Rock snot, a slimy-looking algae that vexes salmon and salmon fishermen, is not an alien invader but a homegrown threat gaining strength in changing water conditions, according to a new study by scientists from Dartmouth and Environment Canada.
Officially known as Didymosphenia geminata, which scientists shorten to didymo, it has existed in portions of Alaska for some eight centuries, said the study published online last week in the journal BioScience.
Only recently, though, has the nuisance algae been noticed, even in sites considered pristine, thanks to aggressive growth that spreads rock snot stalks in lakes, rivers and streams.
"It's in Patagonia. It's in Tierra del Fuego. It's in Alaska," said study co-author Brad Taylor of Dartmouth College. It is also in British Columbia, New York and various northeastern U.S. states. Sweden, Poland and Colorado see it, too.
Alaska officials have waged a crusade against rock snot. Alaska is among several states that ban felt soles in fishermen's waders; the state Department of Fish and Game cites felt's ability to absorb and transport a plethora of unwanted organisms to new places. The Alaska House this year passed a bill that allows state officials to mount a rapid response to eradicate invasive aquatic species and establishes a fund to pay for it; the bill died, however, without Senate action.
Explosion linked to phosphorus
But when it comes to combating rock snot in Alaska, anti-invader strategies may do little, according to Taylor's study. Core samples at Naknek Lake show that rock snot has been there since the year 1200, the study notes. As long as the didymo did not bloom, Taylor said, it went mostly unnoticed.
"Unless you scrubbed a rock and looked in a microscope, you would never know it's there," he said. "Absence of evidence was used as evidence of absence."
In recent times, blooms have been spotted by alarmed fishermen, some of whom were blamed for the algae's presence, Taylor said. "They would see it and they would report it and they were, sort of, guilty by association," he said.
The explosion in didymo blooms, Taylor and research partner Max Bothwell of Environment Canada found, can be traced to a lack of phosphorus in the water. The plant sprouts stalks that stretch out to reach whatever phosphorus is in the water, and those stalks can wind up matting the floors of rivers and lakes, blocking access to food for fish and otherwise altering habitat.
Phosphorus can be a water pollutant, deposited by fertilizer, sewage and other sources. But in the right quantities, phosphorus is good for streams and lakes, Taylor said,
"Too much of it is a bad thing. It turns out that not enough of it might also be a bad thing," he said.
In the past, the timing and flow of the spring snowmelt carried phosphorous into water bodies in a gradual, sustained fashion. That timing has been disrupted, the new study said, by increasingly early springs prompted by a changing climate.
Earlier snowmelt and other changes in precipitation are sending water into streams and lakes in a pulse-like pattern, flushing the phosphorus that exists in the soil at once instead of gradually, the study said. Earlier soil thaw and plant growth also means that more of the phosphorus is being absorbed by plants growing on the land and by microbes in the unfrozen soil, providing less opportunity for the chemical to flow into the water bodies, the study said. Earlier snowmelt and, in some places, reduced snow pack also means that cloud cover in late spring and early summer is reduced, allowing in more of the solar radiation linked to photosynthesis. That radiation combines with lower phosphorus levels in the water to create ideal conditions for didymo blooms, the study said.
'Not a happy ending'
Another study, published in February in the Canadian Journal of Fisheries and Aquatic Sciences, makes similar conclusions about didymo in Quebec. Blooms were first reported in there in 2006 but have exploded since then. That study, led by scientists at Queens University, used core samples to determine that didymo has existed in Quebec since 1970 -- or even earlier. The recent blooms are likely the result of environmental change, not transplant of invasive organisms, the study said.
"The exceptional rise in didymo abundance . . . suggests that there has been an ecologically important change in the river environment towards conditions that favor didymo and the prevalence of bloom states," the study said.
Those findings suggest it will be difficult to control rock snot. "Unfortunately, there's not a happy ending here," Taylor said.
Still, the felt-sole ban remains a worthy policy even if it does not prevent rock-snot invasions, he said. For one thing, "You don't need felt if you're in a stream with didymo because the didymo is like walking on Astroturf," he said.
More importantly, it prevents the spread of other invaders, including a type of worm that thrives in didymo-stalk mats and infects salmon and trout with whirling disease.
Another menacing aquatic invader, first reported in Alaska in 2010, is an unpleasant-looking sea squirt that can cover sections of seafloor and bears the scientific name Didemunm vexillum. Its common name? Rock vomit.
Reach Yereth Rosen at email@example.com.
By YERETH ROSEN