Bay grasses decline 10%
Tangier area hit worst
The Chesapeake’s underwater meadows, often cited as the best indicator of how the Bay is doing, took a beating last year.
The overall acreage of Bay grasses declined by 10 percent — to 63,597 acres — in 1998, according to an annual aerial survey by the Virginia Institute of Marine Science. Although grasses gained ground in some areas, overall coverage was set back to 1991 levels.
The lack of a sustained recovery for grass beds is a blow to Bay Program hopes of achieving 114,000 acres of “submerged aquatic vegetation” by 2005. And some say it is a sign that, if grass beds are to be restored, cleanup efforts may have to be stepped up.
“These recent grasses declines are a clear wake-up call,” said Rich Batiuk, associate director for science with the EPA’s Bay Program Office.
Particularly troubling in last year’s data was the continued dramatic decline of grass beds in the Tangier Sound region, one of the Bay’s most important areas for blue crabs. Since 1992, beds in that area have declined by 63 percent.
Record-high river flows into the Bay during the first six months of last year got some of the blame for the overall decline because high flows typically wash large amounts of water-fouling nutrients and sediments into the Bay.
The Bay has suffered a series of high flows which are partially responsible for reversing what had been an increasing trend in Bay grasses during the late 1980s and early 1990s.
But if that is hindering grass recovery, it may mean more actions are needed to help offset high-flow impacts, especially as some predict a long-term trend toward heavier storms and greater precipitation in the region.
“People say it’s the storms, but if the storms become the norm, then we’ve got to work within that,” Batiuk said. “We’ve got to focus on practices that will withstand those storms.”
And while greater precipitation may be part of what is happening, scientists say it doesn’t explain everything: the Tangier losses have continued through both high- and low-flows years. And even last year, grasses increased — sometimes dramatically — in a number of areas such as the Western Shore of the upper Bay and much of the Potomac.
“The Chesapeake Bay is really too big to make any kind of sweeping statement about it,” said Peter Bergstrom, a U.S. Fish & Wildlife Service biologist who chairs the Bay Program’s Submerged Aquatic Vegetation Workgroup. “It’s so big and complex that we shouldn’t be surprised if it doesn’t all behave in the same way.”
Concern over recent losses may fuel efforts to incorporate goals for grass recovery in a new Bay Agreement, which is to be signed next year.
In particular, some advocate that a light, or water clarity goal be established for the Bay which — if achieved — should allow grasses to grow in areas where sustained comebacks have been elusive.
“That certainly would be something that needs to be considered,” said Bob Orth, a seagrass expert with the Virginia Institute of Marine Science who conducts the annual Baywide survey. “Light is the ultimate factor that is going to influence growth.”
Right now, the Bay Program deals with light indirectly through its nutrient reduction goal — less nutrients mean less sun-blocking algae. But the existing nutrient reduction goals may not be sufficient for grasses in some places. In addition, other factors — such as sediment — can also cloud the water but are not directly addressed in current cleanup strategies.
The Bay Program is completing a new study that establishes the amount of sunlight different plants need in different parts of the Bay, and the study could be used to set nutrient and sediment reductions for specific sites. “We’ve got the minimal light requirements pretty much nailed down,” Batiuk said.
Historically, thick grass beds are estimated to have covered 600,000 acres of the Bay, growing at depths of more than 6 feet. But water clouded with algae and sediment have shut off sunlight to the plants; today they are rarely found at depths of more than 3 feet, and they cover only about one-tenth of their historic area.
The loss of grasses also removes a critical component of the Bay ecosystem. Grass beds provide food for waterfowl, and habitat for juvenile fish, clams and crabs. Young blue crabs seek shelter from predators in grass beds and are found there in greater densities — sometimes 30 times greater than unvegetated bottom — than anyplace else.
Orth said the beds lost in the lower Bay in recent years would have supported 1.7 billion juvenile blue crabs. “These are the numbers of crabs that one would expect to see in the amount of habitat that is no longer there because the grass is gone,” he said.
Because of that habitat value, losses in the Tangier Sound area are of particular concern. Grasses there have declined from a recent high of 18,113 acres in 1992 to 6,612 acres in 1998. But the exact cause of the Tangier problem remains especially murky.
Islands are rapidly eroding in the area, releasing plumes of water-clouding sediment. Computer modeling indicates the Tangier area tends to accumulate material that flows into the Bay from other places, such as the Susquehanna and the Potomac. It’s also immediately downstream of Eastern Shore rivers, which have had increasing trends in nutrient runoff in recent years. The activity of cownose rays, which dig up grasses as they search for food, has also increased.
“We almost have too many hypotheses,” Bergstrom said. “The two things that everybody seems to agree on is that the grasses are going away, and the water quality seems to be getting worse, and we’re faily certain there is a connection.”
But the loss of grass beds in Tangier also illustrates how difficult it is to reverse the loss of a such a key resource.
Grass beds naturally filter water and stabilize sediments. With large amounts of the beds now gone, the ability to do that is greatly reduced. With nothing to hold sediment in place, waves resuspend silt into the water, covering nearby grass beds. “It sort of sets a trajectory that is spiraling downward,” Batiuk said.
The question that looms is whether an irreversible threshold has been crossed for the Tangier beds, or whether there is time for scientists to concretely identify the causes of the decline — and a solution.
“That’s the ultimate worry that some people have,” said Ryan Davis, who oversees Bay grass restoration efforts for the Alliance for the Chesapeake Bay. “Has it just gone over the edge so you can’t get it back? I don’t know the answer to that, and I don’t think anybody does. But I think it’s in the back of everybody’s mind.”
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