Didymo: Freshwater Diatom
River Ecosystems Being Altered by Poorly Understood Algal Bloom
Nicknamed 'Rock Snot', this alga is appearing in rivers around the world, transported on fishing gear and clothing, reducing biodiversity in its wake.
Rock Snot (Didymosphenia geminata) also known as didymo is spreading from its former range in northern latitude, low nutrient rivers but why this change is occurring is not clearly understood. Although it is native to some river systems in North American, didymo is considered an introduced, non-indigenous species in many parts of the world and an important source of freshwater nuisance algal blooms (NABs) that are reducing biodiversity and water quality.
HABs vs NABs
While Harmful Algal Blooms directly affect human health, NABs do not cause illness in humans. Their biggest impact is the alteration of streambed ecology. The damage caused by Didymo is largely from the stalks put out by the diatom which form large mats on streambeds, decreasing clarity and oxygen availability. In some areas mats 20cm thick have been reported.
Damage Done by Didymo
Numbers and types of macroinvertebrates change when there is significant rock snot present. In New Zealand, midge fly larvae were abundant during blooms but few of the caddis-, may- and stoneflies, which are normally associated with healthy rivers. When didymo numbers were lower there was a greater variety of species indicating better overall river health. In Colorado, there was a loss in total number of macroinvertebrates. Why there are different effects in different rivers is one of the areas of investigation.
Fish are affected by didymo indirectly. Some feed largely on macroinvertebrates. Changes to numbers and type of this food source affect fish health. Herbivorous fish cannot thrive on the non-nutritious stalks that dominate the waters during a bloom.
Although historically rock snot blooms were associated with cold nutrient poor waters, recent studies are indicating that areas with high nitrogen and phosphorus levels are seeing increased growth of didymo. In addition, areas that have low water flow are more heavily affected than areas where swift currents cause rocks to move which scours them, reducing the level of bloom.
Preventing the Spread of Didymo
Rock snot can survive out of water for up to 40 days, as long as it is kept in cool, damp conditions. Fishing gear, waders, water shoes, boats act as vectors. Fishermen moving from river to river without disinfecting gear are likely a prime source of unwitting spread of rock snot. To date, copper is the only proven effective biocide but more work will need to be done to determine what other impacts the copper might have on riverine habitat.
In the meantime the focus is on preventing the spread. Campaigns have been launched in areas being affected by didymo to educate recreational waterway users of the potential problem and to encourage preventive action.
It may never be known why this tiny organism has altered so drastically but following preventive decontamination recommendations can help protect streambed biodiversity.
