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Granite Geek: What is "Rock Snot?"

A glob of Didymo

Researchers say an algae called "rock snot" that was thought to be an invasive species in the Northeast is actually native to the northern United States. So if “rock snot” has been here for a long time, why haven’t we noticed it before? To answer this question we turn to Granite Geek David Brooks. He’s a reporter with The Concord Monitor and writer at, and he joined NHPR’s Peter Biello to discuss the matter. 


If “rock snot,” also known as didymo, or didymosphenia geminate, is not invasive but native to northern New England, then why haven’t scientists noticed until recently?

It is one of many algae that live in our waters, and most of the time they live in microscopic form—in little diatoms. They are tiny. If there’s no reason to look for them, there’s no reason you would spot them. We became aware of it 15-20 years ago when blooms started happening. These microscopic algae would form slimy mats that are known colloquially as “rock snot,” because that’s exactly what they look like—it’s like an elephant blew its nose in the river. Suddenly this started cropping up, in New Hampshire and Vermont, among many other places around the world. And nobody had ever seen it, so they said, “Hey this is an algae, it must be invasive!”

What did scientists previously think was responsible for the spread of rock snot?

When you find a new invasive species, you try to figure out why it showed up, and how it was brought here. Nine times out of ten it was brought here by humans. So the assumption was that it was being carried inadvertently from one pristine stream to another—by people. In New Hampshire, the only places that rock snot blooms were found were in the upper waters of the Connecticut River, which are basically nice trout streams. That was very common, compared to what was happening around the world.

There was an influential paper in 2004 that said the most likely vector was fisherman, going from one terrific trout stream to another, accidentally carrying these little diatoms on their feet. Vermont, for example, outlawed felt-soled waiting boots. They are the best kind of boot if you’re an angler because they don’t slip very much, but felt really grabs onto stuff when you step on it. So they outlawed the boots, thinking that was one of the major reasons this algae was being carried from stream to stream.

What do they think now about how it was spread?

After a lot of places started acting as if it was an invasive, some of the biologists involved in the 2004 paper kept looking at the issue. They found some oddities that they didn’t quite understand, and they looked into it further. They looked into core sediments—so basically fossilized mud from thousands of years ago—and started looking for didymo. They had never looked for it before, because there was no reason to. They found it, and realized that it has been around in the northeast and many other parts of the world for a long time. It’s not an invasive at all.

That was the first big surprise. To my knowledge, this is the only time I’ve ever heard of a species that was thought to be invasive turning out to be native. Usually it’s the other way around: you have something you think is native to the area and it turns out it was brought here in 1847 by some guy coming from England.

So the other question was, if it’s been here for a thousand years, how come we are only just noticing these disgusting rock snot blooms now?

That took a fair amount of studying. The most likely explanation is that there isn’t enough phosphorous in these pristine waters. This is doubly unusual, because not only is this an invasive that turned out to be native, but it’s an algae that blooms when there aren’t enough nutrients. Usually we get these algae blooms on lakes and ponds when there’re too many nutrients flowing into it. This is the opposite, which is a bit of a head scratcher.

Does the fact that it’s not an invasive species change the way scientists think that rock snot should be managed or handled?

Absolutely. Vermont, for example, has rescinded its ban on felt waiters because you don’t have to worry about people carrying it from place to place if it’s already in all those places. The question then is, how can you change the conditions that make it bloom?

This is, unfortunately, where the usual culprit shows up: climate change. It’s the most likely cause of these blooms. It’s not absolutely established, but the working hypothesis right now is that there’s been a change in the pattern for snowmelt and spring runoff. It used to be much more gradual, because there was more snow in the mountains and it would slowly melt as winter turned to spring. So the algae would slowly wash down, and in the process of slowly washing into these pristine streams, they would pull phosphorous out of the air and the soil. But these days we don’t get as much snow, and it gets warm fast so all the snow melts faster. So instead of getting a long, slow release of all the phosphorous, you get it— “BOOM”—all at once as everything washes out. The net result is that there’s less available phosphorous in the water, and the algae responds by causing these disgusting blooms.

How do you fix that? That’s a tough one: you have to fix climate change, which, as we know, is not going to be easy. 

Peter Biello is the host of All Things Considered and Writers on a New England Stage at New Hampshire Public Radio. He has served as a producer/announcer/host of Weekend Edition Saturday at Vermont Public Radio and as a reporter/host of Morning Edition at WHQR in Wilmington, North Carolina.

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