After three years of sharp declines, acreage of ecologically important underwater grasses bounced back last year, increasing 24 percent in the Chesapeake Bay and its tidal tributaries over the amount observed in 2012.

Still, beds of submerged aquatic vegetation, or SAV, remain far below their average of the last three decades, and scientists are concerned that most of the comeback consists of widgeon grass, a species notorious for large year-to-year fluctuations.

“The widgeon grass story is great,” said Lee Karrh, a biologist with the Maryland Department of Natural Resources who chairs the Bay Program’s Submerged Aquatic Vegetation Workgroup. “That is good news. It will be interesting to see whether it persists.”

There were other signs of hope: Freshwater grasses in the Upper Bay that had been inundated by huge amounts of sediment in the aftermath of Tropical Storm Lee in fall 2011 showed modest signs of recovery in some places. Eelgrass, which suffered a large heat-related die-off in the lower portion of the Chesapeake in 2010, gained ground.

The overall results from the annual Baywide aerial survey showed 59,927 acres of underwater meadows in the last year, up from 48,195 acres in 2012, but well below the average of 65,468 acres since 1984, and not quite one-third of the Baywide goal of 185,000 acres.

Submerged aquatic plants need clear water to get the sunlight they, like all green plants, need to survive. Because of that tight link to water clarity, the annual SAV survey is one of the most closely watched indicators of how the Bay is doing.

Grass beds are also one of the most critical components of the Bay ecosystem. They pump oxygen into the water, trap sediments, provide food for waterfowl and shelter for fish and blue crabs.

Because of the importance of underwater grasses to the Chesapeake ecosystem, much of the nutrient and sediment reductions required as part of the Bay cleanup are aimed at helping SAV rebound. Sediment, along with algae blooms spurred by excess nutrients, blocks sunlight needed by the plants.

Last year’s rebound halted a three-year slide that saw grass acreage slip by about 45 percent from nearly 86,000 acres in 2009. It was the first time since the aerial survey began in 1984 that grasses declined for three consecutive years. A variety of factors such as Tropical Storm Lee, heat and general poor water clarity were blamed.

All salinity zones of the Bay saw some recovery last year.

  • The tidal fresh salinity zone, which occupies the uppermost portion of the Bay near the mouth of the Susquehanna River, as well as the uppermost tidal reaches of most tributaries, had 13,990 acres. That was 68 percent of the Tidal Fresh Goal of 20,602 acres, and an increase of 1,841 acres from 2012.
  • The slightly salty oligohaline salinity zone, which occupies a relatively small portion of the Upper Bay and tidal tributaries, had 5,590 acres. That was 54 percent of its 10,334-acre goal and a 78-acre increase from 2012.
  • The moderately salty mesohaline salinity zone contains the largest potential underwater grass habitat, stretching from around Baltimore south to the Rappahannock River and Tangier Island, and includes large sections of most tidal rivers, including most of the Potomac. It had 25,579 acres of SAV last year, or 21 percent of its 120,306-acre goal. That was a 5,958-acre increase from 2012.
  • The very salty polyhaline salinity zone is the second largest zone, stretching from the mouth of the Rappahannock and Tangier Island to the mouth of the Bay, and including the lowermost parts of the York and James rivers. It had 14,768 acres, or 44 percent of its 33,647-acre goal. That was a 3,859-acre increase from 2012.

This is the first year scientists presented survey results by salinity zone. Previously, results were presented by geography (Upper, Middle and Lower Bay). But tracking by salinity zones helps explain year-to-year changes as grass species found in various salinity zones often react differently to water conditions in a given year.

For instance, last year’s conditions were particularly conducive to widgeon grass, which drove most of the increase in the mesohaline and polyhaline zones — and is also where most of the recovery took place.

“We were finding widgeon grass in places where I didn’t think it would grow,” Karrh said. The widgeon grass surge resulted in the most grass ever observed in Pocomoke Sound, and the third highest in Tangier Sound, he said.

But scientists were cautious about the significance of the rebound because widgeon grass is notorious for its boom and bust cycles. “We have historically seen areas with incredible amounts of widgeon grass disappear the next year,” said Bob Orth, a researcher with the Virginia Institute of Marine Science who oversees the annual survey.

The dominance of widgeon grass in the mesohaline, which contains two-thirds of the Chesapeake’s potential SAV habitat, is problematic for reaching the Bay’s 185,000-acre goal, Orth said.

Historically, widgeon grass occurred in that area with such species as eelgrass, redhead grass, sago pondweed and others. When widgeon grass occurs with other species, it tends to persist better than it does when it grows by itself. For instance, Orth said, widgeon grass persists in Mobjack Bay in Virginia where it is mixed with eelgrass, but beds of widgeon grass come and go in the nearby Rappahannock River where there is no eelgrass.

“The mesohaline used to have four to six different species,” Orth said. “Now it is essentially dominated by one, which is widgeon grass.”

While cautious about the significance of last year’s widgeon grass comeback, scientists were encouraged that eelgrass bounced back in high-salinity areas.

Eelgrass is the dominant species in that area, but it likes cool temperatures. But as Bay water has warmed over the last decade, the region’s large, stable eelgrass beds have died back. Poor water clarity has increasingly forced eelgrass out of the deeper, cooler water where it thrived and into water less than a meter deep near the shore, which tends to be warmer, Orth said.

Despite last year’s comeback, Orth said, the long-term outlook for eelgrass remains poor barring dramatic increases in water clarity. “We all know that the Bay is not as conducive today as it was in the past for eelgrass growth.”

Scientists were more optimistic about the low-salinity areas, though disappointed they did not see a stronger comeback last year.

The Susquehanna Flats, the largest grass bed in the Bay, expanded modestly in 2013. But, Karrh said, the bed was much denser than it had been in 2012. “I’m hopeful that the density increase will lead to an acreage increase in the future,” he said.

But he and Orth were disappointed that large lush beds in places like the Elk, Bohemia and Sassafras rivers have vanished in the last few years with no sign of recovery. “The Elk River was grass from one end to the other,” Orth said. “We have seen very little recovery in that particular area.”

Karrh said part of the problem in those beds may, as with the case with widgeon grass, be a lack of diversity. While freshwater areas can have the greatest variety of plants, many places now have only two or three species present, he said.

“If we don’t have that diversity, the system can’t respond to changes from year to year,” Karrh said. “Some species do better in a wet year, and some do better in a dry year. To use a sports analogy, if your bench is not very deep, it is hard to adjust to changing conditions.”

Karrh said meeting Bay underwater grass goals would require not only improved water quality, but also greater species diversity. “That point can’t be made enough,” he said.