from above average winter snowfall and spring rains. This runoff was able to pick up nutrients that had built up in surrounding soils during four consecutive years of dry weather. Many parts of the Bay watershed received record monthly rainfall totals between April and June. Virginia's spring rainfall set a statewide precipitation record.

Low-oxygen zones are becoming increasingly common in coastal and estuary areas around the world, largely due to increased inputs of nitrogen and phosphorous from human activity. Reducing nutrient pollution, specifically nitrogen and phosphorus, is one of the major goals of the Chesapeake Bay Program's restoration plan. In 2003, the six Bay watershed states and the District of Columbia agreed to reduce nutrient pollution to about half of 1985 levels. Nitrogen and phosphorous enter the Bay mainly from sewage treatment plants, power plants, vehicle exhaust, and runoff from agricultural, residential, and urban areas.