Last year’s poor water quality, driven by near-record river flows, wiped nearly a third of the Chesapeake’s underwater grass beds from the map, more than offsetting the large gains observed during the proceeding four years of drought.
Overall, the amount of Bay grasses decreased almost 30 percent last year, from 89,659 acres in 2002 to 64,709 acres. It was the largest single-year decline since annual aerial surveys of the grass beds began in 1984.
Scientists blamed the die-off on the higher than normal precipitation which drove huge amounts of nutrients and sediment into the Bay, blotting out the sunlight crucial to the plants.
And, they said last year’s events raise questions about whether the Bay can sustain significantly increased acreage of grass beds until much more is done to reduce pollution.
“When you look at the total Bay grass acreage, we are down to where we were more than a decade ago,” said Mike Naylor, a biologist with the Maryland Department of Natural Resources.
“I think it really highlights the fact that except for the Upper Bay, where we have had a slow upward trend, we really have made minimal progress in the last decade toward achieving our goals,” Naylor said.
The Bay Program last year set a restoration goal of 185,000 acres of submerged underwater vegetation, or SAV, by 2010. Achieving the water quality needed to support those grasses was part of the basis for new nutrient and sediment reduction goals set for the region last year.
But instead of decreasing, the amount of nutrients and sediment entering the Bay rose sharply in 2003. Data from the U.S. Geological Survey show that the Bay’s nine largest tributaries delivered three times as much nitrogen, five times as much phosphorus and 11 times as much sediment to the Bay compared with 2002.
That’s bad news for grasses. Like all plants, they need light to survive, but sediment clouds the water, and nutrients spur algae blooms as well as the growth of epiphytes directly on blades of grass, all of which blocks sunlight.
Naylor said last year’s high flows resulted in a “systemwide decrease in water quality.” In many places, he said, water clarity was the worst on record, and the densities of the algae blooms were the highest ever reported.
“Nature continually reminds us that SAV is very sensitive to water quality,” said Bob Orth, of the Virginia Institute of Marine Science, who oversees the annual aerial survey. “Acreage fluctuations over the past two years reinforce the message that SAV can rapidly rebound when conditions improve, but also decline just as rapidly when conditions worsen as they did in 2003.”
Because of their tight link to water quality, the amount of grasses is one of the most closely watched indicators of how the Bay is doing. They are also one of the most critical components of the Bay ecosystem. Grass beds pump oxygen into the water, trap sediments, provide food for waterfowl and shelter for fish and blue crabs. Densities of juvenile blue crabs may be 30 times greater in grass beds than nearby barren areas.
Last year, grasses declined in all major regions of the Chesapeake:
The Upper Bay, from the Susquehanna River south to the Chester and Magothy rivers, decreased by about 20 percent, from 13,166 acres in 2002 to 10,416 acres.
The Middle Bay, from the Bay Bridge south to the Rappahannock River, decreased by about 41 percent, from 52,973 acres to 30,475.
The Lower Bay, from the Rappahannock River and Pocomoke Sound south to the Bay’s mouth, decreased by 12 percent, from 23,520 acres to 20,802.
Not all of the news was bad. Orth said he was impressed that steady increases over the last decade were maintained in some areas, such as the Severn, Magothy, Middle and Upper Patuxent rivers.
But losses in the Middle Bay are particularly troublesome because scientists say grass beds in that region are particularly important for juvenile blue crabs.
Scientists said much of the drop was caused by the disappearance of widgeon grass in the Middle Bay, a species notorious for wide year-to-year fluctuations.
Widgeon grass is considered a “pioneer” species that can quickly colonize an area when conditions are right, but is less able to withstand setbacks caused by poor water quality. Most of the gains in underwater grasses since the mid-1990s have been the result of widgeon grass regrowth.
“We’ve seen it before,” Orth said. “the plant grows like a weed, and it probably will rebound quite quickly in some of these areas, as long as we don’t have a bad year.”
He and others said the dramatic fluctuations in widgeon grass point to the need to restore not only the grass beds, but also the historic diversity of species once found within many of those beds.
“You like more species because they can respond very differently to water temperature and light, so if one goes down, the other might go up,” Orth said. “I think there is a need to look carefully at the diversity issue.”
Peter Bergstrom, a fishery biologist with the National Oceanic and Atmospheric Administration’s Chesapeake Bay Office, said he reviewed historic ground survey records of SAV species found in grass beds in Maryland from 1971–1990. In areas which are now dominated by widgeon grass, he said, the records show at least one other species was present in previous decades—and sometimes two or three other species were found.
“When widgeon grass started to come back in the late 1980s and early 1990s,” he said, “usually, those other species didn’t come back with it.”
If so, a key challenge is maintaining good conditions for enough years so that species which colonize more slowly can eventually merge into widgeon grass beds. “I think the message is that we need to improve the water quality so that something besides widgeon grass can grow,” Bergstrom said.
But more bad news from 2003 may emerge when this year’s aerial survey is completed. Last year’s data do not reflect any impact form Hurricane Isabel which hit in late September. Most of the aerial survey had been completed before the hurricane hit.
Initially, scientists thought the late-season hurricane was likely to cause only a little damage to SAV beds, as it hit after the growing season. But Orth said on-the-ground surveys early this year showed “significant” losses in some areas, such as the mouth of the York River, although other areas appear to have been spared.
“It’s a mixed message, but there is definitely a significant loss at the mouth of this river,” he said.
Scientists believe 200,000 acres or more of grass beds once covered the Bay, providing huge amounts of habitat for an array of species. But Bay Program efforts to track down historic aerial photographs of the Chesapeake show a steady decline in acreage over the decades as increased amounts of pollution washed into the estuary.
Hurricane Agnes in 1972 was the biggest shock to the grass beds, causing sharp Baywide declines. Grasses bottomed out at an estimated 38,000 acres in 1984, and slowly increased until the 1990s, when acreages leveled off in the 60,000– to 70,000-acre range. Four years of drought conditions starting in 1999 dramatically reduced nutrient and sediment pollution to the Bay, causing grasses to surge to 89,659 acres in 2002—the most observed in recent history.
The Bay Program’s annual Baywide grass estimate, undertaken by the Virginia Institute of Marine Science, is derived from an analysis of more than 2,000 black-and-white aerial photographs taken between May and October.