Didymosphenia geminata



Legal Status: Unwanted Organism - MPI
Status in New Zealand: Investigating
Organism: Marine and freshwater plants

Since the detection of Didymosphenia geminata (didymo) in October 2004 MAF Biosecurity New Zealand has commissioned a number of research reports. The overall objective of the science and technical programme was to determine validated information regarding identification, detection, distribution, containment, impact and control/eradication of didymo. The desired outcome was to reduce uncertainties underlying MAF Biosecurity New Zealand decision-making on response options and long term management options for didymo, as well as to provide management tools that support the actions of our partners to slow the spread and minimise impacts of didymo.

Since the didymo response was transferred to a long term management programme in 2008, programme partner organisations have contributed funding to NIWA and Cawthron research initiatives.

MAF continues to investigate control options for didymo and supports ongoing NIWA research looking into the effects of micro-nutrients on didymo blooms, and Cawthron research to establish the influence of environmental factors on didymo growth.

Key Themes

Worldwide, information on didymo is generally scarce and in some cases these reports provide initial findings and indicate directions for further study. This guide is meant as a brief introduction to the reports available on this page to show where to look for detailed scientific information on some of the more commonly looked for topics of interest. For complete discussion of the topics presented under the headings below, please refer to the reports referenced in the drop down boxes. A summary of the findings of each individual report is available by clicking on the report links provided with each heading below.

Note: For frequently asked questions on general didymo topics, please see the FAQ section: http://www.biosecurity.govt.nz/node/1197/related_faqs

The impact of didymo between 2006 and 2011 is estimated to be $127.8 million. Earlier assessments expected the impacts to be between $58 million and $285 million over the eight years 2004/05 to 2011/12. The main reason for the low impact is due to didymo not spreading to the North Island. Deloitte conducted this assessment in conjunction with the New Zealand Institute of Economic Research and found that the 10-year benefit-cost ratio is 5.9. That is, for each dollar of intervention spending, there are nearly six dollars of benefits, when viewed from 2006 to 2015.

The economic analysis quantifies the cost in financial terms of the potential impacts identified by the didymo TAG (Technical Advisory Group), taking into account the potential range, rate of spread and impacts on the sectors: commercial eel fisheries; municipal, industrial and agricultural water intakes; community, municipal and domestic drinking water; local recreation values; international and domestic tourism expenditure; local and national existence values; and existence values associated with extinction of endangered native species. Economic Impact Assessment

The impacts on economic, cultural and social values are also examined in an earlier risk assessment based on information gathered in stakeholder workshops conducted around New Zealand. The risks to these values were found to be high to extreme. Organism Impact Assessment

International economic impacts were briefly outlined in the preliminary impact assessment. Preliminary Impact Assessment

The species is assumed to be indigenous in Northern Hemisphere boreal or montane regions. Since the mid-1980s its range has gradually expanded in Europe and North America, often forming large nuisance growths. An initial report Preliminary Impact Assessment surveyed the literature and expert opinion regarding the then current knowledge of didymo habitat and biology. This report also considered possible means by which didymo reached New Zealand and concluded that the most plausible explanation is that the species has entered the region on a human vector.

Later DNA testing revealed that the NZ infestation was likely to have originated in North America. DNA Detection Studies
Ongoing monitoring has produced a number of survey reports available below. For a summary of the findings, please see the section: http://www.biosecurity.govt.nz/pests/didymo/where-is-it
Preferred habitat

The preliminary impact assessment Preliminary Impact Assessment summarises the international range and habitat of didymo. The first ecology report attempted to identify the ideal hydraulic habitat for didymo in NZ by studying several sights already infested in the South Island. Didymo was found to thrive in a wide range of environments. Further research was suggested to identify the relationship between water temperature and growth. Habitat was again examined in a follow-up report, which included the effect of floods on didymo growth and the natural growth limitation in spring fed streams (section 7.2). Ecology Studies

Two separate models have been developed to create predictive maps of potential didymo spread. The reports Likely Environments Studies both contain extensive analysis of environmental factors likely to influence suitability of a habitat to didymo establishment. The later model (“Didymo Predictive Maps”) used several environmental indicators already included in the River Environment Classification (REC) system, therefore enabling it to draw on a large established dataset.

Native species

Invertebrates: The ecology reports Ecology Studies include studies on the impacts on invertebrate species. The first compared invertebrate abundance and species composition in affected and non-affected rivers. The second additionally conducted laboratory feeding trials to establish which species might benefit from didymo providing increased food supply, and conversely which species would be disadvantaged by not being able to utilise the resource. Invertebrates were a major focus of the trout impact studies (see below).

Fish: The ecology reports Ecology Studies examined the effect of didymo cover on Native fish species and discuss how changes in invertebrate species and benthic environment might affect availability of this food resource to fish. The preliminary impact Preliminary Impact Assessment assessment also discusses potential effects on native fish.

NB The trout impact studies discuss these issues in relation to exotic trout species (see below).

