Preliminary findings suggest some of the Chesapeake’s underwater grass beds were hurt by heavy rains, heat
Karl Blankenship, BayJournal.com
Portions of the Chesapeake Bay’s underwater grass meadows appear to be headed for steep declines this year, a delayed response to the torrential rains that poured vast amounts of water-fouling sediments and nutrients into the estuary during 2018.
Initial reviews of this year’s aerial survey show significant losses of underwater grass beds in parts of the Mid Bay, where the bulk of the Chesapeake’s underwater grass beds are located.
At the same time, preliminary reviews of the aerial images show that portions of the Upper Bay survived last year’s deluge of muddy water surprisingly well, with grass beds even expanding in some areas.
“It’s going to be a mixed story, as it always is,” said Bob Orth, the Virginia Institute of Marine Science researcher who has been overseeing the aerial survey since its inception in 1984.
The full analysis of this year’s survey, which is still underway, won’t be available until early next year. But the broad picture is starting to emerge as Orth and others pore over the hundreds of aerial images gathered thus far.
Underwater grasses are one of the Bay’s most critical habitats, providing food for waterfowl, juvenile blue crabs and many types of fish. Because they require clear water to survive, they are a closely watched indicator of the Bay’s health.
Grass beds had been steadily expanding in recent years. They covered 104,843 acres in 2017, which was the largest amount observed since Bay cleanup efforts began in the mid-1980s.
Last year, scientists say the Bay grasses may have surpassed that level before the weather turned unusually wet in July, resulting in record-setting rain for much of the watershed.
Rainfall scours sediment off the landscape along with nutrients that spur algae blooms. Carried into rivers and downstream to the Bay, the sediment and algae blooms blot out the sunlight essential for aquatic plants to survive.
The unrelenting rain and ensuing muddy water prevented the aerial survey from being completed. But the 78% of the Chesapeake where images were gathered contained 91,559 acres of underwater grasses. Had the survey been completed, scientists believe total acreage might have topped 108,000 acres.
“I think we would have been well on our way to surpassing the 2017 levels if the last year hadn’t turned into such a muddy mess,” said Brooke Landry, a biologist with the Maryland Department of Natural Resources and chair of the state-federal Bay Program’s Submerged Aquatic Vegetation Workgroup.
The Bay restoration goal is 185,000 acres.
The strongest increases last year were in the mid– and high-salinity regions of the Bay. Those areas showed significant increases, mainly because of the expansion of widgeon grass, which showed up in some areas where it had never been previously reported.
“Baywide, the story [was] really the huge expansion we’ve had in widgeon grass,” said Dave Wilcox, a VIMS scientist who works on the survey. “That’s what is driving our numbers in the last several years.”
Widgeon grass is the most abundant underwater grass species in the Bay, but it is notorious for being a boom-and-bust species that can quickly disappear when water quality declines.
And, Orth said, this year’s survey is showing that many of the widgeon grass-dominated areas of the Mid Bay and some areas of the Lower Bay appear to have suffered significant losses, which likely means overall Bay acreage will be down sharply when this year’s survey is completed.
Places like the Honga River and around Smith and Tangier islands had “luxuriant beds” in 2018, Orth said, “but it’s a different story this year.”
Also, poor conditions and warm water also appear to have taken a toll on eelgrass beds in the Lower Bay, Orth said. “The numbers, I hate to say, I think are going to be really low in those areas,” he said.
But the news isn’t bad everywhere. Farther up the Bay, freshwater species of grasses are doing well in many places this year, including the Choptank, Chester, Severn and Magothy rivers, some of which have more underwater beds this year than they did in 2018, Orth said.
“Everyone was concerned about turbidity from this vast amount of freshwater that we had last year, but it seems to have had a positive impact on the freshwater plants, and they seem to be not nearly as affected by the turbid water as the Lower Bay plants,” Orth said.
Many of the freshwater grass species form canopies that reach to the water surface which may have helped to protect them from the murky conditions, he said.
Images show the large bed at Susquehanna Flats, near the mouth of the Bay’s largest tributary, appears to have survived largely intact even though it was subjected to months of high river flows and muddy water.
“It’s resilience at work,” Landry said. “We had gotten to a point where we had a lot of nice, dense grass beds, and we put them through the wringer last year. I think we’ll see some loss this summer, but it appears as though we have gotten them to some point of resilience where they do withstand more stress.”
Indeed, satellite images from last fall showed that muddy water pouring out of the Susquehanna was not penetrating to the interior of the Susquehanna Flats grass bed. Instead, the dense grass growth was shunting chocolate brown water to either side of the bed.
Still, the final analysis will likely show the high flows diminished the bed somewhat. Cassie Gurbisz, a biologist from St. Mary’s College of Maryland who has been studying Susquehanna Flats for years, visited a number of sites in early August. While all continued to have grasses, she said many seemed patchier than they did before last year’s high flows, but her analysis was still under way.
A major concern involves the fate of eelgrass, at the opposite end of the Bay.
Eelgrass is the dominant underwater grass found in high-salinity areas. Last year’s survey found robust beds in the Lower Bay, but hot temperatures last summer — possibly compounded by an influx of low-salinity water — caused mass defoliation later in the fall. Orth said it appears that high temperatures this year are taking a toll again.
“It’s a concern, and part of the climate change scenario,” he said. That could leave large areas of the Lower Bay, where beds are especially important for juvenile crabs, denuded of vegetation.
The need to restore underwater grass habitat is one reason that the Bay cleanup effort aims to reduce nutrient and sediment pollution, as water clouded by sediment or nutrient-fueled algae blooms can be lethal. Like all green plants, submerged grassed need sunlight to survive.
Grass beds are also a critical component of the Bay ecosystem in their own right. In addition to providing food for waterfowl and shelter for fish and crabs, they pump oxygen into the water, trap sediment and buffer shorelines from the erosive impact of waves.