Ron Rozsa, coastal ecologist with Connecticut’s Department of Environmental Protection, said, as he surveyed a section of the Oyster River salt marsh in Old Saybrook. “We’re talking about a crime scene investigation some forensic ecology, if you will.”

Scientists are calling the mysterious phenomenon sudden wetlands dieback.

The marshes make up about 10,000 acres along Connecticut’s Long Island Sound coast. They are considered the foundation of the marine food chain and buffer the shoreline against flooding and storms. A dieback has also been seen in brackish marshes, which have lower salinity and cover about 3,000 acres in the state. But the problem is not limited to Connecticut. Dieback has been reported in all five of the coastal New England states and is most evident in the marshes of Cape Cod.

Dieback sites have also been documented on Long Island, Virginia, Georgia, South Carolina, Texas and Louisiana where scientists call it brown marsh. It appears to have begun about seven years ago, occurring in isolated but, in some cases, relatively large patches, biologists say. The dieback is causing erosion problems along the shore.

On healthy salt marshes, the smooth cord grass grows in a belt right up to water’s edge, securing the marsh. The death of the grasses effectively means that section of marsh ceases to exist as a productive habitat. “We don’t know what’s causing it, and we don’t know how to stop it. Is it a disease, or a response to a combination of factors?

We want to get a handle on what this thing is,” said Susan Adamowicz, a land management research and demonstration biologist for the U.S. Fish and Wildlife Service.