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Study of Boulder Creek shows link between water flow, nuisance algae

In 2002, Colorado's Front Range was suffering through a historic drought, the worst it had seen in a century. A large snowpack the following year, however, filled the reservoirs and returned life-giving water to streams like Boulder Creek, along with something else local fisherman and environmentalists weren't expecting -- "rock snot."

Rock snot, a form of algae called Didymosphenia geminata or "didymo" for short, is native to the area, according to University of Colorado at Boulder Professor Diane McKnight of the civil, environmental and architectural engineering department. But something has caused the didymo to bloom out of control in parts of the local stream system, prompting researchers like McKnight to label it a "nuisance growth."

"If there's some, that's OK," said McKnight. "But if it covers the stream bed with thick mats from side to side it becomes a problem. The didymo chokes out natural algae in the stream and destroys the habitat for insects on which the fish feed."

McKnight and her colleagues, working with the Niwot Ridge Long-Term Ecological Research Site and Boulder Creek Critical Zone Observatory projects -- both funded by the National Science Foundation -- have discovered higher flow rates in Boulder Creek appear to cause a decrease in the amount of didymo clinging to the rocks.

"When there is a high enough flow, there is some potential destabilization and the rocks move in the stream bed," said McKnight, also a fellow at CU-Boulder's Institute of Arctic and Alpine Research. "That's enough to knock it back."

The findings suggest that controlled flow releases from reservoirs during the summer could be used to limit the impact of this nuisance species in streams in the Colorado Front Range. Flows below Barker Reservoir near Nederland that are above 200 cubic feet per second -- similar to those experienced last year once the reservoir started to spill in late June -- appear to be sufficient to control the didymo.

The team's research was published this month in Hydrobiologia, an international journal of aquatic sciences.

The first sightings of rock snot in the Rockies came in 1994, according to McKnight, but the amounts were so low they didn't raise any concerns. It wasn't until after the 2002 drought that things took a turn for the worse.

"We don't know if it was the drought that caused the explosion of didymo or if it was the man-made changes in the nature of the flow of water after the drought," said McKnight. "But this didymo growth is something that's really changing the stream ecosystems."

McKnight said the findings show there may be a way to slow the didymo before it gets out of hand as it has halfway around the world in New Zealand.

"In New Zealand it's an extreme problem," said McKnight. "It has taken over many of their streams and rivers."

Didymo is an invasive organism in New Zealand and was likely spread by visiting fishermen traveling from stream to stream. The cells of the algae can live for long periods of time without dying, clinging to the bottom of felt-soled waders worn by people while fishing.

"You have to freeze the waders for two days or soak them in bleach to kill the cells," said McKnight.

In New Zealand it is illegal to move from one stream to another without cleaning one's waders. No such regulations exist in the United States.

McKnight said James Cullis, a CU-Boulder doctoral candidate studying water resources engineering, will continue the research over the summer to develop a two-dimensional model to predict how much water flow it would take to create movement in the stream bed at given points in the creek.

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