Researchers John Tanacredi left and Sixto Portilla, show Horseshoe crabs...

Researchers John Tanacredi left and Sixto Portilla, show Horseshoe crabs at their lab in Sayville Sept. 12, 2014. Credit: Newsday / J. Conrad Williams Jr.

Researchers at Molloy College said they have seen a mass die-off of horseshoe crab eggs and larvae on Fire Island this summer, after they were smothered by extreme amounts of macroalgae blanketing the area.

The macroalgae -- basically ulva, or sea lettuce -- washed up on the beaches of Fire Island in late June, just after thousands of horseshoe crabs crawled up to lay their eggs there, said Sixto Portilla, research and technology coordinator for the Molloy College's Center for Environmental Research and Coastal Oceans Monitoring in West Sayville.

The macroalgae stayed on the beaches for six weeks, starving the sediment and water of oxygen and creating fatal conditions for the eggs and the larvae that managed to hatch, Portilla said.

"We were coming up with clutch after clutch of black, decaying horseshoe crab eggs and dead larvae," said Portilla, who coordinates a yearly study of the horseshoe crab population ecology on Long Island. "These are the horseshoe crabs of next year, of 10 years from now. And they're all dead. There's no chance of anything coming out of this beach this year."

Horseshoe crab blood is vital in medical research, and crab eggs serve as a key food source for migrating birds such as red knots, which stop on Long Island to feed during their yearly trip from Brazil. Horseshoe crabs are also harvested on Long Island by baymen to be used as bait.

The horseshoe crab population has been steadily declining on Long Island by about 1 percent a year for more than a dozen years, said John Tanacredi, professor of Earth and environmental studies at Molloy College and director of CERCOM.

Portilla said the macroalgae coverage was especially severe at Davis Park, which he called "one of the most prolific spawning sites on the Island."

"I've never seen this before," Portilla said. "You get algal blooms like this, but over time either they subside or the tides change and this stuff washes away. Not so on many beaches on Fire Island."

The macroalgae likely benefited from unusually clear water this year, which allowed light to penetrate farther into the water, in addition to high nitrogen loads that allowed them to grow faster, said Christopher Gobler, a professor at Stony Brook University's School of Marine and Atmospheric Sciences.

An uptick in the number of mussels, which filter the water, could have led to the increased clarity, Gobler said, while the Old Inlet breach, created during superstorm Sandy, also could have played a role by flushing new water into the bay.

"Before the breach, in most years, you wouldn't get much in the way of macroalgae in Great South Bay because the microalgae were so dense they wouldn't let light penetrate to the bottom of the bay," Gobler said.

Tanacredi said horseshoe crabs prefer to return to the same beaches to spawn -- making the die-off on Fire Island even more of a problem for the population.

"They don't migrate up to Maine or down to Florida," Tanacredi said. "At breeding season, they come back to the basic same beaches."

Patti Rafferty, a coastal ecologist with the Northeast region of the National Park Service, said she is heading up a team that is studying the importance of Fire Island as a spawning ground for horseshoe crabs. That report is due out in several months, she said.

Rafferty said she suspects that the area has become more vital as coastal development reduces the amount of habitat suitable for spawning.

"One of the things the national parks with coastal shorelines can offer are areas that aren't going to be developed," she said.

If the algae returns to blanket the beaches in future years, Portilla said, he's worried the crabs also will be choked out -- permanently.

"If it happens five years in a row, there will be no horseshoe crabs on Fire Island," he said.

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