The Bay’s underwater grass meadows, a critical habitat for crabs and juvenile fish, expanded last year to the highest levels seen since monitoring programs began more than three decades ago.

The gains were widespread, from high-salinity waters in the lower Chesapeake to tidal fresh areas in its uppermost reaches. A number of areas had more grass acreage than had ever been observed, and scientists found isolated patches in places they had never seen grasses before.

The 91,631 acres photographed during the annual aerial survey is nearly half of the Bay Program goal. It also exceeded the 2017 restoration objective two years ahead of schedule — as long as the grasses hang on this year and next.

That’s far from certain, because much of the increase was in the Mid Bay, an area dominated by widgeon grass, a species notorious for its year-to-year fluctuations. Grass beds there more than doubled from 2012 through 2015, and account for more than half of the entire Bay’s acreage.

“Widgeon grass continues to be the story of the Bay in terms of what’s driving these numbers,” said Bob Orth, a scientist with the Virginia Institute of Marine Science who oversees the annual survey.

Orth also cautioned that there were signs of an eelgrass die-back in the lower Bay last fall — something that would not show up until this year’s survey.

Overall, though, the news was good. Grasses increased 21 percent from 2014 levels and have expanded 140 percent since the first survey found just 38,227 acres in 1984 — the lowest ever observed in the Chesapeake.

Like all green plants, submerged grasses need clear water to get the sunlight they require to survive. Because of that link to water clarity, the annual survey of Bay grasses — formally known as submerged aquatic vegetation, or SAV — is a closely watched indicator of how the Chesapeake is doing.

In their own right, grass beds are also a critical component of the Bay ecosystem. They pump oxygen into the water, trap sediments, provide food for waterfowl and shelter for fish and blue crabs.

Brooke Landry, a biologist with the Maryland Department of Natural Resources and chair of the Bay Program SAV Workgroup, noted that the resurgence in underwater grasses in recent years may have aided in the recent blue crab comeback.

“Increased grasses increase the amount of habitat and increase their survival and reproduction,” she said.

Much of the nutrient and sediment reductions required as part of the Bay cleanup are aimed at helping grasses rebound because of their ecosystem value. Sediment, along with algae blooms fed by excess nutrients, block sunlight needed by the plants.

Many, though not all, parts of the Bay experienced improved water clarity last year, which may have aided the expansion, scientists said. But it was the third straight year of Baywide increases, and as beds get larger, they improve nearby water quality, allowing further expansion.

“Big beds are very resilient,” Landry said. She also noted that while doing field work, biologists saw patches in new areas that had been too small to be seen from the air.

“Those beds will eventually grow together and be big enough to be mapped,” she said.

Though grasses improved Baywide, the survey found that increases varied by the salinity of the water in which they were growing:

• The tidal fresh waters at the head of the Bay and in the uppermost tidal reaches of most tributaries, saw a 13 percent increase over 2014, to 17,454 acres. That’s 85 percent of the goal for that salinity zone.

• The slightly salty, oligohaline waters, which occupy a relatively small portion of the Upper Bay and tidal tributaries, experienced a 37 percent increase, to 9,881 acres. That’s 96 percent of that zone’s goal.

• The moderately salty, mesohaline water — the largest area of underwater grass habitat, stretching from near Baltimore south to the Rappahannock River and Tangier Island and including large sections of most tidal rivers — had an increase of 29 percent. The 48,444 acres seen last year reached 40 percent of the goal for that zone.

• The very salty, polyhaline water in the Lower Bay — from the mouth of the Rappahannock and Tangier Island south, including the lower York and James rivers — had 17,772 acres. Though the second largest area in the Bay, it saw the smallest increase — just 5 percent — to 49 percent of
its goal.

The salinity-based goals for the Bay and its rivers are based on the amount of grasses observed in historical aerial photographs, some of which date back nearly a century.

Grasses have already reached or exceeded local restoration goals in some areas, such as the Elk River and upper Chester River on Maryland’s Upper Eastern Shore, the Big Annemessex on the Lower Eastern Shore, and the Bush River on the Western Shore.

The SAV at Susquehanna Flats in the Upper Bay, which was hard hit in 2011 when Tropical Storm Lee sent a flood of sediment and nutrients down the river, continued its rebound and now covers more than 5,200 acres, about half of its former size.

Until last year, the Susquehanna Flats had been the Bay’s largest grass bed. The survey found it had been displaced by a widgeon grass bed exceeding 6,000 acres that straddles the Maryland-Virginia state line between Smith and Tangier Islands.

Widgeon grass can come and go quickly. After spurring the Mid Bay to an earlier peak in SAV abundance in 2002, half of it disappeared the next year. But Landry expressed cautious optimism that if the fickle vegetation can stick around long enough this time, it could create water quality conditions conducive for other, less variable species.

“It kind of sets the scene for other grasses to come in — if it persists,” she said.

Grass beds in the Mid Bay once contained multiple species, which helped them maintain long-term stability. But the other grasses never came back after Tropical Storm Agnes hit the Chesapeake in June 1972, leaving underwater beds devastated in its wake.

“Historically, we know many of these areas prior to Agnes had four or five species,” Orth said. “That diversity allowed the Bay to have SAV regardless of whether it was a high runoff or low runoff year.”

High runoff years often produce a surge of nutrients and sediment that washes into the Bay, which usually cause declines in grass beds. But the last several years have provided favorable growing conditions — below average runoff in 2015, and no more than average before that.

Scientists’ biggest long-term concerns, though, are for high-salinity waters of the lower Bay, where eelgrass dominates. Despite year-to-year fluctuations, that area has seen a general decline over the last two decades.

Scientists say poor water clarity in the Lower Bay and gradually warming temperatures are to blame for the decline. Eelgrass is near the southern extent of its range in the Chesapeake, and it can suffer wide-scale die-offs when water temperatures get too hot.

That happened in both 2005 and 2010, and Orth said eelgrass might have taken another hit last fall.

“We believe that in 2015 there was probably something similar, but not as serious as 2010,” Orth said. “So we are a little concerned that the 2016 eelgrass story may dip a little bit.”

The Bay has about 17 species of underwater grass beds, but most of them live in freshwater or low-salinity areas — regions that have been showing signs of sustained recovery. Many of those are also canopy-forming species that grow to the surface of the water.

“So even if clarity is not the greatest, they can grow up farther to reach the light,” said Becky Golden, a DNR biologist and vice-chair of the SAV Workgroup.

Though Tropical Storm Lee took a toll in 2011 on underwater grasses in many tidal-fresh and low-salinity areas, their growth in some areas now exceeds pre-storm levels. If other areas, such as Susquehanna Flats, keep recovering, fresh and low-salinity regions of the Bay could set new SAV records that would exceed current goals, said Dave Wilcox, a VIMS analyst who works on the survey.

“That’s a pretty positive story.” Wilcox said. “We could get to pretty high numbers if we have the same progression we had from 2004 up to Lee. It would be great.”

That expansion isn’t limited to the Upper Bay, either, Wilcox said. Beds in tidal fresh areas of the Chickahominy, Mattaponi and Pamunkey rivers in Virginia have also been expanding in recent years, he noted.