The amount of grasses in the Chesapeake fell 8.4 percent last year, the second straight year of decline for what is considered to be one of the Bay's best indicators of water quality.
Lingering effects from the large, back-to-back freshets that hit the Bay in the springs of 1993 and 1994 are thought to be the most likely reason for the submerged aquatic vegetation's (SAV) decline.
"It's my gut feeling that those have made a difference in some of the [SAV] populations in the Bay," said Robert Orth, a scientist at the Virginia Institute of Marine Science, who conducts an annual Baywide survey of grass beds. "My feeling is they're feeling the effects."
The amount of grasses declined 5,500 acres last year, to 59,894 acres, according to Orth's survey, which is supported by the Bay Program. In 1994, the amount fell 10.5 percent, down 7,665 acres from the recent high mark of 73,081 acres in 1993.
The decline is not uniform. Some areas, such as the mid Bay, were down sharply. Grass-covered acreage in a region that extends from the Honga River and Barren Island south through Smith and Tangier islands was down 57 percent. Beds in the upper and middle Potomac River shrunk 15 percent, while those in the lower Bay declined 16 percent.
At the same time, there were increases in the Eastern Bay, and other places held steady.
Pinpointing the cause of decline is difficult, Orth said, because any individual grass bed may be affected by local conditions not detected in water quality monitoring data. Water quality is generally thought to be the most important element in determining grass abundance, but other factors - such as biological relationships - may play a role as well, Orth said. For example, he noted that declines in the mid Bay were primarily in beds of wigeon grass. In other areas, where eel grass was mixed with wigeon grass, the grasses showed little impact.
Underwater grasses provide important nursery areas, food and habitat for a wide range of Bay species, from blue crabs to fish to waterfowl. They also help improve water quality by absorbing nutrients, filtering sediment and pumping oxygen into the water. At one time, scientists believe the Bay had 600,000 acres or more of grass.
That amount has declined dramatically in recent decades as pollution overwhelmed the grass beds. Sediment from runoff, along with algae blooms spurred by excessive amounts of nutrients, cloud the water, blocking critical sunlight.
By 1984, the amount of grasses hit a low of 38,135 acres. Since then, they had been making a steady recovery until the back-to-back freshets carried high loads of nutrients and sediment into the Bay during the critical spring and early summer growing season.
Orth said the vast grass beds that once filled the Bay might have at one time been able to withstand such natural events. Such huge beds might have exerted enough control over the local water condition - filtering out sediment and other pollutants while pumping oxygen into the soil and water - to buffer themselves against dramatic impacts.
Today's grass beds, by contrast, are often smaller and may be more susceptible to adverse conditions, he said. "The beds have been completely decimated," he said. "Their ability to transform their environment has been taken away."
Orth said the recent setbacks are no reason to back off from the goal of restoring grass beds throughout the Bay. In 1993, the Bay Program set a restoration goal of 114,000 acres of Bay grasses - a goal officials had hoped could be achieved by the year 2005."We screwed the Bay up for centuries, and now we want things to turn around very quickly, and it's not," Orth said. "The Bay is like an engine that is not running very smoothly. It starts and it stops and it coughs a little bit, and improves for a little while and then gets sick. You've just got to keep on trying."