Smelly algae crowd into prime pond on Vineyard

A stinking apron of green algae is clinging to the shores of Edgartown Great Pond this summer, a mysterious ropey bloom that poses problems for small boats and that is a sign of disruptions in the pond's sensitive ecosystem.

Martha's Vineyard environmental officials first noticed the algae last year, but the bloom was much larger this year when it rose to the surface in July, covering as much as half of the pond.

"It's a thready, slimy algae that's made up of numerous very small-diameter threads. As it washes up on shore and begins to dry out, it kind of has the feel of felt, almost," said Bill Wilcox, water resource planner for the Martha's Vineyard Commission.

And when the algae begin to decay, they stink. The sulfuric smell drifts along Edgartown's beaches and around vacation houses that sell for millions of dollars.

Last week, Wilcox collected a sample from shore and sent it the Smithsonian Institution, which identified the organism as Enteromorpha clathrata. Its growth apparently has been fueled by excess nitrogen in the pond, which is fed by a freshwater spring and is also tinged with salt water that floods in from occasional manmade breaches in the barrier beach on Martha's Vineyard's south side.

When the water is at its high point, the pond surface is roughly 850 acres.

The green mat, Wilcox said, ranges in thickness from 1 to several inches.

People may find it annoying to wade through the gooey glop to get to their sailboats or other small craft, but the algae mass poses a bigger threat to eelgrass, which needs sunlight to thrive on the pond's floor, and the shellfish whose beds could be fouled as the bloom decays and sinks into the water's depths.

Wilcox said the threat to Great Pond's health is disappointing, given the comeback it has made in the past several years.

Shellfish, meanwhile, have had their own struggles in the pond, and that may exacerbate the algae growth.

The parasitic disease Dermo has decimated Edgartown's oysters in recent years, at times killing off more than half the population. Algae that harm the shellfish beds would be one more hurdle for those working to replenish the supply of oysters.

The oysters, in turn, could help tamp down the algae problem. Oysters, Wilcox said, "are like little filtering factories" that help cleanse the pond.

"Without the oysters, the system has a little excess . . . nitrogen," he said. "Nitrogen's a fertilizer and I think it's safe to say it's stimulating the growth of the algae we're dealing with now."

Growing underwater at first, the algae tend to break loose in July and float to the surface, where wind blows the growth to the shores. Wilcox noticed the algae growth last summer, but it has been more pronounced this year and the source is not clear.

There are algae in Jacobs Pond, which empties into Edgartown Great Pond, Wilcox said. But that does not mean Jacobs Pond is the source. The organisms could also have floated in with the tide during the three times a year a temporary channel is cut from Great Pond into the ocean and saltwater flows in and out for several days.

Water resource officials are deciding what to do about the algae, Wilcox said, and are considering ways to rake the growth from the surface, rather than risk letting it decay and settle to the pond's bottom.

Until then, along with the smell - which Wilcox described as rich, moist, and organic even before the algae begin to decay - the green mass also can create hazards for boaters when it tangles a rudder or an engine.

"Then there's the whole aesthetic sense," he said. "When the pond's producing algae, there's the sense that it's not right and it's not as it should be."