For years, Didymosphenia geminata (Didymo, for short) has been on many states’ high-priority aquatic invasive species lists. Didymo, a freshwater diatom, has the potential to form dense mats on stream and river bottoms, making recreational activities difficult and giving affected waterways an unsightly appearance. Didymo blooms began in Canada in the late 1980s, and have since occurred around the globe in places like New Zealand, Chile, and across the northern hemisphere. Didymo first began to appear in Northeastern states including Vermont in the mid-2000s.

Since the appearance of nuisance blooms around the world, some national and U.S. state governments have implemented regulations aimed at curbing the spread of invasive species with Didymo being the primary focus. Early research into the cause of blooms implicated fishermen as the primary vector. This research suggested that Didymo was picked up “on the boots of fishermen” and brought to places such as western Canada and New Zealand where it is now at nuisance levels. Felt-bottomed boots, which have a porous and difficult to dry sole, were considered the main culprit. In response, seven states including Vermont banned the use of felt-soled wading boots, and numerous other states considered similar legislation.

However, new research suggests that the previous assumptions about Didymo and its spread may be incorrect. Interestingly, this is coming from the same researcher who wrote the original piece that cited fishermen and wading boots as the main perpetrator responsible for worldwide Didymo proliferation.

Last month, research scientist Dr. Max Bothwell presented his current research on Didymo to a regional group of biologists and managers in Montpelier. Employed by Environment Canada in British Columbia, Dr. Bothwell has extensively studied Didymo since its appearance in British Colombia in 1989. He has conducted research on populations in New Zealand, South America, and throughout North America. That previous research was what led him and his fellow scientists to believe that Didymo, like many invasive species, had been spread around the globe by humans, and that recent introductions were causing these problematic blooms.

Dr. Bothwell’s presentation focused on his most recent study, in which he refutes the notion that Didymo is truly an invasive species (a non-native species that has deleterious effects outside of its native range). Bothwell suggests that Didymo has actually always been found globally, and has evidence that Didymo cells existed in British Columbia and New Zealand rivers long before nuisance blooms ever appeared. He also demonstrates that Didymo cells exist in many places worldwide where no bloom has ever been recorded.

Instead, Bothwell asserts that blooms are caused by a rather simple environmental condition – a lack of available phosphorus.

Yes, you read that correctly. Problems with Didymo are being caused by phosphorus levels that are too low, says Bothwell. To a crowd of water quality professionals from the Northeast, this concept raised quite a few eyebrows. This is the same element that has led to so many water quality issues in Vermont, and has led to countless remediation projects.

Didymo cells thrive and reproduce as long as phosphorus levels are above a minimum threshold, which is typically quite low. In this scenario, Didymo cells are abundant, invisible to the naked eye, and never have any discernible effect. Bothwell insists that this has been the case in many systems around the world for millennia. When phosphorus falls below the minimum threshold, however, Didymo cells attached to substrates begin to grow “extracellular stalks.” Presumably, production of these starchy stalks gives the Didymo cells more access to the limiting nutrient (phosphorus) by thrusting the live Didymo cells off the bottom. To use Bothwell’s analogy – think of the stalks like trunks on a tree, pushing energy-producing leaves closer to the sunlight. When viewing a Didymo bloom, such as the one shown here, you are actually looking at these organic, non-living stalks.

Bothwell believes that the appearance of Didymo blooms in the last quarter-century is due to a variety of factors, including atmospheric changes in phosphorus and silvicultural practices that introduce nitrogen-based compounds into riverine systems, which limits available phosphorus through chemical interactions. It is because of these reasons, and not unintentional anthropogenic introductions, that Didymo blooms are now commonplace in areas where they were never documented before.

There are still several questions that remain to be answered. Because of the difficulty in detecting Didymo even with modern methods, there is great uncertainty about where the diatom currently exists. If the locations where blooms occur represent only a fraction of the worldwide range, then we have little information concerning the extent of the species. The potential still exists for Didymo to be transferred from one water body to another. In theory, if brought to streams with extremely low phosphorus levels, then more nuisance blooms could begin to pop up. It does seem clear, though, that the threat of overland transport was initially exaggerated, as Didymo occurs naturally in many systems where no blooms have ever been documented.

Despite new information on Didymo, felt-soled wader bans around the country and in Vermont remain intact. This is due in small part to the uncertainty that still swirls around Didymo, and its current range and ability to cause blooms in locations that may not have native populations. Bans persist mainly because of other aquatic invaders such as New Zealand Mudsnails and the parasite that causes Whirling Disease. Research has shown that these tiny creatures can become trapped and persist in the porous spaces in felt, and therefore can be carried from one waterbody to another. Whirling disease has caused losses to trout and salmon populations in states surrounding Vermont, and New Zealand Mudsnails have (not coincidentally) appeared in some of the most heavily fished streams in the country, both in the West and much closer to Vermont. And while Didymo may not be a non-native invasive, those species and countless others that can be carried by felt soles definitely are.

Article by Josh Mulhollem, Vermont Department of Envionrmental Conservation