Trout fisheries

The impact on trout fisheries have been considered from the earliest stages. Trout are included in the economic and recreational values in the initial impact reports. The ecology reports Ecology Studies identified the likely shift in invertebrate taxa and abundance in didymo infested waterways and raised the question of whether this would benefit or hinder trout populations. A study was commissioned to look specifically at this question, which used data on invertebrate conditions in a trout bioenergetic model to determine potential effects on trout size. The study produced an interim and final report. Trout Impact Study

Water chemistry

The effect of didymo on dissolved oxygen and pH in whole river reaches was investigated in the second ecology report Ecology Studies by comparing heavily and moderately affected rivers with didymo free rivers. At a microscale, patterns in chemistry and metabolism associated with didymo mats were measured with electrochemical microsensors and a pulseamplitude-modulated fluorometer to examine the means by which didymo achieves rapid growth and sustains high biomass levels in oligotrophic river water.
There are currently no approved treatment options to remove or control didymo in waterways (see section http://www.biosecurity.govt.nz/pests/didymo/control). The control strategy is prevention of spread between waterways. Part of this strategy is underpinned by cleaning protocols which have been developed from 2 studies that evaluated a variety of solutions and treatments for effectiveness in killing didymo, and what conditions didymo can survive on various materials likely to be transferred between waterways. Research into methods to remove or kill didymo from waterways has been undertaken. Initially a structured 3-stage investigation was carried out to identify potential didymo control agents. Control Trial Studies From these trials the most promising agent was shown to be a chelated copper formulation manufactured under the name Gemex™. Two further studies were conducted to examine the effects of longer term treatment regimes with Gemex™ firstly on didymo, and secondly on non target species including trout, koura and native fish and invertebrates, and the residual level of copper expected to accumulate in the environment. Following a new detection of didymo in the Takaka River in December 2006 a feasibility study was conducted to establish whether a trial of Gemex could be conducted in the river. The report did not recommend treatment of the river at that stage for a variety of reasons. Takaka Feasibility Study

The effect of spring-fed creeks (as opposed to waterways fed by tributaries) on didymo has been studied to try to determine if factors causing the observed limitation of didymo growth in these creeks could be identified and developed into a treatment method for water bodies generally. Spring Fed Creek Control Study
Coverage of didymo is rated on a scale (Kilroy Biomass Index or KBI) which incorporates a visual estimate of area covered and the measured thickness of the cover. This scale has good correlation with laboratory measurements periphyton biomass, and is used as the default method of estimating didymo biomass in a waterway.

Within affected rivers, environmental controls on didymo biomass have not yet been conclusively identified. It is possible that light, nutrient and/or temperature levels exert physiological control, and may produce a seasonal growth cycle. At a multi-year time scale, differences between rivers and river reaches in didymo proliferation may be due to multiple factors, including differences in light, temperature, flow regime, nutrient availability and grazing intensity. The ecology reports Ecology Studies investigate these factors. Bed mobilising flooding does appear to be one of the more significant factors in limiting growth. A study in the second report indicates that increased nutrient loading to affected rivers is likely to be followed by increased growth of didymo. Several years of data would be required to establish the extent of seasonal cycles.

Factors influencing nutrient levels in steams were also included in the predictive map model, Likely Environments Studies as these were recognised as influencing algae growth.


A full list of reports is provided below where work projects have been divided in to the three main categories of Control (including decontamination, containment, suppression, elimination, and eradication), Distribution (including detection, distribution, mapping, and sampling and analysis protocols), and Impacts (including impacts on trout, the economy and the environment).


Description: Gemex™ concentrations were assessed for the short term effects of Gemex™ on a range of non-target species. The exposure regimes were based on those expected to occur during a Gemex™ river treatment. Non-target species showed lower sensitivity when exposed to high concentrations of Gemex™ for short periods than when exposed to lower concentrations of Gemex™ for longer periods. Amphipods were the most sensitive of the species tested, followed by trout and bullies.

Water hardness also had a significant influence on copper toxicity to non-target species with survival being greatest at higher water hardness levels. Predictions were that even in soft water there would be minimal impact of Gemex™ on dwarf galaxias, koura and snails. Results confirmed earlier predictions that trout survival would be higher at higher water hardness. It remains unknown whether increased water hardness will decrease the toxicity of Gemex™ to didymo.

There was no significant bioaccumulation of copper in adult koura exposed to high concentrations of Gemex™ for up to 8 hours, suggesting that risks to humans and other predators consuming koura after a Gemex™ treatment of a waterway is likely to be minimal.
Report: Effect of Gemex™ use as a control agent for Didymosphenia geminata on non-target aquatic species, including trout, native fish, crustaceans, molluscs and insect larvae. 2008 S. Clearwater et al. Download Report Link to PDF document (514 KB) (77 pages)
Description: An assessment of long term (10-11 months post -treatment) effects of Gemex™ was undertaken at the site of the small river trial of early 2007 (Control Trials: Stage 3 Link to PDF document (1876 KB)). The review found that there was no evidence of long-term adverse effects of a single dose of Gemex™ on the non-target species tested. Although trout were affected in the short-term, trout densities had returned to near pre-treatment levels 11 months afterwards. Copper concentrations in the water and fish were also within acceptable drinking water, human health and protection of ecosystems guidelines by this time.

