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Invasive ‘rock snot’ turned out to be a native – so why is it suddenly a problem? 

  • A glob of didymo. N.H. DES


Tuesday, June 28, 2016

An invasive freshwater algae with blooms so nasty they are called “rock snot” has provided an unusual (maybe even unique) surprise in the struggle against imported species: Scientific sleuthing found that it’s not invasive after all, but has actually been lurking unnoticed in New England longer than humans have been here.

This doesn’t make the algae less of a problem, as anybody who has encountered rock snot in the upper Connecticut River can tell you. But it raises an interesting question. If the source of rock snot – Didymosphenia geminata, usually shortened to “didymo” – isn’t new, why haven’t we ever noticed it before?

This is where the science fun shows up.

The story goes back to just before the start of the century, when anglers from New Zealand to Western Canada to New England starting reporting big, slimy mats of algae in back-country streams. It soon was found in the upper Connecticut River in New Hampshire and some waters in Northern Vermont.

The mats were gross – as if an elephant with a cold had blown its nose in the water – and mystifying. Microscopic analysis showed that a species of algae had suddenly gone crazy, its tiny separate diatoms growing enormously long stalks that became intertwined, creating water-clogging mats.

Blooms are standard practice for aquatic algae, as the summertime appearance of nasty blue-green algae mats on New Hampshire lakes reminds us, but these were new and surprising.

“There are plenty of native algaes, but we were seeing these mats showing up that look different from other typical algae blooms. It was identified as didymo and everybody said, ‘Oh, we don’t have didymo. It’s a new introduced species,’ ” said Shawn Good, a fisheries biologist with the Vermont Fish and Wildlife Department.

Biologists have gotten depressingly experienced in reacting to a new invasive species, so both states kicked into gear, launching educational campaigns.

Vermont went further and banned felt soles on wading boots. Soles made of felt are beloved by anglers because they provide the best grip on slippery rocks – the ban was very controversial – but felt is very good at snagging debris, unlike plastic or rubber. These soles were thought to be a likely route used by didymo to spread from stream to stream.

Vermont’s action was prodded by a 2009 scientific paper titled “On the Boots of Fishermen: The History of Didymo Blooms,” by a British Columbia scientist named Max Bothwell. He argued that blooms were being spotted around the world due to a rise in back-country fishing for trout, which was spreading the algae.

“A lot of state agencies saw that paper and took notice,” said Good. “If this thing is spreading literally on the boots of fishermen, we need to take steps to address that vector. . . . We have similar laws here in Vermont in terms of (containing spread of invasive plants) – it’s illegal to pull your boat or trailer out of a boat launch if it’s got plants wrapped around it.”

So far so good, or so bad.

Now fast forward a few years, as a number of other studies came out from New Zealand, Chile and other fishing locales that raised some questions. Enter Bothwell again.

“He started looking at where they were showing up, how they were showing up, the water chemistry where they were seeing the blooms – started digging a little deeper around here,” Good said. Scientists began hunting through ancient sediment cores and other evidence of what plants existed millennia ago and got a surprise.

“They find that didymo has been present in north temperate regions of North America – in many of these rivers – long ago,” Good said.

And so in 2014, Bothwell and others released a new paper, titled “The didymo story: The role of low dissolved phosphorus in the didymo blooms.” In the best scientific tradition, Bothwell looked at the evidence and changed his mind. Politics may deride this as flip-flopping, but researchers celebrate it as – well, science.

The 2014 paper said the sudden appearances of rock snot isn’t happening because the algae has just arrived but because the conditions have changed.

Specifically, and very unusually, these didymo blooms seemed to be caused by a shortage of phosphorus leaching into water from the surrounding soil and environment.

Yes, shortage. This is kind of crazy because algae blooms usually occur when algae encounter too much of a nutrient such as phosphorus and start to multiply like crazy. Why is didymo doing the exact opposite?

Here’s how the New Hampshire Department of Environmental Services explains it:

“Didymo appears to have a special adaptation of producing a stalk that allows it to regulate its depth in the water column, to optimize its ability to obtain low levels of phosphorus from the water. Much like a plant can extend its stalk to grow higher above neighboring plants to capitalize on sun exposure, Didymo secretes a stalk (which is viewed as the ‘bloom’ material on river bottoms), perhaps for a very similar purpose, but for nutrient acquisition.”

Okay so far. But we’re talking about pristine waters, far from human activity, so what causes a relatively recent change in the amount of phosphorus available to didymo?

You guessed it: The most likely culprit is climate change.

“It’s not necessarily the amount of phosphorus that’s causing the blooms – it’s the timing of the spring melt and the runoffs and the volume of the spring runoff . . . that changes the (availability) of phosphorus,” Good said.

“Before, the spring melt was four, five, six weeks – rivers would come up and stay high for a while,” he said. This relatively slow process drew out phosphorus present in the soil and atmosphere and moved it into the river.

“The way springs tend to be now, it seems, is it all happens at once – from winter to early summer in a short amount of time. . . . What that does for phosphorus input . . . now we’re getting flush, boom, then it’s over,” he said. “That kind of robs these rivers of the phosphorus that they’re used to having over a longer period of time. This results in a long-term trend of phosphorus levels in water bodies being lower overall. It seems like didymo is thriving in these conditions.”

So there you have it: Microscopic didymo has been lying around, unnoticed, in many of our streams and ponds for centuries or more. Now that winters are weakening due to our own actions (me, too – I drove a car to work) the chemistry of the water is changing so that these invisible diatoms occasionally join hands, so to speak, and become gobs of slimy, disgusting goo.

As with acid rain or mercury deposition, a byproduct of our society creates unpleasant effects in even the most remote parts of the state.

Since there’s no easy way to add phosphorus to remote water bodies, it looks like we’re stuck with rock snot.

So the next time you don your felt waders and head into the beauty of a New England trout stream, don’t be surprised if something gross gets in your way. It’s probably our fault.