An outbreak of potentially fatal algae in Buzzards Bay has prompted the state to ban shellfishing in waters south of Cape Cod until further notice, including Wareham.

The ban includes waters from the Rhode Island border to Nantucket Sound and is one of the most expansive bans to have been announced ever.

“I don’t remember a ban this significant since I’ve been here,” said Wareham Harbormaster Garry Buckminster, who has worked for the town since 1995.

The culprit for the ban is a phytoplankton called Pseudo-Nitzschia that can contain a biotoxin that concentrates in filter-feeding shellfish. The algae were found in very high concentrations in samples collected on Oct. 6 throughout Buzzards Bay, causing the Massachusetts Division of Marine Fisheries to issue the ban. The closure will not affect the harvest of whelks, bay scallops or sea scallops for purposes of extracting and selling or consuming.

Buckminister said the state did not make the decision lightly.

“The quality of the testing that the Division of Marine Fisheries performs is second only to drinking water standards,” said Buckminster. “Public health is the number one issue.”

Consuming infected shellfish could result in amnesic shellfish poisoning, with symptoms that can include vomiting, cramps, diarrhea and incapacitating headaches followed by confusion, disorientation, permanent loss of short-term memory, and in severe cases, seizures and coma.

As a result of the closure, digging, harvesting, collecting and/or attempting to dig, harvest or collect shellfish, and the possession of shellfish, is prohibited.

Buckminster said Harbormaster Department employees have posted signs at shellfishing locations throughout town announcing the ban. In a few cases, people digging during the ban were told to release their catch.

“We’ve had to stop some people already and have them dump their catches,” said Buckminster.

Buckminster said the Division of Marine Fisheries will continue to test waters in Buzzards Bay, Vineyard Sound, Nantucket Sound and Cape Cod Bay for the phytoplankton. Once shellfish are free of toxic levels of the algae areas will be reopened. However, Buckminster said currently there is no timeframe.

“The state is taking all the necessary precautions,” he said.