Bay grass restoration threatened by warming, scientists say
Greater cleanup seen necessary to keep eelgrass from succumbing to heat, murky water
The Bay region is unlikely to meet its underwater grass restoration goals unless it clears up the Chesapeake’s water beyond what is now targeted, scientists warned in a recent journal article.
If more action is not taken, they warn that eelgrass — the primary underwater grass species found in high-salinity portions of the Bay — may face a “catastrophic” decline in the Chesapeake because of a combination of warming temperatures and murky water.
As a consequence, they predict populations of blue crabs and many other fish will also decline as areas with once-lush grass beds convert to muddy bottoms. They project that the resulting economic impacts from that loss of habitat could reach $1.5 billion to $2.5 billion annually.
Nor is it just a problem for the future, the scientists said in a paper published in the Feb. 3 issue of the journal Global Change Biology. Over the last half-century, eelgrass has been eliminated from nearly half the area it once occupied in the Bay. It rebounded slightly in the late 1980s, but since 1991 — a period when grass beds have come back in many other areas — eelgrass acreage has declined 29 percent.
“It is happening now, and it is happening rapidly,” warned Jonathan Lefcheck, a post-doctoral researcher at the Virginia Institute of Marine Science, and lead author of the paper.
Underwater grass beds are one of the most critical habitats found in the Bay. They provide shelter for juvenile crabs and fish, as well as food for waterfowl. They also protect shorelines from the erosive force of waves, and help filter sediment and nutrients out of the water.
Like all plants, underwater grasses need sunlight to survive. In the wake of Tropical Storm Agnes in 1972, grass beds suffered dramatic declines as the Bay filled with sediment and nutrient-fueled algae blooms, hitting a low point of 38,0000 acres in 1983.
Since then, they have made a comeback in many places, reaching 92,315 acres throughout the Chesapeake and its tidal rivers in 2015, the most recent year for which data is available. That’s about half of the Baywide goal of 185,000 acres, which is based on observations made in the decades prior to Agnes.
Eelgrass, though, has declined. That’s a concern because unlike low salinity area of the Bay that can support multiple species, eelgrass is the only seagrass that can survive in much of the lower Chesapeake. In most high salinity areas of the Bay, there is nothing that can take its place. The paper pins eelgrass loss on two factors: loss of water clarity and warming water temperatures.
In many of the eelgrass-dominated areas, water clarity has generally worsened since 1997, the paper said. Eelgrass was once commonly found at depths of more than 1 meter, but murkier water means plants no longer get enough sunlight to survive at such depths.
Meanwhile, gradually warming water temperatures are adding stress to the plants, which are near the southern edge of their range in the Bay. Eelgrass does not tolerate hot temperatures and suffered sharp diebacks after hot summers in 2005 and 2010.
In effect, scientists say, poor water clarity is squeezing eelgrass into shallower areas, but those are also warmer.
Further, there is not enough shallow water habitat available to restore historic levels of underwater grass in high salinity areas where eelgrass is the dominant — and typically only — species, said David Wilcox, a data analyst at VIMS who was a co-author of the paper.
“Unless we get the deep beds back, it would be hard to drive that up,” he said. “It is hard to imagine getting that deeper grass without the clarity that would support that.”
Scientists say they expect further decreases if past trends continue.
The paper said that the impact of warming temperatures alone in the next 30 years would lead to a further 38 percent decline in eelgrass cover. Similarly, if water clarity trends in the Lower Bay remain unchanged, , eelgrass would decline 84 percent. If both trends continue, 95 percent of eelgrass beds would be lost in the Chesapeake in 30 years, the paper said.
Such a loss would reverberate throughout the ecosystem, as there is no other species that would fill the void, resulting in declines of blue crabs, silver perch and a host of other species highly dependent on grass beds in the lower Bay.
“If you’re a guy who wants to take his son fishing on the weekend, you can expect a lot fewer fish out there,” Lefcheck said. “The eelgrass habitat is going away, so all these critters are going to have no place to live.”
Scientists also worry that a catastrophic loss may not be decades away. Eelgrass suffered huge diebacks after previous hot summers: 55 percent after 2005 and 41 percent after 2010.
In both cases, the beds rebounded, but scientists said that likely would not be the case if there are two consecutive hot years — the odds of which increase as average temperatures continue to rise.
The reason eelgrass might die back permanently with a prolonged hot spell stems from the method by which it reproduces. It has root-like structures called rhizomes, which produce new shoots that spread over the bottom, but if the plant is killed in late summer, when water temperatures are at their warmest, the rhizomes die, too.
Eelgrass beds also produce seeds in the spring, which can still produce a recovery the following year even if the plants die during the summer. But if a plant-killing heat spell hits for a second year in a row, neither the seeds nor the rhizomes would be available to spur a comeback in the third year.
In fact, that appears to be what happened at an eelgrass restoration site in the Piankatank River during two consecutive hot growing seasons in the early 1990s, said Bob Orth, a longtime underwater grass researcher at VIMS and co-author of the paper.
“Because there were no seeds, in that third year there were no plants left in the Piankatank,” Orth said, noting that the eelgrass has been largely absent from the river since. “We had an open window into what could happen if we had significant Baywide heat events back-to-back.”
The paper has significant implications for Bay cleanup efforts. Chesapeake Bay water clarity standards are designed to return underwater grass abundances similar to those observed the mid-1900s throughout the Bay. Meeting those clarity requirements requires nitrogen, phosphorus and sediment reductions to ensure that enough light reaches grasses to allow their return.
But, scientists say, those clarity goals never accounted for the impact of warming temperatures on eelgrass.
Eelgrass can withstand “moderate increases in temperature,” the paper said, but only if water was clearer than in the past, so plants would not have to work as hard to get energy from the sun — thereby offsetting some of the stress on the plant caused by the heat.
“We’re pretty certain that if we want eelgrass to return to its previous habitat, you are going to have to get more clarity,” Orth said. “It is a physiological fact.”
Rich Batiuk, associate director for science with the EPA’s Bay Program Office, said that before water clarity standards can be changed, scientists need to determine just how much clearer water would need to be to support the eelgrass restoration in the face of warming temperatures. Then, he said, the state-federal Bay Program partnership would have to determine whether those goals are achievable.
“We may have to rethink what is possible in a Chesapeake that is going to have warmer summers in Virginia’s portion of the Bay,” Batiuk said.
That sets up a tough choice for the region, he added, because losing eelgrass in the Lower Bay would have consequences for the entire ecosystem. For instance, juvenile crabs that find shelter in eelgrass beds later spread throughout the Chesapeake.
“One change there can reverberate around the system, not just in Virginia itself, because it is such an integrated system,” Batiuk said.
Besides Lefcheck, Wilcox and Orth, other authors on the paper include Rebecca Murphy of the University of Maryland Center for Environmental Science, and Scott Marion, of the Oregon Department of Fish & Wildlife.
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