Multiple doses of Gemex™ should be more effective at controlling didymo than a single dose but may also elevate impacts on non-target species. This needs to be taken in to account when considering the use of Gemex™ to control didymo in a river or stream.
Report: Review of Ecological Effects of Gemex™ in a Small River 2008 K. Shearer et al. Download Report Link to PDF document (2584 KB) (51 pages)
Description: A significant new detection of didymo was discovered in the Takaka River, Nelson, in late December 2006. NIWA was commissioned to conduct an assessment into the feasibility of eliminating didymo using an experimental chelated copper control tool that was still under development at the time of the detection. Consideration was given to the logistics of using the chelated copper treatment, the likelihood of success, the impacts on the river ecology and river users, and the possible effects on groundwater and Te Waikoropupu Springs.

Treatment of the Takaka River was not recommended, as the probability of successfully eliminating didymo with one treatment of chelated copper was low and there was insufficient data available to develop a dosing strategy that would have a high probability of success in a system as large as the Takaka River. In addition, treatment of the lower Takaka River was likely to exceed Ministry of Health safe drinking water levels in the aquifer below Takaka township.

Further detail is required regarding the accumulation and potential impacts of the chelated copper formulation on the aquatic environment over short and long-term time frames.
Report: Takaka river didymo elimination feasibility study. 2007. R. Wells et al. Download Report Link to PDF document (867 KB) (33 pages)
Description: A structured 3-stage investigation was carried out to identify potential didymo control agents and conduct a range of experimental trials to assess their effectiveness on cell mortality and biomass degradation, likely impacts on other components of the ecosystem, feasibility of application, risks which could affect success, duration required for effective control and cost. NIWA was commissioned to lead the work, with sub-contracting (Stage 1) to Michigan Tech University (MTU), USA.

Stage 1: Product screening – Ten algaecides/biocides were selected for initial screening based on a review of the international literature. Screening trials were conducted in small-scale in-situ experiments to evaluate the biocides mainly for their toxicity to didymo at different contact times. A parallel work stream was sub-contracted to MTU to identify agents that degrade the non-living stalk material that characterizes didymo mats.

Results from Stage 1 (screening of biocides and disrupters of stalk formation) and Stage 2 Phase 1 (biocide testing) indicate that of all products screened and tested, chelated copper and Organic Interceptor™ showed promise as biocides against didymo, while enzymes were the only treatment which showed good potential as a degrader of didymo stalk structures.
Report: Didymosphenia geminata experimental control studies: Stage 1 (Screening of biocides and stalk disrupting agents) and Stage 2 Phase 1 (biocide product testing). 2006. P. Jellyman et al. Download Report Link to PDF document (2789 KB) (121 pages)
Stage 2: Product testing – Four products selected on the basis of Stage 1 results were tested in stream-side channel trials to further evaluate their toxicity to didymo and also to evaluate their effects on non-target biota. This latter part was also done using laboratory toxicity bioassays with flora and fauna that would normally reside in the target rivers.

Of the four products tested in the static cobble trials, Gemex™ (a chelated copper product) showed the most promising results. Intensive trials in artificial flow-through channels indicated that an application rate of 20mg Cu/L is recommended for a field trial of Gemex™ in order to maximise effectiveness against didymo while minimising effects on non-target species. The data also indicated that multiple treatments are likely to be necessary to eliminate or suppress didymo in natural waterways.
Report: Didymosphenia geminata experimental control trials: Stage Two, Phase Two (testing the effectiveness of Gemex™, a chelated copper formulation) 2007. S. Clearwater et al. Download Report Link to PDF document (1435 KB) (99 pages)
Stage 3: Control/eradication trial - A control/eradication trial was conducted on Princhester Creek in Southland, using a chelated copper formulation (Gemex™) identified as the lead agent from Stage 2 results.

The Princhester trial indicates that Gemex™ could be an effective D. geminata control treatment, particularly for early stage infestations, with the following essential treatment modifications: (i) sequential multi-location downstream dosing of Gemex™ for river segments of 2 - 3 km; (ii) repeat dosing of the river system after approximately 21 days to ensure sustained suppression or possibly eventual elimination.
Report: Didymosphenia geminata experimental control trials: Stage Three Phase One (assessment of the effectiveness of Gemex™, a chelated copper formulation, in a small river) 2007 S. Clearwater et al. Download Report Link to PDF document (1876 KB) (101 pages)
Description: A comprehensive Refworks© literature database has been developed containing international bibliographic references on control techniques for didymo, other diatoms, algae and other aquatic micro- and macro-organisms. Citations have been gathered from all appropriate sources. Abstracts are housed in an online database available to MAF Biosecurity New Zealand staff and contractors. Commissioned to NIWA.
Report: Bibliographic database on controlling benthic algae relevant to New Zealand. 2006. M. Gee & R. Wells Download Report Link to PDF document (80 KB) (10 pages)
Description: An observation that Didymosphenia geminata did not appear to thrive in spring-fed tributaries of affected rivers, even though the tributaries are periodically exposed to colonisation, suggested that a factor or combination of factors within spring-fed creeks might have potential for controlling didymo. Comparisons were made between the survival of D. geminata on artificial substrates placed in predominantly spring-fed creeks to that on substrates at adjacent sites in three rain-fed rivers in the South Island of New Zealand where D. geminata had become established since October 2004.

Didymosphenia geminata cells died and/or disappeared from colonies growing on the substrates placed in the spring creeks, whereas cells generally remained healthy in the main river sites. Water chemistry analysis showed that the spring creek sites generally had higher nitrate, alkalinity (measured as calcium carbonate), sodium, calcium, and to a lesser extent magnesium. Lower concentrations of organic carbon were found in the spring creeks. Macroinvertebrate abundance was variable with no clear differences between spring creeks and the main rivers.

