The amount of underwater grasses in the Bay — often considered the best indicator of the Chesapeake’s health — remained almost stagnant last year.
Although much of last year was marked by low stream flows into the Chesapeake, which typically means less water-fouling nutrients and sediment, a dense algae bloom blocked sunlight from reaching grass beds in much of the mid-Bay.
Huge losses of submerged aquatic vegetation in that area — some rivers lost all of their grass beds — offset gains in the upper and lower parts of the Bay.
Overall, the annual aerial survey of grass beds showed the Chesapeake and its tidal tributaries contained 69,126 acres of the underwater meadows last year, roughly a 1 percent increase from 1999.
During the last decade, there has been little Baywide change as the total amount of grass beds remains frozen at about a tenth of their historic level.
“The Baywide numbers have sort of hit a plateau where you have localized declines and recoveries, but the overall effect is kind of flat,” said Peter Bergstrom, a Bay grass expert with the U.S. Fish and Wildlife Service’s Chesapeake Bay Field Office.
“It tells us that we need to do some additional work if we want that trend to really increase,” said Bill Street, a scientist with the Chesapeake Bay Foundation.
Expanding grass beds was a goal of the Chesapeake 2000 Agreement because they are considered 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.
The agreement specifically calls for improving water clarity in the Bay enough by the end of the decade to allow a return of grasses to levels that existed in 1930s. An analysis of that amount is now under way, using historic aerial photographs.
To make that recovery happen, the Bay Program in June will release draft water quality criteria that — once adopted — should help spur grass recovery. In particular, the Bay Program is developing a first-ever water clarity criteria to help achieve conditions that will allow grasses to return, by controlling nutrients, which spur algae blooms, and reducing sediment, which muddies the water.
By the end of the year, the Bay Program will determine — river by river — the amount of additional nutrient and sediment reductions needed to achieve that criteria, as well as new criteria for dissolved oxygen and chlorophyll a, a measure of algae. Initial estimates suggest nearly twice as much nutrient reduction will be needed than was accomplished in the past 14 years.
Last year’s grass figures, scientists say, offer a clear relationship between water quality and grass beds. Aided by low stream flows, upper and lower parts of the Bay enjoyed relatively good water quality which allowed grass beds to expand in many ares.
But last spring, an unusually dense mahogany tide persisted for weeks in much of the mid-Bay, totally blocking the light from the grass beds. Some areas lost all of their grasses, including hundreds of acres that vanished in the lower Chester River and thousands of acres in the Eastern Bay.
“Usually, we have a much more difficult time explaining what happened, but we think it is pretty clear in the data that we collected in 2000,” said Mike Naylor, a biologist with the Maryland Department of Natural Resources who also chairs the Bay Program’s SAV Workgroup. “We think that algae bloom contributed to the reductions in light availability, which translated into catastrophic losses in some places.”
Last year’s figures show sharp differences by region:
- In the Upper Bay, which extends from the Susquehanna River to the Bay Bridge, 14,814 acres of grasses were mapped, an increase of 3,001 acres, or 36 percent.
- In the Middle Bay, which stretches from the Bay Bridge to the Rappahannock River and Pocomoke Sound (including the Potomac River), acreage declined by 3,418 acres to a total of 33,465, a loss of 9 percent.
- In the Lower Bay, 20,847 acres of grasses were mapped, an increase of 934 acres, or 5 percent.
The loss of grasses in the mid-Bay was made worse because the beds consisted primarily of widgeon grass, a species notorious for widely fluctuating from year to year.
The “flip-side,” Naylor said, is that widgeon grass may also rebound more quickly than other species. “We’re not nearly as concerned with the loss of widgeon grass as we would have been with other species,” he said.
Such dramatic fluctuations could also be averted, some suggest, if more grass beds contained a mix of species — as was likely the case historically in much of the Bay — rather than being dominated by a single grass type.
“Anytime you get a more diverse bed, it is going to be a more resilient and stable bed,” Street said.
Bergstrom noted that small patches of other species, such as redhead grass, managed to persist in areas where widgeon grass disappeared.
Besides improvements in the Upper and Lower Bay, scientists were also pleased to see increases for the second straight year around Tangier Sound, one of the Bay’s most important settlement areas for larval blue crabs as they enter the Bay.
From 1992 through 1997, nearly two-thirds of the grass beds in the sound had disappeared, leaving only 6,612 acres by 1998. In 1999, grass beds returned to 10,618 acres, and last year the area gained another 2,859 acres.
“We were extremely pleased to see these big increases in Tangier Sound as that is such a critical area for the crabs,” Naylor said. “That was particularly encouraging.”
The annual Baywide grass estimate, made for the Bay Program by the Virginia Institute of Marine Science, is derived from the analysis of more than 2,000 black and white aerial photographs taken between May and October.