RENEE MONTAGNE, host:

Many streams have been invaded by a very slimy algae. It's commonly known as rock snot and it's been spreading through the western part of the country for at least a decade. This year it went national.

NPR's John Nielsen has the story.

JOHN NIELSEN: Rock snot blooms began when a microscopic Dr. Jekyll turned into a Mr. Hyde. Ecologist Sarah Spaulding of the U.S. Geological Survey says this change begins when one-celled organisms called diatoms attach themselves to stream rocks, then gigantic slimy stocks burst out of the cells into the water.

Dr. SARAH SPAULDING (U.S. Geological Survey): So here's where the snot comes in. It makes a mucopolysaccharide stock, so this is essentially slime, and it can be hundreds of times the length of the cell, and that's what really causes all the problems.

NIELSEN: Rock snot blooms can blanket pristine stream beds with thick wads of goo that kill the bugs that big fish like to eat.

Angler Paul Dresher says these blooms can stretch for miles.

Mr. PAUL DRESHER (Fisherman): You put a lot in the water and try to reel back in and you end up with a giant wad of white, gooey, cottony material on the end of it. And it makes it essentially unfishable.

NIELSEN: Until the 1990s, blooms like these are few and far between in North America. Researcher Sarah Spaulding says the diatom that made them, didymosphenia geminate, or didymo for short, were so that researchers went to great lengths just to see one.

Dr. SPAULDING: People would say we ought to rent a van and drive to Lake Superior so we can get some didymosphenia in our collection. So it was something that was a trophy and didymosphenia is especially beautiful when you look at it through the microscope.

NIELSEN: But then something happened and the diatoms got more explosive and more aggressive. Researchers like Andrea Kirkwood of the University of Calgary say it may have been a more aggressive European strain of didymo that was accidentally brought to Canada.

Dr. ANDREA KIRKWOOD (University of Calgary): Or some sort of genetic event, an even where something happened in the species itself that now allows these massive growth formations that we see in cool, cold water habitats.

NIELSEN: In any case, it wasn't long before big blooms of didymo were mucking up prize fishing streams in western states like Idaho, Wyoming and Colorado. And then last summer, rock snot showed up for the first time in New England.

Leslie Matthews, a scientist with the state of Vermont, says it slimed at the Connecticut River in June.

Dr. LESLIE MATTHEWS (Vermont Agency of Natural Resources): We had an infestation that covered miles of the river and was two or three centimeters thick and bank to bank.

NIELSEN: Before the summer ended, similar didymo blooms had fouled streams in New York, New Hampshire, Tennessee, Missouri and Virginia. Fisherman Paul Dresher of New Hampshire says there's one thing that is now clear: anglers who move from stream to stream have helped spread these blooms around.

Mr. DRESHER: The anglers and recreationists travel worldwide now and when they bring their equipment back and don't adequately sanitize it, they run the risk of spreading this material, and that's pretty certainly how it got into northern New England.

NIELSEN: And to all those western states and into well-known fishing streams half a world away on the south island of New Zealand. Experts say there's no good way to stop an established rock snot bloom, but officials in New England say they are hoping to slow the rate at which the blooms are spreading.

Fisherman Paul Dresher says they've done it partly by posting lots of signs that urge anglers to clean and dry their boots and gear.

Mr. DRESHER: So if you travel around northern New England right now, you're going to find these little posters nailed to telephone poles and trees at every spot where anglers know other anglers go, telling them don't spread rock snot.

NIELSEN: Is that what they say, stop rock snot?

Mr. DRESHER: That's what it says.

(Soundbite of laughter)

Mr. DRESHER: It catches people's attention.

NIELSEN: Whether these signs will do any good won't be known until early next spring, when scientists go out looking for new rock snot hotspots.

John Nielsen, NPR News, Washington. Transcript provided by NPR, Copyright NPR.