Where do red tides come from?

Answers to your questions about the news.
June 16 2005 4:35 PM

Where Do Red Tides Come From?

And will the algae wreck my lobster bisque?

Click on image to expand. Red tide at morning, that's a warning.
Red tide at morning, that's a warning

On Thursday, the federal government cordoned off 15,000 square miles of shellfish beds on the Massachusetts coast because of a devastating red tide outbreak. This year's toxic algae bloom is the worst since 1972; Massachusetts Gov. Mitt Romney estimates that it's costing the state's shellfish industry $3 million a week. Why is the red tide in Massachusetts so bad this year?

Wet weather, high winds, and a bumper crop of algae cysts. New England's snowy winter and rainy spring caused an influx of fresh water into coastal regions. This fresh water is packed with nitrogen, phosphorous, and organic acids known as humate—the perfect habitat for red tide seeds, or cysts. The flood of fresh water also created a top layer of lighter, less saline-rich water, a hospitable environment for cysts that normally lurk in mud at the bottom of the ocean. Lots of late-spring sunshine encouraged the algae to bloom.

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On the East Coast, red tides are caused by overgrowths of two types of single-celled algae, Alexandrium fundyense and Alexandrium tamarense. During the spring and summer growth season, each cell replicates around 1 million times. As billions and billions of algae accumulate, shallow seawater may take on the shade of the algae's red pigment. (While outbreaks in Florida often cause rust-tinged water, the choppier, deeper waters up north usually keep red tides invisible.)

Last fall, scientists monitoring Massachusetts's coastal waters noticed a much greater number of cysts—millions more per square meter—dormant in the ocean floor. This bumper crop joined up with the cysts that drift southward from the Gulf of Maine every year. (The Bay of Fundy, tucked into the gulf's northeast corner, has a particular bounty of cysts.) More often than not, these blooms drift off to sea before they get close to the Massachusetts coast. This year, though, May's Nor'easter winds seem to have carried the algae to Cape Cod.

Bivalves like clams and oysters feed themselves by filtering nutrients from water. The poison naturally emitted by Alexandrium, saxitoxin, isn't harmful to clams, oysters, or mussels. But when humans consume an oyster that's been contaminated by saxitoxin, they can come down with paralytic shellfish poisoning, an often-deadly neurological disease. (So far, no one seems to have gotten sick from this latest outbreak.)

This year's red tide is expected to last at least another month. Although the poisonous algae don't affect lobsters, crabs, and fish, state officials have noticed that people are avoiding all seafood, deepening the economic fallout. Scientists are also troubled by the prospect of the red tide dropping massive numbers of cysts in the water, thus ensuring another outbreak next year.

Explainer thanks Don Anderson of the Woods Hole Oceanographic Institution and Jerome Naar of the Center for Marine Science.

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