Preliminary sampling and analysis from researchers with the University of New Hampshire have revealed high levels of dangerous toxins from cyanobacteriain many of the Island’s ponds.

A person who went crabbing in Chilmark Pond was sickened by what appears to be a neurotoxin from blue/green algae blooms in the water, according to Chilmark health officials.

It’s been bad news for shellfishing on Martha’s Vineyard and beyond in recent days, from a state-ordered closure to the discovery last week of a heavy algae bloom in the Tisbury Great Pond.

A state-ordered shellfish closure due to toxic algae blooms was extended to Martha’s Vineyard late Sunday. All Island ponds and harbors are closed to shellfishing — except for bay scalloping.

Cochlodinium, the rust-colored algae bloom that has turned up in Cape Pogue and Sengekontacket Ponds, has now invaded Lagoon Pond, Martha’s Vineyard Shellfish Group director Rick Karney confirmed this week. The algae was found in the west arm of the Lagoon.

Volunteer Edgartown shellfishermen worked the tides last week to transfer young bay scallops out of harm’s way at Cape Pogue Pond, after an algae bloom seen a year ago returned.

Cochlodinium polykrikoides, a single-cell dinoflagellate, staged a late-summer comeback in the large, pristine bay that lies north of the Dike Bridge on Chappaquiddick. The algae is not harmful to humans but can be toxic to shellfish.

On Thursday morning all was right with the Lagoon Pond. The water was clear, blue-green crystal, by all appearances the very picture of estuarine health. Just a day before, the water was clouded by an unsightly yellowish-brown fog from the head to the mouth of the Lagoon. It was an explosion of prorocentrum, an algae, and the largest one that Martha’s Vineyard Shellfish Group director Rick Karney has ever seen.

Experts are mystified by the bloom of an unknown type of algae this summer on the Edgartown Great Pond that has covered acres of the pond’s surface, choking out light to eelgrass beds and then sinking onto shellfish beds.

A sample of the algae was sent this week to the Smithsonian Institution after attempts to positively identify it through records at the Polly Hill Arboretum and through the Woods Hole Oceanographic Institution were inconclusive.