|
|
Pictures worth 1,000 acres
Citizens help to survey submerged aquatic vegetation
Growing Respect for Grass
Underwater grass beds last year rebounded from their losses of 2003, led by a strong recovery in the Upper Bay, which contained more of the vital habitat than anytime since annual surveys began two decades ago.
But the good news was not Baywide. The lower Bay suffered losses—thought to be caused by Hurricane Isabel, which hit in late 2003—and contained the lowest amount of grasses observed since 1987.
Overall, the annual aerial survey found 72,935 acres of grass beds in the Bay and its tidal tributaries during 2004, a 14 percent improvement over the previous year.
Still, that represents only 39 percent of the Bay Program’s restoration goal of 185,000 acres, which scientists believe represented the amount of grass beds that existed in the Bay only a few decades ago.
The most exciting news, scientists said, was the explosion of grasses in the Susquehanna flats. Vast beds in that area once drew huge waterfowl populations, but the grasses largely vanished in recent decades as nutrient and sediment flows into the Bay increased. The resulting algae blooms and muddy water blocked sunlight critical to plant survival.
“This was a waterfowl capital of the world,” said Bob Orth, a seagrass expert with the Virginia Institute of Marine Science, who oversees the annual aerial survey. “When we first started looking at this 20 years ago, you could hardly see a blade of grass anywhere.”
Last year, the photos showed that the flats, near the mouth of the Susquehanna River, had a dense grass bed that was more than 2 miles by 2 miles in size.
Neighboring rivers in the upper Bay, including the Elk, Sassafras and Bohemia, also flourished. “We set records all over the place,” said Mike Naylor, a biologist with the Maryland Department of Natural Resources. He noted that the Back River, which historically has had such bad water quality that it doesn’t even have an underwater grass restoration goal, contained 30 acres of grasses last year.
Scientists cited a number of factors for Upper Bay improvements, which have been trending up for years. Not only has water quality in that area improved, but the low- salinity plants found there tend to be more tolerant of murky water than other aquatic grasses. Also, their methods of seed dispersal favor rapid recovery, as the plants create large “seed banks” in the sediment that can rapidly come to life when conditions are right.
For example, beds around the Chester River, which were almost gone two years ago, greatly expanded last year. Such areas, Orth said, teeter on the edge of having adequate water quality to support grasses. “If they get good conditions they can take off.” he said. “If they fall below that edge, they die.”
Although 2004 was a wetter than normal year—which is usually bad news for plants—most of the rain didn’t come until after the critical spring growing season. “A lot of the high flow in 2004 didn’t start until July, so it wasn’t a factor in the spring,” said Peter Bergstrom, a fishery biologist with the National Oceanic and Atmospheric Administration’s Chesapeake Bay Office.
In contrast to the Upper Bay, grasses in the high-salinity lower Chesapeake fared poorly last year. Scientists believe that was the result of Hurricane Isabel, which caused extensive shoreline erosion in the southern part of the Bay, washing out nearshore grass beds.
Results by region last year show:
- The Upper Bay, from the Susquehanna River south to the Chester and Magothy rivers, increased from 10,416 acres in 2003 to 21,654 acres last year.
- The Middle Bay, from the Bay Bridge south to the Rappahannock River, increased from 30,475 acres to 33,719 acres.
- The Lower Bay, from the Rappahannock River and Pocomoke Sound south to the Bay’s mouth, decreased from 20,802 acres to 17,561 acres.
Because of their tight link to water quality, the amount of grasses is one of the most closely watched indicators of the Bay’s health. 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. Juvenile blue crab densities may be 30 times greater in grass beds than nearby barren areas.
Scientists believe 200,000 acres or more of grass beds once covered the Chesapeake, providing huge amounts of habitat for an array of species. But Bay Program efforts to track down historic aerial photographs show a steady decline in acreage over the decades as increased amounts of pollution washed into the estuary.
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—only to be knocked back 30 percent by extremely wet conditions in 2003.
The annual Baywide grass estimate is derived from an analysis of more than 2,000 black-and-white aerial photographs taken between May and October.
