Tropical storms and hot temperatures proved to be a lethal combination for the Chesapeake's underwater grasses, which declined 22 percent Baywide last year, according to the latest aerial survey.

Much of the damage was inflicted by an unusually wet spring followed by record-setting September flows from the Susquehanna River after Tropical Storm Lee and Hurricane Irene that left much of the Upper Bay awash with sediment, nutrients and debris.

That's bad news for grasses. Like all plants, they need light to survive. Sediment clouds the water while nutrients spur algae blooms and the growth of epiphytes directly on blades of grass, all of which block sunlight.

Also contributing to the decline were the warmer than normal temperatures during summer 2010 that led to a die-off of eelgrass in the lower Bay. The die-off wasn't evident until the 2011 aerial Bay grass survey, which is conducted by scientists from the Virginia Institute of Marine Science.

The actual amount of submerged aquatic vegetation, or SAV, observed in last year's survey was about 58,000 acres, the lowest level seen since the late 1980s. But water conditions remained so muddy for such a long time last fall that some areas could not be surveyed.

If they had been, scientists think the actual amount of grasses might have been closer to 62,800 acres, which would have been the third lowest level since 1990.

Because of their tight link to water quality, the acres 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.

One of the goals of the Chesapeake Bay Total Maximum Daily Load, or "pollution diet," is to restore water quality to allow 185,000 acres of underwater grasses to thrive in the Bay and its tidal tributaries.

Last year's setbacks were a stark reminder that cleanup efforts to date cannot offset nature's ability to set back, if not totally erase, beds that took years to return.

While individual beds in some areas did well, grasses declined overall in the Upper, Middle and Lower Bay.

But scientists were hopeful that freshwater grasses in the Upper Bay, which have been trending strongly upward for the last decade, could swiftly resume that trajectory. Despite the declines, scientists said there was evidence of strong seed production in many areas, which could set the stage for recovery.

"There is plenty of capacity for these grasses to bounce back really quickly," said Lee Karrh, a biologist with the Maryland Department of Natural Resources who chairs the Bay Program's SAV Workgroup.

Scientists were especially pleased that the Susquehanna Flats, the largest grass bed in the Bay, survived the record September and October deluge from the Susquehanna River, although it appears to have lost about a third of its acreage.

But scientists were gloomy about the situation for eelgrass, the dominant species in high-salinity areas, which has been trending downward for more than a decade. Grass acreage in the Lower Bay was the second lowest observed since the aerial survey began in 1984, primarily because of eelgrass declines.

"We are losing an important component of the Bay," said Bob Orth, the VIMS scientist who oversees the aerial survey.

Scientists were also hoping that this year's weather - either in the form of high river flows or high temperatures - does not deliver another setback for grasses.

The 2011 declines followed a 7 percent decline detected in the 2010 survey. It was the first time since 1994 and 1995 that grass beds suffered back-to-back Baywide setbacks. Taken together, more than a quarter of the 85,800 acres of SAV present in 2009 no longer exist.

Last year's data shows no major region was spared:

  • In the Upper Bay, which stretches from the head of the Chesapeake to the Bay Bridge, underwater grasses decreased 38 percent, from 21,353 acres in 2010 to 13,287 acres last year. While most areas saw significant losses, grasses more than doubled in the Chester River and Eastern Neck Narrows.
  • In the Middle Bay, which stretches from the Bay Bridge to the Rappahannock River and Pocomoke Sound, acreage decreased 4 percent from 35,446 acres to 33,867 acres, based on estimates. But some areas thought to have sustained losses, including the middle and upper Potomac, as well as parts of the Patuxent river, were not surveyed because of poor fall conditions, so the figure may understate losses, scientists cautioned. The figure also reflects large losses of eelgrass in Tangier Sound as well as some large gains of widgeon grass in other areas.
  • In the Lower Bay, which is south of the Rappahannock River and Pocomoke Sound, beds decreased 32 percent from 22,685 acres to 15,645 acres, largely as a result of a widespread die-off of eelgrass that was stressed by warm temperatures in 2010. But some lower salinity areas, including the Chickahominy River, saw increases.

There were some slivers of good news last year. The survey found the first grass bed in the fresh tidal portion of the James River, near the mouth of the Chickahominy River.

It also found sharp increases in the Eastern Bay and Choptank River. But that increase consisted of widgeon grass, a species notorious for year-to-year fluctuations.

"There were good points," Orth said. "Unfortunately they were small good points."