The decline in both proportions and estimated weight of viable D. geminata cells to extremely low levels in all the spring creek sites suggests that poor survival of D. geminata in spring-fed creeks may be a general phenomenon. However, no single variable common to the spring-fed creeks could be identified as being responsible for poor survival of D. geminata. A combination of factors is likely to be responsible for the observed declines.

Further trials are required to determine which factor, or combination of factors, is responsible for reducing the survival of this invasive alga in New Zealand spring-fed waters. Such trials may provide clues towards developing control methods for D. geminata applicable to a wide range of freshwaters, not just spring-fed creeks. Commissioned to Fish & Game.
Report (new): The survival of Didymosphenia geminata in three rivers and associated spring-fed tributaries in the South Island of New Zealand. 2007. S. Sutherland, M. Rodway, Cathy Kilroy , W. Jarvie, G. Hughes Download Report Link to PDF document (1333 KB) (41 pages)
Description: After previously determining how to quickly kill didymo with decontamination treatments (see Decontamination Study, Kilroy 2006), the next priority was to determine how long didymo might survive if removed from a river and left untreated. The Survival Study aimed to determine the survival time of didymo cells and mat fragments under a range of levels of environmental factors of light, temperature, moisture and water quality expected to be encountered in a wide range of indoor and outdoor gear storage or cell/colony transport situations. Once the optimum temperature–light–moisture regime was identified, trials to determine didymo viability under varying water quality conditions were conducted. Several chemicals and products were also tested for their effect on the survival of didymo cells. Lastly, an assessment of didymo survival and appropriate decontamination measures was made for felt-soled waders which had been identified as a prime candidate for transfer of didymo between rivers and possibly into New Zealand due to their high absorbency and potential for close contact with didymo. This work was commissioned to NIWA.

The results showed that didymo is capable of surviving outside its natural environment for much longer than previously thought. Under optimum conditions of low temperature (approx. 9 °C), low light and adequate moisture, survival was over two months. Lower rates of survival in damp conditions versus wet conditions were due to desiccation, which occurred faster at higher temperatures. Rapid death by either desiccation or extreme temperatures was translated into operational decontamination recommendations.

The optimal pH range for didymo was pH 4.0 to 9.5 which covers the natural range of freshwaters in New Zealand. Extrapolating the results to physical conditions in bird guts (low pH, high temperatures, darkness) indicated that didymo cells were extremely unlikely to survive passage through bird guts. The relative risk of transport of didymo cells between river catchments on bird's feathers or feet was estimated to be small compared to transport by humans, because of the natural tendency of birds to groom and clean their feathers, the natural tendency of feathers to shed water and debris and the estimated rapid desiccation of didymo cells during flight under dry weather conditions.

The study provided data to expand the current recommendations for the Check Clean Dry procedure and provided methods for using lower dilutions of various decontamination agents for longer soaking times, to especially serve those high volume/commercial users who wish to soak their gear overnight at less expense to themselves, their business and the environment. A summary (Table 4) was provided ranking all effective products and methods based on their relative effectiveness, and the following additional factors: availability, cost, toxicity/irritation to humans, corrosiveness, possible effect on other organisms and biodegradability.

Absorbent items required longer soaking times to allow thorough saturation. Decontamination solutions containing surfactants (nappy cleaner, dishwashing liquid) saturated absorbent items faster than decontamination solutions which did not contain surfactants. Full strength sea water required four hours treatment time for 100% mortality. Dilute seawater and dilute bleach were not effective even after long soaks. Cleaning products marked as “Environmentally Friendly” were less effective at killing didymo than commonly available dishwashing detergents.

Options were provided for using extreme temperatures to decontaminate gear where practical. The temperature for hot tap water was recommended to be no less than 45 °C, with soaking times of at least 20 minutes for non-absorbent items or at least 20 minutes plus whatever additional time is required for thorough saturation of absorbent items. Domestic water heating cylinders often deliver water to outlet taps at between 45 and 55 °C, but this may vary, so those wishing to use hot water treatment to decontaminate gear should check the temperature at the beginning and end of the treatment to ensure that it remains at no less than 45 °C. Previous recommendations for using very hot water for rapid treatment (at least 60 °C for at least one minute) still apply where practical for temperature resilient items and where temperature compliance is assured.

Combining the power of heat with the power of detergent was highly recommended for decontaminating felt-soled waders due to the inability of decontamination solutions at ambient temperatures to passively soak quickly into felt soles. The entire felt sole needs to be completely immersed for at least 30 minutes in hot tap water at no less than 45 °C (uncomfortably hot to touch) containing 5% dishwashing liquid or nappy cleaner. If hot water alone is used, careful attention is needed to ensure the temperature of the water is maintained at no less than 45 °C for 40 minutes to ensure the interior of the felt reaches a sufficient temperature. Alternatively, freezing any item until solid will also kill didymo.

