Among fly anglers and conservationists, few words stir more confusion—and concern—than Didymo. Officially known as Didymosphenia geminata, this stalk-forming freshwater diatom (a type of algae) can form thick, wool-like mats across the bottom of rivers and streams. When it blooms, Didymo can coat cobbles and gravel with dense brown growths that resemble wet tissue or felt—earning it the nickname “rock snot.”
But behind the headlines and photos of blanketed riverbeds, science is still unraveling what actually drives these blooms, how long Didymo has been among us, and what anglers can really do about it.
A Native and Global Resident
Unlike an invasive species newly introduced to North America, Didymo already has a wide native range—both within and outside of the United States (Blanco & Ector 2009; Taylor & Bothwell 2014). In fact, research shows that it can exist quietly in a stream for decades or even centuries without forming a noticeable bloom (Kunza et al. 2018). And in some waters, dense blooms have appeared in places rarely—if ever—visited by humans (Schweiger et al. 2011).
This means that Didymo’s recent prominence is not solely the result of anglers, waders, or felt soles carrying it from one river to another. Although humans can introduce Didymo to new streams (Kilroy & Unwin 2011; Bothwell et al. 2009), the bigger picture is more complicated.
Why It Blooms: The Nutrient Paradox
Most algae thrive in nutrient-rich environments. Didymo is the opposite. It becomes a problem in waters that are too clean. At extremely low levels of dissolved phosphorus (typically below 2.0 parts per billion), Didymo responds by lengthening its stalks—forming the thick mats anglers see as blooms (Bothwell & Taylor 2017; Kilroy & Bothwell 2012; Bothwell et al. 2014; James et al. 2015).
This paradox has led scientists to propose two major hypotheses for the recent global rise in Didymo blooms:
- New introductions into waters where it didn’t previously exist.
- Environmental change, where long-established populations suddenly begin blooming due to shifts in nutrient levels or climate (Bothwell et al. 2009; Taylor & Bothwell 2014; Kunza et al. 2018).
The evidence increasingly supports the second hypothesis: that Didymo has long been present in many streams but is responding to modern changes in water chemistry—particularly declining phosphorus availability due to atmospheric deposition patterns, improved wastewater treatment, and landscape shifts. Still, scientists acknowledge that phosphorus limitation alone doesn’t tell the whole story (Kirkwood et al. 2007; Kunza et al. 2018).
Impacts Beneath the Surface
Blooms of Didymo can reshape the living community at the streambed. Several studies have shown increased overall invertebrate density (Gillis & Chalifour 2010; Larned & Kilroy 2014), yet the mix of species often shifts away from the mayflies, stoneflies, and caddisflies that trout prefer to eat (Anderson et al. 2014; Jellyman & Harding 2016; Clancy et al. 2021). These classic “indicator species” of clean, cold water may decline under thick Didymo mats, replaced by more tolerant organisms.
When it comes to trout themselves, the story becomes even murkier. Peer-reviewed studies remain few, and their conclusions are mixed. One study found that trout were actually more abundant where Didymo was present (Clancy et al. 2021), another found no difference in trout biomass (James et al. 2016), and a third documented a negative relationship between Didymo and trout numbers (Jellyman & Harding 2016). In other words, science hasn’t reached a consensus on whether Didymo harms, helps, or simply coexists with trout populations.
Angler Awareness and Decontamination
Even if Didymo isn’t always “invasive” in the strict sense, it can be spread to new waters on gear, wading boots, and boats. Fortunately, anglers can take meaningful precautions. Laboratory testing shows that soaking waders and boots in a 5% dish soap solution for at least one minute kills over 90% of Didymo cells (Root & O’Reilly 2012). However, this treatment isn’t perfect—cells embedded in clumps of organic material can survive longer—so scrubbing and drying remain critical steps.
For anglers who wade from river to river, especially across regions, basic decontamination remains one of the simplest and most important acts of stewardship.
What We Still Don’t Know
Scientists continue to study why Didymo blooms occur in some rivers and not others—and why they suddenly appear after decades of dormancy. Are global nutrient trends the key? Are there subtle genetic differences among Didymostrains? Or are we simply observing a natural oscillation in a species that has always been here?
For now, the best approach for anglers is vigilance and humility: respect for the science still unfolding, and a commitment to clean gear and clean water—no matter where we fish.
Youtube link to underwater footage of a Didymo Bloom on the upper Manistee:
References
- Anderson, C. et al. (2014)
- Blanco, S. & Ector, L. (2009)
- Bothwell, M. L. et al. (2009, 2014)
- Bothwell, M. L. & Taylor, B. W. (2017)
- Clancy, C. et al. (2021)
- Gillis, C. & Chalifour, M. (2010)
- James, D. A. et al. (2015, 2016)
- Jellyman, P. & Harding, J. (2016)
- Kilroy, C. & Bothwell, M. L. (2012)
- Kilroy, C. & Unwin, M. (2011)
- Kirkwood, A. E. et al. (2007)
- Kunza, A. E. et al. (2018)
- Larned, S. T. & Kilroy, C. (2014)
- Root, H. T. & O’Reilly, C. M. (2012)
- Schweiger, K. A. et al. (2011)
- Taylor, B. W. & Bothwell, M. L. (2014)
- Brian Hodge, Trout Unlimited (2022)