“Rock snot” refers to Didymosphenia geminata, a microscopic organism formally known as Didymo. This species is a serious environmental concern because its dense, fibrous mats smother the bottom of freshwater rivers and streams globally. Didymo’s proliferation, known as a nuisance bloom, severely alters aquatic ecosystems, making its prevention and management a priority worldwide.
The Organism Behind “Rock Snot”
Didymo is a single-celled diatom, a type of microalgae. Diatoms are unique because their cell walls, called frustules, are made of silica, the same material as glass. This siliceous structure gives the mats a rough, cotton-like texture, despite the misleading “snot” descriptor.
Blooms appear as thick, extensive mats attached firmly to rocks and submerged vegetation. These mats are formed by non-photosynthetic mucopolysaccharide stalks secreted by the living diatom cells. While alive, the stalks appear light brown, but they turn white or gray as they die and decay. It is the bulk of these tenacious stalks, not the microscopic cells, that create the large, flowing, rope-like streamers.
Didymo is native to the cool, low-nutrient waters of the Northern Hemisphere, including parts of Europe and North America. Since the 1980s, however, it has exhibited invasive behavior, forming dense blooms even within its native range and aggressively colonizing new areas like New Zealand, South America, and Australia. This shift from a benign native to a bloom-forming nuisance is not fully understood, but it is now one of the most significant freshwater biosecurity threats worldwide.
Ecological Consequences
The physical presence of massive Didymo mats fundamentally changes the habitat structure of the streambed, known as the benthic zone. These thick coverings smother the rocky substrate, eliminating the microhabitats aquatic invertebrates rely on for shelter and feeding. The dense mats also trap fine sediments, which further degrades the river bottom habitat.
This smothering effect has a cascading impact on the entire aquatic food web, particularly affecting the insect community. Sensitive macroinvertebrate groups, such as mayfly and stonefly larvae, show significant declines in abundance when Didymo is present. These insects are replaced by more tolerant organisms, like midge larvae and worms, which shifts the composition of the stream community.
The reduction in sensitive insect populations directly impacts fish species, such as trout, which depend on these invertebrates as a primary food source. The thick mats can also impede water flow near the substrate, affecting oxygen movement and disrupting important fish spawning areas. Though not a direct health risk to humans, the biomass of a Didymo bloom diminishes the aesthetic and recreational value of pristine waterways, interfering with fishing and swimming.
Preventing the Spread
Human activity is the primary vector for Didymo spread, as the microscopic cells are easily transported between waterways on gear. A single drop of water or a tiny clump of material on waders, fishing equipment, or boats is enough to start a new bloom. A mandatory biosecurity measure known as the “Check, Clean, Dry” protocol is used to mitigate transmission.
The first step, Check, requires the removal of all visible plant material, mud, or clumps of Didymo from all gear before leaving the water’s edge. Any material removed should be left on the bank or placed in the trash, not washed down a drain.
Next, the Clean step involves decontaminating all exposed equipment using approved solutions that kill the cells.
Non-Absorbent Gear
Effective methods include soaking in hot water at 60°C for at least one minute, or at 45°C for a minimum of 20 minutes. Chemical options include a 2% bleach solution for one minute or a 5% to 10% solution of dishwashing detergent, applied for up to ten minutes.
Absorbent Gear
For materials like felt-soled waders, a 5% bleach solution for one hour or a salt solution made from 500g of salt dissolved in five liters of warm water for four hours is recommended.
Finally, the Dry step is crucial because desiccation effectively kills Didymo cells. All gear must be dried completely to the touch, inside and out, before being used in a different body of water. To ensure complete mortality, the equipment should then be left to dry for an additional 48 hours before entering a new stream or river.