Related FAQs
Yes. Although didymo prefers a river environment with clear water flowing over stable substrate at depths sufficient for light penetration to drive photosynthesis, blooms have been found in South Island Lakes. Lake Wakatipu, for example, contains visible didymo on some parts of the lake shore.
Yes. Didymo will likely continue to spread to unaffected areas within New Zealand. Research on the environmental variables which control didymo's growth (water depth and flow rate, nutrients, light, invertebrate grazing, etc.) has helped identify which habitats and locations it is most likely to establish. Ongoing passive and active surveillance will help determine rate and range of spread. Human activities are considered the most likely source of spread of didymo between rivers and catchments.
We will most likely never be able to determine exactly how didymo first entered the river system. The most plausible method of introduction was the unintentional transport of microscopic didymo on recreational or industrial equipment from affected areas overseas.
Scientists are not certain when didymo first arrived in New Zealand. The first visible blooms were identified in the lower Waiau and Mararoa catchments in October 2004. However, credible retrospective reports indicate didymo was visible in the Mararoa River as early as 2001.
A didymo likely environments model by NIWA shows that most of the highly suitable New Zealand rivers for the establishment of didymo are in the South Island. However, parts of the North Island are also suitable for didymo establishment and growth, particular in the mountain regions and the central plateau. There is no evidence yet of any spread to the North Island, but rivers could be affected. For more information about likely environments for didymo, view the maps on our website. Environments suitable for the establishment of didymo include cool water, stable riverbed and waterflows, high light availability, moderate flow velocity, and pH neutral or slightly alkaline.
For information on where didymo has been found, click here.
Didymosphenia, derived from the Latin 'didymos', means double. Likewise, geminata, derived from the Latin word 'geminus', means twin born, paired or double. Both of these words refer to the bilateral microscopic shape of the organism which looks like a curved bottle with paired bulbous ends. This is due to the pairing of two halves of the silicified cell wall which fit together like a Petri dish, with one half slightly smaller than the other.
Didymo is a member of the group of single-celled aquatic plants (freshwater algae) known as diatoms. Although it is microscopic, didymo can form dense colonies called algal blooms which can be seen with the naked eye. Diatoms are unique in that their cell walls contain silica (a major element in sand), which is why didymo feels gritty when touched. Didymo also contains chlorophyll, a pigment which enables it to make its own food by using energy from the sun. Under optimum growing conditions, didymo cells ooze large amounts of a mucus-like substance (mucilage) which attaches didymo firmly to underwater surfaces. Young colonies look like raised pimples on the surfaces of river rocks, but as the mucilage elongates to form stalks, the colonies form impenetrable mats which form thick strands and can cover all surfaces, including other plants, logs and debris.
Didymo's native distribution is limited to cool temperate regions of the Northern Hemisphere, including the rivers of northern forests and alpine regions of Europe, Asia and parts of North America. It has never been found previously in the Southern Hemisphere. Over the past twenty years, the distribution of didymo appears to be gradually expanding outside its native range. Furthermore, even within its native range, there have been reports of excessive growths in areas where it previously existed only in low concentrations.
Yes. In New Zealand, didymo exhibits several characteristics of an invasive species. In a relatively short period of time, it has grown prolifically as evidenced by the significant increase in biomass throughout the Mararoa and lower Waiau Rivers since October 2004 when it was first reported.
Didymo is an exotic organism to New Zealand, which means it is non-native and was introduced from another geographic region. Exotic species are not necessarily harmful. In fact New Zealand has many exotic valued species such as apples, maize, and sheep. However, when a non-native species occupies lands or waters, particularly in natural communities, it can often cause negative ecological or economic effects because the insects, diseases, or other grazers/predators that help control it in its native range may not be present in the new location.
A number of environmental factors influence how long it will take for didymo to bloom after introduction to a waterway, or whether it will bloom at all, but the process is not well understood. It seems to vary according to river conditions, in some rivers it is visible fairly quickly and in others it is only visible under a microscope.
Didymo populations grow mainly by asexual cell division. Sexual reproduction is likely, but not yet observed in didymo. The presence of a dormant or resting stage has never been reported. Little is known about which factors trigger asexual versus sexual cell division. Nor is it known what role the mucilaginous stalks have on the lifecycle of didymo especially compared to the more common diatoms which do not produce such extreme amounts of mucilage. It is likely the mucilage protects individual didymo cells from short term dehydration and therefore could aid in dispersal. Free-floating cells of other species of freshwater diatoms have been found to survive only a few hours when completely out of water, but if kept moist, they can survive longer.
Didymo is not considered a significant human health risk.
Freshwater fish such as trout, salmon, native galaxiids (whitebait) and bullies are most likely to be affected. Recent science results have suggested the impacts to native fish will be smaller than first thought. Science has shown that didymo isn't as negative as first predicted on invertebrates, and this should be good news for fish stocks.