Relying on ambient drying as a stand-alone treatment for decontaminating highly absorbent risk goods such as felt-soled waders is not recommended in situations where use between waterways is frequent (daily, weekly or even monthly). Desiccation to below 83% moisture has previously been shown to kill didymo (see Decontamination Study), but slightly moist didymo can survive for months. Drying should only be relied upon as a decontamination treatment if great care is taken to actively and completely dry the felt (such as by using a heat source where temperatures around the felt are assured of reaching 30 °C). Once the felt appears dry, complete dryness must be confirmed by a tactile inspection of the felt pile to the base of the fibres. Once completely dry, items must remain dry for at least 48 hours before use in another waterway. These findings for felt soles can be extrapolated to other absorbent materials.
Report (revised): Studies on the survivability of the invasive diatom Didymosphenia geminata under a range of environmental and chemical conditions. 2006. C. Kilroy et al. (Revised May 2007) Download Report Link to PDF document (1813 KB) (121 pages)
Description: Based on the results of these early experimental studies by NIWA on the effectiveness of various chemical and physical methods to kill Didymosphenia mats and cells on gear, the first validated protocol to decontaminate risk goods was released in February 2005 as a fact sheet. This protocol was updated in November 2005 and nicknamed Check Clean Dry. Later, the Survival Study (Kilroy et al. 2007) provided additional information for the Check Clean Dry procedure. This work was commissioned to NIWA.
Report: Tests to determine the effectiveness of methods for decontaminating materials that have been in contact with Didymosphenia geminata. 2005. C. Kilroy (Revised 2006) Download Report Link to PDF document (1125 KB) (36 pages)


Description: In November 2007, MAF and the didymo regional partner groups conducted a national delimiting survey to monitor the spread of didymo. In total, samples were analysed from 145 sites, including 74 North Island sites and 71 South Island sites. Ten positive or suspect positive new sites were found, all in the South Island.

The majority of the ten South Island new sites are very close to other waterways that already are infected with didymo. Typically they are geographically close to infected sites, downstream, as is the case with the Clarence River in North Canterbury, or directly connected to a waterway that is infected with didymo, as is the case with the Rakaia River in Canterbury.

The survey followed a different model compared to earlier delimiting surveys. Previous surveys had been contracted to an external organisation to coordinate sample collection and analysis. This November survey instead had significant input from the regional partner groups, and used a web based database to coordinate sample collection and analysis. The model used for this survey supports a transition from MAF quarterly national surveys to partner driven, regionally based surveying.
Report: Didymo National Delimiting Survey, MAF Biosecurity New Zealand Download Report Link to PDF document (409 KB) (15 pages)
Description: The first mapping study to predict in which river reaches in New Zealand didymo was most likely to establish was conducted in early 2005, based on the environment of the single affected New Zealand river system known at the time and on information available about the organism's environmental preferences overseas. This early mapping study (known as the Likely Environments Map - LEM) found that over 50% of New Zealand's river area was predicted to be highly suitable for the establishment of didymo, with the majority of that river area located in the South Island. Note that didymo cells are likely to be able to survive, but not necessarily establish and grow, in nearly all New Zealand waterways.

Over the following two years, didymo spread to over 40 rivers spanning the length of the South Island. Updated predictions of the potential extent of didymo throughout New Zealand were made by basing models on field data of percentage cover and thickness collected from 145 sites throughout the South Island. The dataset includes 21 locations where didymo was absent.

Variables for the 2007 updated model (known as the Didymo Predictive Maps - DPM) were selected from a suite of over 120 variables in the New Zealand River Environment Classification (REC), a database which classifies individual sections of New Zealand’s river network on the basis of a hierarchy of controlling factors, including climate, topography, geology and land cover of the catchment of each section, and attributes of the section itself.

The DPM model can be used to generate colour-coded maps predicting percentage cover and thickness of didymo in New Zealand's river reaches. It is anticipated that these maps will assist in assessing potential impacts in currently unaffected rivers, in identifying and managing high-risk sites, and in identifying the important environmental factors that influence didymo growth. A protocol will be available for ongoing monitoring to provide suitable data for updating the model again in the future to continuously improve our knowledge of the relationship between didymo and environmental variables.
Report: Likely environments in which the non-indigenous freshwater diatom, Didymosphenia geminata, can survive, in New Zealand. 2005. C. Kilroy et al. Download Report Link to PDF document (1025 KB) (42 pages)
Report: Predicting the suitability of New Zealand rivers and lakes for colonisation and growth of the invasive, non-indigenous diatom, Didymosphenia geminata: an update. 2007 C. Kilroy et al. Download Report Link to PDF document (6247 KB) (98 pages)
Description: Didymo is a single celled organism which is difficult to detect in the early stages of invasion. Early detection of didymo increases the potential for successful mitigation of impacts and enables more efficient targeting of containment, control and public awareness measures. The aim of this study commissioned to the University of Waikato has been to develop sensitive and specific molecular detection methods for use in routine surveillance of didymo.
Phase 1: Develop methods for efficient sample collection, preservation and DNA extraction, and design and implement a highly specific and sensitive PCR-based detection capability for D. geminata.

A unique genetic fingerprinting tool and specific field collection and preservation techniques for didymo were developed during Phase 1. A didymo-specific DNA sequence was identified which allowed the detection of didymo from amongst a complex environmental community with a high degree of sensitivity. A phylogenetic tree was developed showing the relationship between didymo and related New Zealand diatoms.
Interim Report: A sensitive genetic-based detection capability for Didymosphenia geminata: Interim Report 2006 Cary et al. Download Phase 1 Report Link to PDF document (2269 KB) (69 pages)
Phase 2: Design and validate laboratory-based, high-throughput quantitative polymerase chain reaction (QPCR) detection and enumeration assays for D. geminata and begin a global phylogeographic survey of D. geminata.

