Underwater grass beds covered an estimated 78,260 acres of the Chesapeake last year, expanding their area by 7 percent over levels observed in 2004. The coverage was 42 percent of the state-federal Bay Program’s 2010 restoration goal for underwater grass beds, considered one of the Bay’s most critical habitats.

But the 2005 aerial survey figures, released by the Bay Program in May, need an asterisk. That’s because a massive die-off of eelgrass, the dominant species in high salinity areas, took place after the survey was conducted in those areas.

It will be months before scientists know whether the die-off is a short-term setback, or a long-term catastrophe.

Before succumbing to unusually warm and still conditions in Bay water last year, the eelgrass produced a bumper crop of seeds. If their seedlings survive this summer, the grass beds could bounce back soon. But if they die off because of warm temperatures, poor water quality or other problems, there will be few plants next year as seedlings don’t produce seeds of their own until their second year.

“If we get a second really hot, still summer, and we don’t get good seed production, that would be something that really could take a long time to recover from,” said Mike Naylor, a biologist specializing in underwater grasses with the Maryland Department of Natural Resources.

Scientists continued to be impressed by the explosion of grasses in the Susquehanna flats. The bed, which had only patchy plant coverage a few years ago, has mushroomed into the largest, densest grass bed in the Chesapeake, featuring a diverse mix of species—just the characteristics biologists like to see in a healthy bed.

“When you look at that huge area of the upper Bay, from 1999 through now, it has gone from really patchy to really dense,” said Bob Orth, an underwater grass expert with the Virginia Institute of Marine Science, who oversees the annual aerial survey. “That is pretty phenomenal.”

Overall, results by region last year show:

  • The Upper Bay, from the Susquehanna River south to the Chester and Magothy rivers, decreased from 21,673 acres to 19,464 acres last year, a decline of 10 percent.
  • The Middle Bay, from the Bay Bridge south to the Rappahannock River, increased from from 33,709 acres to 39,575 acres, an expansion of 17 percent.
  • The Lower Bay, from the Rappahannock River and Pocomoke Sound south to the Bay’s mouth, increased from 17,560 acres to 19,220 acres, an increase of 10 percent.

Scientists downplayed the decrease in the Upper Bay, noting that the survey showed grass beds in that region greatly increased in density from 2004 to 2005, as areas with only scattered plants became much more thick and lush.

“In reality, there is actually much more grass, even though some of the numbers show there is a little less total area,” Orth said.

That’s important, scientists say, because dense beds are considered to be more ecologically valuable in providing habitat, holding bottom sediment in place and reducing the energy of waves before they hit the shoreline.

“All of the positive influences that the Bay grasses have are a function of their density,” Naylor said. “As the density increases, the functions that SAV provide increase with that density change.”

The change in the middle Bay primarily consisted of widgeon grass, a species which is notorious for year-to-year fluctuations.

Much of the increase in the lower Bay was a continued recovery from Hurricane Isabel, which severely damaged underwater grass beds in that area when it hit in fall 2003. Many of those areas, though, were the ones affected by the die-off.

Underwater grass beds depend on clear water to receive sunlight. Because of their tight link to water quality, the amount of underwater grasses—formally known as submerged aquatic vegetation, or SAV—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. The amount began dropping in midcentury as sediment from the land and nutrient-spurred algae blooms began clouding the water.

Grasses bottomed out at an estimated 38,000 acres in 1984, and slowly increased until the 1990s. Their recent peak was 89,659 acres in 2002, after four years of drought sharply reduced the amount of nutrients and sediment in the water.

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.

Maryland Sea Grant has produced a new 80-page guide, “Underwater Grasses in Chesapeake Bay and Mid-Atlantic Coastal Waters” to help citizen volunteers, students, and others interested in learning about the critical plants.

The book is 8.5 inches wide and 5.5 inches tall and is printed on water resistant paper. It features more than 100 color photographs, 55 line drawings, and descriptions of 16 of the most common SAV species, along with other aquatic species people may encounter. It was produced in collaboration with the National Oceanic and Atmospheric Administration’s Chesapeake Bay Office, the Alliance for the Chesapeake Bay and the Maryland Department of Natural Resources.

Copies are available for $29.95 from Maryland Sea Grant, plus shipping and handling. For information, visit its web site at www.mdsg.umd.edu/store.