In 2009, a Canadian researcher by the name of Max Bothwell published a paper that would rock the angling world on its felt-soled heels. The paper was titled “On the Boots of Fishermen: The History of Didymo Blooms on Vancouver Island, British Columbia.” In excruciating detail and embedded with convincing 12-cylinder words, Bothwell laid out how wading anglers had spread the dreaded invasive algae Didymosphenia geminata across Canada, and by inference, the entire planet. The vector was via the felt on boots that, ironically, were specifically designed to cut through the slippery algal slime to gain traction on the clean footing below.
With detailed maps, Bothwell showed that the invasion of the algae, affectionately referred to as “Didymo,” or more descriptively, “rock snot,” took exactly the path followed by trout and steelhead anglers. He proved that porous felt on the anglers’ wading boots not only would inhale the snot, but keep it moist and alive long enough to be transported across watersheds. His paper was so vivid and so commonsensical that angling publications, environmental organizations, government agencies, and conscientious fishing-product manufacturers rose to the occasion to devise and promote “clean angling practices.” One major company, Simms, in an incredibly bold and laudable move, went so far as to quit selling its highly popular and profitable felt-soled wading boots altogether.
To be sure, Didymo wasn’t the first of our invasive aquatic hitchhikers. Whirling disease and New Zealand mud snails had long since been connected with wading anglers. But unlike whirling disease and mud snails, which were out of sight and thus out of mind for the casual angler, Didymo was a face punch to even the most oblivious.
The sallow, fetid wretch that is Didymo looks worse than bad — it looks absolutely toxic. It coats riverbeds in mucilaginous veils of green, yellow, and gray. It covers everything like a gooey sponge and waves sickly tentacles of slime, dar-
ing anyone or anything even to touch it. When you do gather the courage to touch it, it is surprisingly unslimy. It feels like soggy, cheap toilet paper, such as might be found in a Forest Service outhouse. Some have described it as having the texture of wet wool.
Didymo is a diatom. Its cell wall is composed of silica, the stuff of glass and sand. The silica is what gives Didymo its characteristic grittiness. Diatoms are ubiquitous, among the most common types of phytoplankton, and are a basic ingredient in the aquatic food web. When diatoms die, their silica shells (called valves) create a sediment that is mined and sold as diatomaceous earth. We encounter dead diatom skeletons in our daily lives in the form of abrasives in toothpaste and plastic polish, in pool filters, in kitty litter, and as a stabilizer of dynamite.
Alive, though, Didymo is far less user friendly than its skeletal remains. For one thing, it’s not much good as food. The foothill yellow-legged frog population is in steep decline. The California Department of Fish and Wildlife has listed them as “species of special concern.” The larvae (tadpoles) feed on algae and convert their carbohydrates into protein at an amazing rate. When born into an environment with mixed species of algae, the tadpoles feed on each with equal gusto. Unfortunately, Didymo provides little, if any, nutritional value to the tadpoles, and they cease to grow on a diet of the stuff. In an environment overwhelmed by Didymo, the besieged tadpoles have to carry yet another straw on their weary backs.
Didymo also creates a hostile environment for most caddisflies, mayflies, and stoneflies. The algal stalks are simply too dense for them to crawl through, and they quietly disappear. In their place come midges, scuds, and aquatic worms. The total biomass remains pretty much the same, but the size of the individual organisms drops considerably. Trout expend a greater amount of energy snipping out these tiny organisms than they did dining on the standard EPT (Ephemeroptera, Plecoptera, and Trichoptera) diet. Around the globe, anglers confronted with Didymo are learning to fish small flies and redefining the art of patience as their imitations come to hand draped in gooey snot, cast after cast after cast.
Five short years after Max Bothwell dropped the hammer on the wading angling community, he and others came to realize that his assumptions and conclusions were largely wrong. Didymo isn’t the invasive species it had been branded, he discovered. Instead, he found it to be a widespread and native alga that lived unnoticed just about everywhere with the possible exception of New Zealand. (A paper published by Bothwell in the May 2014 journal BioScience suggests that Didymo is not a recent immigrant to New Zealand, either, but has been there all along.) In only the past 20 years, Didymo has created “nuisance blooms” from Canada to Chile, throughout Europe, and on the South Island of New Zealand. From apparently nowhere, it has flash-mobbed the world.
A vexing problem for those who were trying to prove that Didymo is an invasive and running rampant in new-found homes is that it might create a nuisance bloom several years in a row and then completely go away for a few years, only to return at a later date. That’s not how a species invasion works.
So, what happened? If Didymo has been hiding under cover of broad daylight for thousands of years, why hadn’t it bloomed like this before? Well, it has. There is ample evidence that points to prehistoric blooms even before humans set foot on North American soil. In Europe, there are documented reports of Didymo blooms over a century ago. When Bothwell pointed out in 2009 that Didymo seemed to flourish where trout fishers converge, he was onto something, but the link was casual, rather than causal. Trout fishers go where there are trout, and in many instances, where there are trout, there is Didymo. Didymo requires ultraclean water, and in fact, just like trout, it is a biological indicator of pristine water quality.
Didymo also requires the mineral phosphorus to survive. But unlike most other living things that grow in response to nutrients, Didymo blooms in its absence. When phosphorus levels drop to a nearly untraceable two parts per billion (in 2009, Bothwell didn’t have access to a method of testing that was sensitive enough to measure these infinitesimal levels of phosphorus), Didymo responds by growing a long, hair-thick stalk that provides increased water contact and thus is able to extract a bit more phosphorus. A commonality among Didymo blooms worldwide is that they are occurring in waters where winter snowfall has been scant and spring has come early.
When runoff occurs early or not at all, very little phosphorus is entrained in the water, and what small amount does become mobilized gets consumed by plants before it ever reaches the streambed. Nitrogen can also negatively affect Didymo’s ability to metabolize phosphorus, and in years with little spring melt, normally benign background levels of nitrogen can be enough to trigger a bloom. Not too many years, ago President Bush suggested that instead of worrying about climate change, we should simply learn to adapt. Didymo is doing just that, and like so many other events that are evolving in our increasingly warm climate, we are ill-equipped to predict, much less to deal with the consequences.
Does all this mean we can go back to wearing felt-soled boots with impunity? Not really. Didymo provides a nearly perfect incubator for the tubificid worms that can harbor whirling disease. While the Didymo trapped and thriving in your felt sole is probably of little concern, the associated worms can wreak havoc on a fishery. Bothwell’s 2009 condemnation of felt-soled wading boots was the right response, but for the wrong reason. He is excused.