Phase 3: Design a DNA-compatible long-term surveillance capability, develop and validate field compatible DNA protocols, and transfer the technology to end users through workshops and a field manual.

The didymo PCR amplification method has been refined to a more sensitive and specific quantitative and fully validated level. The Taqman QPCR detection method has a powerful negative predictive value and is, therefore, recommended as the primary surveillance tool in the North Island, where, at the time of publishing the report, didymo has not been detected. Positive results with the DNA detection method can be validated with other molecular methods (electrophoresis gel, high resolution melt (HRM), sequence) or by the microscopic method, to rule out contamination (and thus false positive results) and provide high confidence in the extremely low level DNA detection capability.

New didymo DNA detection protocols (including didymo sample collection, decontamination and DNA denaturation methods for field equipment, DNA extraction and QPCR analysis) have been developed to achieve a field-based high-throughput capacity, with rigorous quality control and quality assurance protocols.

Phylogeographic work identified genetic markers that could distinguish between different origins of didymo in New Zealand. Preliminary results indicate that D. geminata in New Zealand is more likely to have originated from North America than from Europe.
Report: A sensitive genetic-based detection capability for Didymosphenia geminata (Lyngbye) M. Schmidt: Phases Two and Three. 2007. Cary et al. Download Phases 2 and 3 Report Link to PDF document (2968 KB) (96 pages)
Description: This research identified ways to improve the efficiency of Didymosphenia sampling methodology for ongoing surveillance operations. Commissioned to NIWA.

Overall results from all surveys undertaken as part of this study indicated that detailed benthic sampling and filtration methods from the water column (drift sampling) enabled detection of didymo at sites farther downstream than the methods currently used in delimiting surveys.

A comparison of the costs and effectiveness associated with all sampling methods tested showed that filtering samples from the water column was the most efficient method for detecting didymo at low densities. A five to 10 minute deployment of the net was recommended as the standard methodology for future didymo surveillance.
Report: A comparison of sampling methods for the detection of the invasive alga Didymosphenia geminata in New Zealand. 2006. C. Kilroy & M. Dale Download Report Link to PDF document (762 KB) (52 pages)
Objective: Determine the distribution of didymo at high risk sites throughout New Zealand during the summer of 2006-07. Three national delimiting surveys were undertaken for the 2006-07 summer. Each survey targeted sites that were most likely to be infected with didymo and which provided conditions most favourable for its establishment (based on the Likely Environments Study Kilroy et al. 2006). Sites were distributed throughout the North and South Islands. Research developments were used to modify and refine sampling and analysis protocols. Commissioned to NIWA.

November 2006: Only two of the 109 sites sampled in the November 2006 survey tested positive: the lower Waihao River near SH1 and the lower Clutha River near Kaitangata. No didymo was detected in the North Island samples.

Februrary 2007: The seven additional rivers in the South Island that tested positive as part of the February 2007 survey were: Greenstone, Haast, Lindis, Kawarau, Motueka, Tekapo and Upper Mararoa. The West Coast region of the South Island in general remained free of didymo, as did the majority of Canterbury. All rivers tested positive are identified on the MAFBNZ website map. Additional positive sites were identified external to the February 2007 survey as part of MAFBNZ’s ongoing passive surveillance programme and through surveys undertaken by other organisations. These sites are detailed in the report. No didymo was detected in the North Island.

May 2007: Didymo was detected in a further nine rivers during the May 2007 survey: Hurunui, Rangitata, Tasman, Young, Cardrona, Matakitaki, Kakanui, Maerewhenua, and Dart. No didymo was detected in the North Island and it continues to be clear of any known infestations. The report contains a summary list of rivers that MAFBNZ considered positive as at 31 May 2007.

