Getting to Know Didymo

Getting to Know Didymo

Didymosphenia geminata is a microscopic alga currently causing headaches for biologists and anglers in two hemispheres. Often called "rock snot" for its

Didymosphenia geminata is a microscopic alga currently causing headaches for biologists and anglers in two hemispheres. Often called "rock snot" for its slimy appearance and gooey consistency, didymo is an emerging problem that could have bad effects on trout rivers in both North America and New Zealand.

Didymo is native to North America, and until recently was thought to inhabit only high-

latitude, high-elevation lakes and streams. However, during the past 20 years its distribution has changed, and scientists are increasingly finding didymo in rivers that do not fit this description. In fact, rivers as far south as Texas and Arkansas now see blooms of didymo, and at this point there are more questions than answers about the slimy invader.

The central question is why this alga, which is endemic to North America, has suddenly spread and become such a problem. Dr. Max Bothwell, a biologist based in Canada, has researched didymo for nearly 30 years, and he tells us one hypothesis for the proliferation is global climate change. He explains that extended drought in the Rocky Mountain West has reduced seasonal floodings that would flush out the streambed of didymo colonies. Another related hypothesis Bothwell has examined links the spread and size of blooms to increased levels of ultraviolet light hitting the earth (some diatoms,

including didymo, thrive in high exposures of UV light).

But compounding the problem is the presence of didymo Down Under. Didymo is not known to be native to New Zealand, but in 2005 it was found in several South Island rivers. The assumed culprits are globe-trotting fly anglers or kayakers. The New Zealand government has responded by quarantining those rivers to prevent the organism from spreading.

The assumption in New Zealand is that didymo is becoming abundant there because it is an exotic, invasive species with no natural controls. If this is true then, according to Bothwell, "the reason it is spreading in New Zealand is different than in North America."

Although unsightly, odiferous and potentially hazardous to wading anglers (the stuff is very slippery), there is no definitive answer about how didymo affects trout. But it inarguably disrupts the river ecosystem during its massive blooms. "The organism has the ability to impact upper trophic levels as well as change the sediment deposition, allowing more fine sediment to be deposited on the streambed," US Environmental Protection Agency biologist Sarah Spaulding says. For instance, larval insects can't navigate through the dense mass of didymo stalk material. But at the same time, according to Bothwell, North American rivers with high densities of didymo often have an increase in chironamid populations.

As for preventing the spread of didymo, Spaulding said anglers are advised to completely dry their gear for at least 24 hours after fishing in a contaminated stream. In addition, a 5- to 10-percent bleach solution will kill didymo cells. In an effort to raise the profile of this problem among anglers, the Federation of Fly Fishers will host a conference on didymo in 2006.