Scientists heartened by baywide increase in underwater grasses

Karl Blankenship
Posted 8/10/15

The amount of underwater grass beds in Chesapeake Bay surged 27 percent last year, one of the largest single-year increases since monitoring of the critical habitat for fish and crabs began three …

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Scientists heartened by baywide increase in underwater grasses


The amount of underwater grass beds in Chesapeake Bay surged 27 percent last year, one of the largest single-year increases since monitoring of the critical habitat for fish and crabs began three decades ago.

While a rebound was seen Baywide, much of the recovery in 2014 was driven by a huge expansion of widgeon grass in the Mid Bay. Widgeon grass is notorious for its boom-and-bust cycles, which means it can disappear as quickly as it pops up.

Bob Orth, a researcher with the Virginia Institute of Marine Science, who oversees the annual aerial underwater grass survey, said 2014 was interesting because widgeon grass was found in areas where it had never before been mapped.

He and others were so surprised when the photos showed beds in certain areas, such as parts of Pocomoke Sound and the Honga River, they had people visit the sites in person. “Sure enough, it was,” Orth said.

Meanwhile, beds in freshwater and low-salinity areas, while having an increase in grasses, remain at half the record-high levels observed in 2010, before the devastating impacts of Hurricane Irene and Tropical Storm Lee in 2011. Those storms pushed huge amounts of sediment-filled water into the Bay, blotting out grass beds.

In the high-salinity areas of the lower Bay, eelgrass is rebounding after a heat-related die-off in 2010, but scientists are concerned that the critical species remains in long-term downward trajectory.

The overall results from the annual Baywide aerial survey showed 75,835 acres of underwater grass beds in the Chesapeake and its tidal tributaries last year, up from 59,711 acres in 2013.

“What we do know now is that restoration and conservation efforts to clean up the Bay’s waters are paying off,” said Brooke Landry, a biologist with the Maryland Department of Natural Resources and chair of the Bay Program’s SAV Workgroup.

It was the second straight year of increase for submerged aquatic vegetation, or SAV, after a dramatic three-year decline from 2010 through 2012 that saw underwater grass acreage plunge to its lowest levels in the Bay since the 1980s.

Still, SAV acreage remains well below its recent highs of 89,659 acres in 2002, and is just 41 percent of the Chesapeake Bay Program partnership’s ultimate goal of 185,000 acres. But, Landry said, shorter term Bay Program goals of 90,000 acres in 2017 and 130,000 acres in 2025 are achievable.

“I think we have a good chance of meeting both of these interim goals if we continue efforts to cut pollution that runs into the Bay,” she said.

Submerged aquatic plants need clear water to get the sunlight they, like all green plants, need to survive. Because of its tight link to water clarity, the annual survey of Bay grasses — formally known as submerged aquatic vegetation, or SAV — is one of the most closely watched indicators of how the Bay is doing.

“I can’t stress enough the importance of underwater grasses as a measure and indicator of the Bay’s overall health,” Landry said.

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.

All salinity zones of the Bay saw overall increases 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 15,305 acres last year, a 9 percent increase over 2013. That’s 74 percent of the goal for that zone.

• The slightly salty oligohaline salinity zone, which occupies a relatively small portion of the Upper Bay and tidal tributaries, had 7,413 acres last year, a 32 percent increase from 2013. That’s 72 percent of the goal for that zone.

• The moderately salty mesohaline salinity zone contains the largest potential underwater grass habitat. It stretches 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 37,260 acres of grasses last year, an increase of 47 percent over 2013. That’s 31 percent of the goal for that zone.

• 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 includes the lowermost parts of the York and James rivers. It had 15,857 acres last year, an increase of 8 percent from 2013. That’s 47 percent of the goal for that zone.

While the overall increase is good news, scientists remain concerned that the increases in recent years have been disproportionately driven by widgeon grass in the midsection of the Bay. That species often disappears as rapidly as it pops up.

Once, those vast grass beds included a mix of other species, such as eelgrass, red head grass, sago pondweed and others, which gave the beds more stability.

“In the historical past, it was mixed with other species, so when widgeon grass flamed out, other species picked up,” Orth said. “Now if it flames out, you have nothing.”

Scientists remain concerned about high-salinity areas of the Bay, where eelgrass is by far the dominant species. While acreage has risen, eelgrass covers only about half the acreage it did in the early 1990s.

Eelgrass, which is at the southern end of its range in the Chesapeake, is sensitive to heat and suffered major die-offs during the hot summers of 2005 and 2010. Although grasses bounced back after each of those summers, it never returned to previous levels.

“There are whole sections of rivers that don’t have eelgrass anymore,” Orth said.

Its recovery is further hampered by poor water clarity which 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.

“There is a downward trend happening there from 2005 to where we are now,” said Dave Wilcox, who works on the annual aerial survey at VIMS. “We are a little concerned that we are leveling off now at a level that is below where we were in 2010.”

Low-salinity areas continued to rebound as well, particularly in the Upper Bay which was especially hard hit by Tropical Storm Lee in 2011.

In the wake of Lee, the Susquehanna Flats, which at about 10,000 acres was by far the largest grass bed in the Bay, was reduced by half. But grasses on the flats, which are particularly important for waterfowl, continued to rebound, and rose about 7 percent last year as grasses began expanding into deeper water.

Overall, Susquehanna Flats has about 5,700 acres of grasses, still the largest of any single bed in the Bay.

This year’s aerial survey is under way. Preliminary results, primarily from the lower part of the Bay, suggest continued SAV gains this year, though increases appear smaller than those seen last year, Orth said.

“What we’ve seen seems to be a slight increase in some areas over last year,” he said. “It won’t be a huge increase.”

But, he added, the survey has yet to cover the Mid Bay area dominated by widgeon grass, and those results will greatly impact Baywide numbers. “That could drive the changes,” he said. Distributed by Bay Journal News Service

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