For years, those fishing in East Coast waterways have faced bans on felt-soled boots and urgings that they scrub their gear to combat the spread of a pervasive algae. But a recent Dartmouth College study could turn such thinking on its head.
Didymo, a species of diatom that produces threadlike stalks called "rock snot" blooms, long has been believed to represent an invasive threat to bodies of water across the nation, but the algae is largely native to those areas, according to the study.
After reviewing records documenting didymo's presence in rivers around the globe, a team of researchers led by Dartmouth professor Brad Taylor found that cells of the algae had been present in those bodies of water for centuries — in some cases, even thousands of years — but environmental conditions triggering the rock snot blooms had been rare or absent.
"A lot of the response to didymo has been, 'How can we keep it from spreading?' " Taylor said. "Our work suggests that in a lot of areas, in the areas just up the road in Pennsylvania and just south of Maryland and Virginia, didymo has been there at least 50 years. In some cases, in the case of Pennsylvania, it's been there thousands of years."
The team began the study in summer 2012, when Candian government agency Environment Canada researcher Max Bothwell visited Taylor at a field site in Colorado, Taylor said.
"I told him that didymo has always been in these rivers — we've known that for a long time — and it only blooms in some of them," Taylor said. "We started talking more about perhaps didymo is native and just had gone unnoticed in large parts of the world. … It's been found all over the world and for a long time, not just the past 10 years."
The extensive growth of rock snot blooms that river users and environmental groups have witnessed over the past several years hasn't resulted from its spread from river to river, as widely believed; it is caused by a shortage of phosphorous and other factors resulting from climate change, Taylor said.
"When phosphorous concentrations are extremely low, didymo produces this stalk to push itself up into the water column to access nutrients," Taylor said. "It's very similar to beanstalks; if you're going to grow a bean plant … when you starve the plants of light, they grow tall and spindly."
Blooms of rock snot, so named for its mucuslike appearance, harm salmon and trout populations by suffocating their habitats, allowing the insects that the fish feed on to burrow into the stalk mats.
The research team now is concentrating its efforts on determining how humans are contributing to rock snot blooms' growth, Taylor said.
"We spew nitrogen out of our cars and coal-fire power plants, and then that rains down and fertilizes systems with just nitrogen," he said. "You then have plants that are fertilized by nitrogen but not phosphorous, and so that results in a decrease in phosphorous going into lakes and rivers."
Additional environmental factors resulting from climate change also deprive didymo of phosphorous, Taylor said.
"A shift toward earlier growing seasons and an earlier pulse of springtime stream flows — apparently, in the Baltimore region, some heavy snowmelt occurs higher up in the Delaware drainage — those occur earlier in the year, and the rivers are basically starved of phosphorous later in the year," Taylor said.
Taylor said the team's discoveries indicate that multimillion-dollar algae eradication efforts and fishing restrictions won't prove effective in combating rock snot blooms.
In 2011, in the hopes of stopping didymo's spread, Maryland became the first state to ban felt-soled wading boots; algae can nestle into felt fibers and be deposited in other bodies of water. Maryland's Department of Natural Resources also set up wader washing stations for those fishing on the river to wash their gear.
"If it's already there, it's not going to do any good to try and stop the spread of it, obviously, and so the action might be better put into figuring out what's causing it to bloom and then ways to regulate that or ways to simply adapt to it and deal with it," Taylor said. "The idea of trying to poison it is one I think isn't going to be doable, and it's probably not going to solve the root problem."
Theaux Le Gardeur, Gunpowder Riverkeeper and fishing guide, said didymo is primarily visible in the river from February through April, when the algae's stalks are blooming.
"We still have some didymo present, but the river is flowing at about twice the median flow over the past 13 years, so accessing the river is a real problem for the majority of river users," LeGardeur said. "It still is a nuisance for anglers because it fouls the hooks of the flies and the weights."
Ronald J. Klauda, DNR director of monitoring and assessment of nontidal waters, said what Maryland scientists have seen so far of didymo doesn't necessarily square with the study's conclusion that the organism has been widespread but scarce in many streams, and that it has been triggered to grow aggressively by changing stream conditions.
Klauda said DNR noticed didymo only after anglers started reporting it. The agency first got reports of it in the Gunpowder River in 2008, he said, followed in subsequent years by sightings in other rivers and creeks. Despite looking for it in streams where anglers haven't reported it, scientists have yet to detect it, he said.
"Whether it's native or non-native is more maybe of an academic question in my mind, because it's doing something it hadn't done before," Klauda said. "Assuming it's been here forever, why are we now seeing blooms occurring?"
The researchers' suggestion that didymo might bloom in waters low in phosphorus doesn't appear to apply in Maryland streams, especially the Gunpowder and Savage rivers, Klauda said. Phosphorus levels in those two streams rarely get as low as those in New Zealand streams where blooms have occurred, he said.
Klauda said his greater concern is whether rock snot blooms in early spring are having an ecological impact on streams, especially their fish populations. While impacts have been reported from didymo blooms in New Zealand, Klauda said Maryland scientists haven't seen any here yet. Research is continuing, he said, and in any case, it's a "seasonal nuisance" to anglers.
Klauda said Maryland followed the "precautionary principle" in urging all fishermen to clean their waders and fishing gear and in banning felt inserts. The felt ban was instituted not just to prevent transfers of didymo from one stream to another, but also to guard against the spread of other potentially harmful diseases and organisms, including "whirling disease."
"There were some studies that [showed] these were very efficient transporters of organisms," Klauda said of the felts. "At the time, it was the right thing to do. I still think it was the right thing to do."
While some anglers have complained about it, Klauda said the ban has created far fewer ripples than he expected.
"I think most anglers really want to do what's best for the system," he said. "How can they hurt anything by having clear gear?"
Such misgivings aside, Taylor said didymo could prove a valuable case study into climate change's effects on native species.
"Didymo may be really sensitive to the climate-induced changes and environmental conditions, and so it may be a good canary in the coal mine, if you will, [for] other changes that are going to happen in rivers," Taylor said. "Here's this native species that's being affected by changing environmental conditions and causing a nuisance, and that's something I think we need to be more aware of in the future, and it may be more common."