Algal invasion
Science Centres: Aquatic Biodiversity and Biosecurity
Didymo growth in the lower Waiau River, showing how the alga completely covers stones and aquatic vegetation.
It’s been dubbed ‘rock snot’ and described as ‘looking like wet toilet paper’. A northern hemisphere diatom (a single-celled freshwater alga) which NIWA identified in two popular trout fishing rivers in Southland is proving ominous.
The diatom Didymosphenia geminata (didymo) was initially spotted in the lower Waiau River, Southland, during a survey by NIWA and Southland Fish & Game staff for Meridian Energy. ‘The whole river is infested for well over 100 km. The plants are very thick in some places and coverage is reported to be expanding all the time,’ says NIWA scientist Cathy Kilroy, who made the discovery. Didymo has also been found in the Mararoa River, which flows into the lower Waiau.
Didymo is not considered a significant human health risk, but Biosecurity New Zealand has classified it as an unwanted organism, which means there’s a penalty of up to $100,000 or 5 years’ jail for knowingly spreading it.
Stopping its spread
NIWA is working with Biosecurity New Zealand, Environment Southland, Agriquality NZ, Southland Fish & Game, and other agencies to establish didymo’s extent, and to assess its potential effects. In mid December, a survey of over 60 sites in other Southland rivers found no trace of it. A priority now is to investigate the conditions in which didymo will establish and thrive.
Biosecurity New Zealand is asking people to avoid the affected rivers. It’s also asking anyone who does use or enter the river to thoroughly wash all items exposed to the water with nappy cleaner, 10% bleach solution, disinfectant skin cleanser, strong detergent, or soak in a salt water solution for two hours. NIWA is currently testing to find out which is the most effective method against didymo.
How to spot didymo
Light micrograph showing live didymo cells and stalks.
NIWA scientists use a microscope to positively identify the alga, but the main visible features are:
- Forms a thick ‘mat’ which is beige, brown, or white. Not green.
- Feels spongy, scratchy, woolly. Not slimy to touch.
- No distinctive smell.
- Very strongly attached to river rocks and other surfaces. Does not fall apart when rubbed between your fingers.