Survey methodology including site selection, sampling and analysis protocols, and results from the three 2006-07 summer surveys are covered in the report.
Report: New Zealand-wide surveys in November 2006, February 2007 and May 2007 for the presence of the non-indigenous freshwater diatom Didymosphenia geminata in high risk sites. M. Duncan Download Report Link to PDF document (3093 KB) (139 pages)
Description: This quarterly survey tested 108 highest risk sites in the South and North Islands for the presence/absence of D. geminata. Five South Island locations tested positive: Clutha R downstream of the Roxburgh Dam, Mataura R near Athol, Hamilton Burn, Omarama Stream, Matukituki River. No didymo was detected in the North Island samples. Commissioned to NIWA.
Report: A New Zealand-wide survey in August 2006 for the presence of non-indigenous freshwater diatom Didymosphenia geminata in 108 high risk sites. 2006. M. Duncan Download Report Link to PDF document (1598 KB) (69 pages)
Description: This quarterly survey tested 139 highest risk sites in the South and North Islands for the presence/absence of D. geminata. Four South Island locations tested positive: Gowan 1 km upstream from the confluence with the Buller R, Twizel R at SH8, Ohau R downstream of its confluence with the Twizel R, and Upukerora R near Lake Te Anau township. No didymo was detected in the North Island samples. Commissioned to AgriQuality and sub-contracted to NIWA.
Report: A limited New Zealand-wide survey for the presence of non-indigenous freshwater diatom Didymosphenia geminata in sites most likely for its introduction and establishment. 2006. M. Duncan & P. Wilkin Download Report Link to PDF document (1054 KB) (90 pages)
Description: A quarterly survey of a subset of the highest risk sites from the October 2005 nationwide survey detected D. geminata in one South Island river (Waitaki) out of 119 sites tested in both the South and North Islands. No didymo was detected in the North Island samples.
Report: A limited NZ-wide survey for the presence of non-indigenous freshwater diatom D. geminata in sites most likely for its introduction and establishment. 2006. M. Duncan Download Report Link to PDF document (757 KB) (38 pages)
Description: This analysis and reporting of results of a nationwide delimiting survey, where microscopic analysis of 982 sub-samples of stone scrapings, collected at 492 sites throughout the North and South Islands where Didymosphenia was likely to be introduced by human activity and where it was most likely to establish due to environmental suitability, found didymo in one river (the Von) where it had not been previously identified from the ongoing passive surveillance programme. No didymo was detected in the North Island samples.
Report: A New Zealand-wide survey in October 2005 for the presence of the non-indigenous freshwater diatom Didymosphenia geminata in sites most likely for its introduction and establishment. 2005. M. Duncan et al. Download Report Link to PDF document (2077 KB) (116 pages)
Description: When didymo was first discovered in the Mararoa and lower Waiau Rivers in October 2004, algal mats were abundant and there was concern that cells may have been transferred to neighbouring catchments. After a visual search of adjacent rivers failed to detect macroscopic mats of didymo, a local delimiting survey was initiated to detect low levels of the alga by microscopic analysis of benthic rock scrapings and drift net samples. Sixty-two sites in the Upper Waiau, Von, Oreti, Aparima and Matuara Rivers were surveyed in December 2004. No traces of didymo cells or stalks were found in any of the approximately 550 sub-samples.
Report: A survey to investigate the presence or absence of Didymosphenia geminata in selected Southland rivers. 2004. C. Kilroy Download Report Link to PDF document (495 KB) (19 pages)
Description: The Waiau Arm flow cap model was validated in Aug 2005, when didymo frustules entering the Waiau Arm as a result of water diversions from the Mararoa River were found by NIWA to have settled out of the water column and deposited on the bed of the Arm channel predominantly in Zone 1 near the MLC. A repeat survey in Dec 05 after the diversion of Mararoa water was suspended for four months showed that 99% of the didymo sampled was dead, compared to 25% dead in the Aug 05 survey. (MAF Biosecurity New Zealand thanks Meridian Energy for funding this work.)

Benthic survey of the Waiau Arm to determine the distributional extent of Didymosphenia geminata. 2005. D. Sutherland et al. Download Report Link to PDF document (464 KB) (26 pages)

Evaluation of the state of Didymosphenia geminata in the Waiau Arm following a four-month 'park and flush' regime. 2005. D. Sutherland Download Report Link to PDF document (467 KB) (16 pages)
Description: Based on hydrodynamic modelling and an examination of historical flow levels, NIWA estimated that when the Mararoa flow was under 42 m3/sec, Didymosphenia would settle to the bottom in the Waiau Arm in the region nearest the MLC structure and farthest from Lake Manapouri. (This work was jointly sponsored by MAF Biosecurity New Zealand and Meridian Energy.)
Report: Preventing possible Didymosphenia transport into Lake Manapouri: Estimated flow limits for the Waiau Arm and Mararoa River. 2005. B. Biggs et al. Download Report Link to PDF document (765 KB) (19 pages)
Description: Meridian Energy, which has a resource consent permitting the diversion of water from the Mararoa River into Lake Manapouri via the Waiau Arm, agreed to monitor for the microscopic presence/absence of didymo along the margins of the Waiau Arm and entrance to Lake Manapouri to validate previous reports that no macroscopic growths of didymo had been observed along the Waiau Arm, even though water had been diverted from the didymo-affected Mararoa River for several previous years when didymo was presumably present in the diversion water.

The Waiau Arm Margin Surveys were conducted by NIWA five times from April to December 2005. The surveys showed that outside of the Manapouri Lake Control (MLC) area (zone 1) where didymo was expected to accumulate, there were no live or dead, microscopic or macroscopic didymo colonies along the river margins either in zone 2 (the main reach of the Waiau Arm) or in zone 3 (the Arm near the entrance to Lake Manapouri and along the Lake shore). Individual didymo cells, either dead or alive, were found extremely rarely along the margins in the main reach of the Waiua Arm (zone 2) and were not considered to indicate established colonies. (MAF Biosecurity New Zealand thanks Meridian Energy for funding these surveys.)