There have been limited scientific studies on the effects of didymo overseas in areas where algal blooms have been reported. However, anecdotal reports from overseas indicate negative effects on fish and invertebrate habitat, and that didymo behaves as an invasive species. Similar effects are probable here, although recent science results have shown that the numbers of invertebrates increases but the size of the invertebrates decrease. There seems to be minimal effects on fish in didymo infected rivers. Short term impacts have not been as severe as expected. Research on this has been conducted by Biosecurity New Zealand and partner organisations
Surveillance for didymo is ongoing and has been an important part of the response since day one. Formal surveys were first conducted in Southland and Otago regions in December 2004. This was supported by a nationwide call for all organisations regularly working or conducting research in other New Zealand rivers to keep an active look-out for signs of didymo.
More recently the responsibility for ongoing sampling and monitoring work has moved to organisations with regional responsibilities. They select the most important places for sampling in their area and decide how often to sample. Regional groups can store their sample results in the natural didymo samples database.
Biosecurity New Zealand has been the lead agency and is coordinating resources from stakeholder agencies on the ground. This has included communications, enforcement of controlled areas, and water sampling. We now have a Long Term Management Programme in place that involves a number of our key partners working together with Biosecurity New Zealand. The objectives of the Long Term Management Programme are:
- to slow the spread of didymo and other freshwater pests throughout New Zealand;
- protect valued sites and at risk species;
- mitigate impacts of didymo on affected sectors; and
- maintain the North Island free of didymo for as long as possible
Long-term management programme 
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No, there are no plans to eradicate. Past experience with invasive species indicates that eradication of didymo is unlikely. No country has ever attempted to control or eradicate didymo. In New Zealand, a range of management options are being investigated including control, reducing the spread, minimising impacts and protecting high value sites. We are mindful that any control action must not create more environmental problems than it might solve.
No. Eradicating any microscopic organism from a natural environment is virtually impossible, especially in an aquatic environment. We know of no systematic attempts to eradicate invasive blooms of didymo. This is likely because of its widespread distribution and because in a number of countries it is considered a native species.
While we work to understand more about the physical, chemical and biological factors which control didymo, our efforts will continue to be focused on reducing the spread from known affected river systems.
Internationally there is very little information on didymo - its biology, ecology, impacts, surveillance or control techniques. When didymo was first found in New Zealand, the available information upon which to base incursion response decisions was primarily anecdotal due to the absence of validated scientific studies. The recent work in New Zealand led by MAF Biosecurity New Zealand has made us the world authority on this organism.
If you see something you suspect is didymo in an unaffected river, please report the location of the find to Biosecurity New Zealand on 0800 809 966.
Didymo can be distinguished from other species of algae on the basis of:
- Colour - didymo is beige/brown/white but not green.
- Touch - Although it looks slimy, it doesn't feel slimy, but rather spongy and scratchy like cotton wool.
- Odour - Live didymo has no distinctive odour.
- Strength - didymo is very securely attached to river stones and does not fall apart when rubbed between your fingers.
However, didymo can easily be confused with other broth stalked algae, so microscopic analysis is required to confirm identification. Please phone Biosecurity New Zealand on 0800 80 99 66 if you think you have found didymo.
It is the responsibility of organisations holding events to ensure good river hygiene and to bear the costs associated with any decontamination actions.
However, Biosecurity New Zealand can provide advice, information and material on the practical steps for groups wishing to hold waterway based event to minimise the spread of didymo.
Approved decontamination methods are available on the website - including advice on how to apply these methods to specific items that are likely to come into contact with water from rivers and lakes (e.g. fishing gear, jet boats, kayakers, people, animals, vehicles etc). Fact sheets also contain the approved cleaning methods which are available from numerous locations.
Low risk doesn't mean no risk. Low risk areas may not have big blooms but if people don't clean they risk spreading didymo to other waterways. Rivers that are suitable for recreational activities such as fishing and kayaking, also tend to be those that are suitable for establishment of didymo. In its microscopic form it is hard to detect so we need everyone to treat all waterways as if they are affected.
No it's about protecting all of our waterways. To date didymo has not been detected in the North Island; however people still need to Check, Clean, Dry to ensure rivers do not become affected.
Scientists believe that humans are the biggest risk of spreading didymo. There is no correlation between bird nesting areas or colonies with didymo finds. There is however a strong correlation between recreational river use and didymo infestations.
Aquatic weeds like Hydrilla and Lagarosiphon.
Didymo is hard to detect in its microscopic form, so it is invisible in some waterways. People need to treat all waterways as if they might have didymo. People need to Check, Clean, Dry everytime they use a waterway.