If one species of underwater grass is irreplaceable in the Chesapeake, it is the one that is in the most trouble today: eelgrass.

Its absence would doom many areas of the Bay to barren bottom, taking a toll on such species as post-larval blue crabs, which depend on eelgrass beds near the mouth of the Bay for hiding places when they return after spending months in the coastal ocean.

“The first place they settle in are the grass beds,” said Bob Orth, a Virginia Institute of Marine Science researcher.

It is the only perennial grass to persist in large amounts over the winter, providing a year-round source of food and refuge for waterfowl and other aquatic species. Other grasses in the Bay are annual plants, coming to life each spring as temperatures warm up.

“That provides a lot of services in colder weather,” said Ken Moore, a VIMS seagrass scientist, “particularly for waterfowl, which come in and can eat it. It provides sediment control during winter storms that you wouldn’t have if it wasn’t there.”

Depending on how one makes the distinction, eelgrass is the only true seagrass in the Chesapeake. (Scientists debate whether widgeon grass is a seagrass, or a freshwater grass.) Among the distinctions—other than a tolerance for salt water—is that most seagrasses pollinate underwater, while freshwater plants pollinate in the air or on the surface.

“Seagrasses are adapted for growth at great depths,” Moore said. “We have seagrass, eelgrass, growing quite shallow because the water is too turbid. But if you go up to Martha’s Vineyard, or off Wood’s Hole, it is growing 30 feet deep. It is just that the water is much clearer.”

In the Bay, eelgrass has to make do in murkier conditions. Evidence suggest it once commonly grew to depths of 2 meters or a bit more; in today’s cloudy water, they rarely grow in depths greater than a meter.

If eelgrass were to sharply decline, there is little to take its place.

Widgeon grass, which flourishes in the mid Bay, can tolerate higher salinities, but does not grow as deep as eelgrass, nor is it as tolerant of waves as eelgrass, so open areas of shoreline with high wave action could be off-limits. That means many habitats filled by eelgrass today will not be filled with widgeon grass tomorrow.

Another possibility is that Halodule, or shoal grass, could migrate into a warmer Cheapeake from North Carolina. But like widgeon grass, shoal grass tends to come and go. Shoal grass also prefers salinities even higher than eelgrass, so much of the Bay could be less-than-optimal habitat.

“I don’t think it would come very far up the Bay, even if we had it in the Chesapeake,” said Peter Bergstrom, a fisheries biologist with the National Oceanic and Atmospheric Administration’s Chesapeake Bay Office. And, he cautioned, “we don’t know how it would do in the Bay. It might displace the eelgrass in places.”

Grasses found farther south, such as turtle grass—an important species in Florida—would find Chesapeake winters too cold to survive. In short, Moore said, there is no “super seagrass” to thrive in high salinity places if eelgrass is lost. “It’s eelgrass or nothing, for the most part.”