Necropsies on some of the diamondback terrapins point to saxitoxin, a biotoxin produced in algae blooms that has been found in the water at 10 times the normal level.

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JAMESPORT, N.Y. — Hundreds of small turtles have washed up dead on the eastern end of Long Island in the past month, a die-off scientists blame on waterborne toxins that have reached unprecedented levels for reasons that aren’t clear.

Necropsies on some of the more than 200 diamondback terrapins found on the island’s North Fork point to saxitoxin, a biotoxin produced in algae blooms that has been found in the water at 10 times the normal level. The poison collects in shellfish, which are eaten by the turtles in brackish bays and estuaries, quickly causing paralysis and death.

“We’re seeing bodies washing up in perfect condition. This has never happened before. It’s an alarming thing,” said Karen Testa, executive director of Turtle Rescue of the Hamptons, whose volunteers have collected dozens of the dead turtles and sent them to state officials for analysis.

She says all signs point to saxitoxin.

“There’s no other explanation for what’s causing the die-off of these poor animals,” she said.

Christopher Gobler, a professor at Stony Brook University’s School of Marine and Atmospheric Sciences who has studied algal blooms off Long Island for more than 20 years, said saxitoxin is normally detected in the region’s waters, but he has never seen saxitoxin this high and never seen it cause such a wildlife die-off.

Red-algae blooms produce the saxitoxin, which state officials have called a “dangerous neurotoxin” that can damage or impair nerve tissue. Shellfish filter the toxic algae cells from the water and when other creatures chomp down on the shellfish, they can become paralyzed.

Saxitoxin can also cause paralytic shellfish poisoning in humans, which typically results in numbness and tightening in the face and a loss of coordination. In most cases, patients make a full recovery in a few days, but rare cases have resulted in death.

The Centers for Disease Control and Prevention (CDC) estimates 30 cases of poisoning by marine toxins are reported each year, but officials have been unable to give a precise number because there is no requirement that health-care providers report the illnesses. The CDC says an average of one person dies every four years from toxic seafood poisoning.

A spokesman for the New York State Department of Environmental Conservation, which runs a marine toxin-monitoring program and sent representatives to collect the dead turtles, said all signs from initial necropsies point toward saxitoxin, but the agency is sending the turtle’s organs for further testing. Those results won’t be available for several weeks.

“This is a serious threat to public health,” said Adrienne Esposito, executive director of the Citizens Campaign for the Environment. “It’s not a joke anymore. When you have a saxitoxin that can kill humans, you need to address the cause.”

Gobler and Esposito both believe the increase in saxitoxin levels may be related to nitrogen in the water caused by leaking septic tanks and sewage that makes its way into bays, though there appears to be no explanation for why the levels are higher than ever before.

Experts say the damage already done to the eastern Long Island turtle population, coming during the breeding and egg-laying season, could have long-term consequences.

“We’ve seen very few instances like this before,” said Dr. Russell Burke, the chairman of the biology department at Hofstra University, who also studies turtles on Long Island. “It can take decades to recover.”