The Bay’s underwater grass beds, one of the most important habitats in the Chesapeake, increased by 8 percent last year, bouncing back from the beating they took in 1998.
Results from the annual grass bed survey are one of the Bay Program’s most closely watched indicators of the Chesapeake’s health because the abundance of submerged aquatic vegetation — or SAV — is closely tied to water quality.
Scientists were particularly cheered by the rebound in and near Tangier Sound, one of the Bay’s most important areas for blue crabs. Since 1992, nearly two-thirds of the grass beds in the sound had disappeared, leaving only 6,612 acres by 1998. Last year, grass beds returned to 10,618 acres.
“I think the Tangier recovery is the best news,” said Peter Bergstrom, a scientist with the U.S. Fish and Wildlife Service’s Chesapeake Bay Field Office. “It had been going down every year since 1993. That’s a long string of declines. It’s pretty encouraging that it started going up again.”
Besides being a good indicator of water quality, grass beds also provide crucial habitat for many Bay species. They provide food and shelter for juvenile fish, clams and crabs — densities of young blue crabs can be 30 times greater in grass beds than in unvegetated areas. Grasses are also important food for waterfowl.
Today, the Bay has only a fraction of the grass beds that once existed; some estimates put the historic number at 600,000 acres. Water clouded by sediment and algae has reduced sunlight in the water, causing dramatic declines in recent decades.
The Bay Program’s new Chesapeake 2000 Agreement calls for improving water clarity in the Bay by the end of the decade to allow for the return of grasses to levels that existed in 1930s. An analysis of that amount is now under way, using historic aerial photographs.
The exact amount of grasses in the Bay last year is unclear because poor weather delayed completion of the annual aerial survey, conducted for the Bay Program by the Virginia Institute of Marine Science. By the time some areas were surveyed, the seasonal die-back of freshwater grass species had already begun, leaving the picture for 4,000 acres unclear.
The survey counted 64,689 acres of grasses, an 8 percent increase over the 1998 numbers for the same area. If that increase held true for the unmapped areas, about 68,125 acres of grasses may have been present in 1999.
The 1998 survey counted 63,597 acres, a 10 percent decline from the previous year.
Last year’s trends varied by region:
- In the Upper Bay, which extends from the Susquehanna River to the Bay Bridge, 8,768 acres of grasses were mapped, a 2 percent decline from 1998.
- In the Middle Bay, which stretches from the Bay Bridge to the Rappahannock River and Pocomoke Sound (including the Potomac River), 36,676 acres of grasses were mapped, a 22 percent increase.
- In the Lower Bay, 19,246 acres of grasses were mapped, a decrease of 9 percent.
Much of the annual fluctuations are thought to be weather-related. The 1998 decline was blamed on unusually high flows the first half of that year. Strong flows tend to flush more sediment and nutrients — which spur algae blooms — into the Bay where they cause reduced water clarity.
Last year’s drought, by contrast, resulted in improved water quality in many areas, including Tangier Sound. But the drought brought its own trouble: salinities that hit near-record levels during the summer growing season in the upper parts of the Bay and upper tidal portions of tributaries caused die-backs of some freshwater grass species.
“We observed drastic shifts in grass populations, possibly driven by higher than normal salinity levels,” said Bob Orth, a VIMS scientist who conducts the annual survey. “While the more salt-tolerant widgeon grass and eel grass expanded, we recorded die-back in some of the freshwater species such as millfoil that was unexpected at the beginning of the season.”
Effects of the drought may linger into this year because most freshwater species grow from the seeds or tubers produced the previous summer, said Mike Naylor, a biologist with the Maryland Department of Natural Resources and the chair of the Bay Program’s SAV Workgroup.
“If we got lower seed production last year, we might see some pretty substantial declines in some areas this year,” he said.
He and other scientists remain worried about Tangier Sound, despite the apparent improvement. The sound likely benefited from low flows that began in the last half of 1998 and continued through last year, which sent less water-clouding sediment into the area’s water. In that case, another high-flow year could cause a setback.
Also, the sound contains two types of grasses: eel grass and widgeon grass. At least some of the gain came from eel grass, which is more stable. But if much of the comeback was by widgeon grass, which can fluctuate widely from year-to-year, it would be less significant than a comeback of eel grass.
“That’s something you don’t get a sense of in the aerial survey,” he said. Increased monitoring is planned for the area this year to better understand what is happening.
Baywide, grasses have rebounded from their low of 37,253 acres in 1984. But they still remain below their recent high of 73,082 acres in 1993. In fact, for the entire decade of the 1990s, there has been no Baywide trend — probably in part because of several high-flow years — with levels mostly fluctuating between 60,000 and 70,000 acres.
On smaller scales, though, scientists say there have been noticeable changes. While areas like Tangier Sound have suffered declines, other places — such as parts of the Patuxent, Severn and Potomac rivers, Mobjack Bay in Virginia, and several smaller Western Shore tributaries in Maryland have seen recoveries.
The common denominator, Naylor said, is water quality.
“I’ve been very surprised at how well the SAV trends have followed the water quality trends in those areas,” Naylor said. “You see that the SAV has increased very dramatically in areas with substantial water quality improvements. And when you look at areas where water clarity has gone down and the total suspended sediment levels have gone up, like Tangier Sound, you see that the SAV has really gone downhill.”