Historically, widgeon grass beds were mixed with other native grasses, which helped to provide more overall stability. If widgeon grass died back, the other species remained.

"Because it is the only species in these systems, there is nothing else to replace it when it dies back," Orth said.

That loss of diversity, especially in the Middle Bay, is an illustration of how much the SAV situation in the Bay has changed over the decades.

No one knows how much SAV once existed in the Bay. Examination of aerial photographs of the Bay from the early to mid-1900s suggest that as much as 200,000 acres may have existed at that time.

Increased nutrient and sediment pollution contributed to underwater grass loss over time, and Tropical Storm Agnes's massive deluge in 1972 obliterated many of the grass beds that remained, especially in the Upper Bay, which was smothered by sediment from the Susquehanna River.

The startling loss of underwater grasses was one of the major problems cited in multiyear EPA study that led to the creation of the Bay Program in 1983. In 1984, when annual aerial surveys started, just 38,000 acres of grasses were seen in the Bay. That number gradually increased to about 60,000 acres in 1990 - roughly the acreage found today.

For the next decade, SAV acreage generally hovered between 60,000 and 70,000 acres. But twice in the past 10 years, grasses surged to more than 85,000 acres, only to be knocked back by severe weather events - a reminder while the Bay cleanup has made progress, it has yet to reach the point where it can offset the influence of natural events.

While the acreage of grasses in the Bay last year was similar to what it was two decades ago, the distribution has changed dramatically.

In 1990, grass was sparse in the Upper Bay and other low-salinity areas, while the salty lower Bay had the largest, most dense grass beds, primarily consisting of eelgrass.

But grasses have recovered dramatically in the Upper Bay and in low-salinity portions of a number of tidal rivers. The Susquehanna Flats, which was mostly barren a decade ago, has mushroomed into the largest single grass bed in the Bay, covering several square miles even after last year's setback.

Also helping the Upper Bay recovery: Low-salinity beds include a mix of multiple freshwater species, each adapted to slightly different conditions. Many are prolific seed producers, which provides more opportunities for a comeback.

Meanwhile, the Lower Bay has seen a decline over the last decade driven by the demise of eelgrass, and scientists are increasingly gloomy about the prospect of a robust comeback. Eelgrass, which is near the southern edge of its range in the Bay, is stressed by warm temperatures, which were blamed for triggering large die-offs in both 2005 and 2010.

Because eelgrass is the only grass that thrives in high-salinity portions of the Bay - where most of the potential underwater grass habitat is located - its demise would likely make the Bay goal of restoring 185,000 acres of underwater grasses unattainable.

As recently as the 1960s, eelgrass was found as far north as Eastern Bay near Kent Island and the Patuxent River, but its range has shrunk considerably, with its farthest outposts now near the mouth of the Potomac.

Eelgrass' loss would also have significant ecosystem ramifications. Extensive eelgrass beds once provided habitat corridors for juvenile crabs, fish and other species moving up and down the Bay's shallows. Loss of that shelter can make species more vulnerable to predation.

"You are not going to have those continuous corridors," Orth said. "I think we are probably going to end up with eelgrass persisting in remnant populations, and in many places they may coexist with widgeon grass."

Underwater grasses are also important for removing sediment and nutrients from the water. Continued losses of eelgrass would make it even more difficult to meet water quality goals in the Lower Bay.

Eelgrass could tolerate slightly warmer temperatures if water clarity were better, because it would make it easier for the plant to get the sunlight needed for photosynthesis and to gain more energy.

"It puts more of an emphasis on trying to improve water clarity for grasses because that could be the game changer if we have warmer conditions," Orth said. "You just need more light."

But clarity tends to be poor in most areas with eelgrass, he noted.

The full impact of last year's high flows won't be known until surveys are conducted this year. About 7 percent of the Bay and its tidal tributaries could not be mapped late in the year because of murky water. Also, it's possible that areas mapped before the September deluge may have died back later in the year.

In the meantime, scientists will be anxiously watching the weather to see if it brings less rain - which means less nutrient and sediment runoff - and moderate temperatures that will not stress eelgrass.

"We are kind of at the mercy of what happens this spring," Karrh said. "But we are primed for a strong recovery in the Middle and Upper Bay."

In mid-March the long term outlook from the National Weather Service said it was likely that spring would be warmer than normal for the region, but the precipitation outlook was uncertain, with there being equal chances of above, below or normal amounts of rainfall.

Details about the survey, including aerial photos of grass beds from around the Bay, are found at www.vims.edu/bio/sav.