Didymosphenia geminata in the Waiau Arm and Lake Manapouri: A survey to investigate current distribution, and recommendations for ongoing monitoring. 2005. C Kilroy and N. Blair Download Report Link to PDF document (962 KB) (35 pages)

Waiau Arm and Lake Manapouri Survey for Didymosphenia geminata: Ongoing Monitoring 1 June 2005. 2005 C. Kilroy Download Report Link to PDF document (890 KB) (3 pages)

Waiau Arm and Lake Manapouri Survey for Didymosphenia geminata: Ongoing Monitoring 4 July 2005. 2005 C. Kilroy Download Report Link to PDF document (1124 KB) (3 pages)

Waiau Arm and Lake Manapouri Survey for Didymosphenia geminata: Ongoing Monitoring 25 July 2005. 2005 C. Kilroy Download Report Link to PDF document (866 KB) (3 pages)

Waiau Arm and Lake Manapouri Survey for Didymosphenia geminata: Ongoing Monitoring 17 August 2005. 2005 C. Kilroy Download Report Link to PDF document (288 KB) (3 pages)


Description: Since didymo was first detected in New Zealand in 2004, extensive mats of the alga have developed in many river reaches. The presence of thick algal mats over large areas of river channel poses a risk of substantial changes in ecological properties (e.g., species diversity, population sizes, nutrient pools), and ecological processes (e.g., ecosystem metabolism, nutrient cycling, animal behaviours). Studies on the effects of didymo on river ecosystems and the environmental factors that affect didymo growth and biomass have been underway since 2005. Studies of the effect of didymo on river ecosystems may help to identify species and processes at high risk, and aid in prioritising conservation efforts. Studies of the effects of the external environment on didymo may contribute to control efforts.

In 2005, three separate ecological studies were undertaken by NIWA, based on four surveys over time at sites in the Mararoa and lower Waiau Rivers.

(1) The hydraulic habitat study showed that didymo thrived in a wide range of hydraulic conditions from very slow moving, shallow waters to beyond the range of depths and velocities that commonly occur in New Zealand rivers, with a minor peak in biomass at water velocities of approx. 0.5 m/s.

(2) The temporal study on didymo biomass and condition in relation to hydrological changes and water chemistry showed that biomass accumulation is faster in higher nutrient and lower temperature waters along the ranges experienced in the two rivers studied, and that moderate-sized floods do not scour didymo very effectively unless there is considerable bed sediment movement.

(3) The invertebrate study showed that didymo-affected sites had much greater densities of all classes of benthic invertebrates than the didymo-unaffected sites. However, the invertebrate community structure in the didymo-affected sites was altered, with lower proportions and lower mean size/biomass of "good" invertebrates (mayflies, stoneflies, caddisflies) desirable for fish food. Further studies (planned for 2006-2007) are needed to determine the effects of the larger absolute number, but lower relative number and lower mean size/biomass of "good" invertebrates on higher trophic levels.
Report: Ecological studies on Didymosphenia geminata. 2006. C. Kilroy et al. Download Report Link to PDF document (1071 KB) (79 pages)
In 2006-2007, a follow-up study was conducted to determine the effects of didymo on native fish, invertebrates (benthic and drift), and water quality (dissolved oxygen and pH), as well as determining didymo biomass dynamics under different hydraulic, nutrient, light and disturbance conditions. In general, didymo proliferations led to increased invertebrate abundance and increased diversity, but the populations shifted from a predominance of EPT taxa to a predominance of crustaceans, non-EPT insects, and worms. These results are consistent with results in the Ecology 2006 study Link to PDF document (1071 KB). No effect of didymo on the sizes of common invertebrates was detected in the current study.

The highest density of New Zealand native galaxiids among the sites was measured at one of the Oreti River sites with moderate didymo cover. Few other differences in fish populations between didymo-affected and non-affected sites were detected.

Results of the metabolism study indicated that high levels of didymo biomass can significantly alter the chemistry of river water. Elevated pH appears to pose a greater risk to river biota than reduced dissolved oxygen levels.

The time between bed-mobilising floods is an important determinant of didymo biomass levels. This has application for managing didymo biomass levels in dammed rivers, provided the intentional water release can be of sufficient magnitude, frequency and duration to remove a large proportion of periphyton growth from the river bed.
Report: Ecological studies of Didymosphenia geminata in New Zealand, 2006-2007. 2007. S. Larned et al. Download Report Link to PDF document (5585 KB) (140 pages)
Description: Initial investigations into the ecological effects of the invasive diatom Didymosphenia geminata (didymo) suggested that, although didymo was associated with high benthic invertebrate density, the average size of these invertebrates appeared to be reduced. This prompted concerns that trout foraging and growth rates might be negatively impacted, by a reduction in the quality of their invertebrate prey. This was investigated using a combined drift foraging and bioenergetics model for brown trout, which would predict net rate of energy intake and growth potential, based on observed invertebrate drift and water temperature. Commissioned to Cawthron Institute

Provisional results (2006) from the Mararoa River indicated that under moderate to heavy didymo biomass, invertebrate drift density and biomass were low and large aquatic invertebrates that are preferred by trout were rare. However, the results were based on sampling one reach on only one occasion (April 2006) and so should be regarded as provisional pending further research.
Interim Report: Invertebrate drift and trout growth potential in a didymo (Didymosphenia geminata) affected reach of the Mararoa River. 2006 J. Hayes et al. Download Interim Report Link to PDF document (875 KB) (43 pages)
Further results (2007) based on data collected from two rivers in autumn and winter indicated no significant negative effect attributable to didymo on invertebrate drift density and biomass. Among the didymo-affected sites, small drifting aquatic invertebrates were more dominant in rivers with highest algal biomass.

Results from brown trout drift foraging and bioenergetics modelling did not support the hypothesis that didymo alters invertebrate drift sufficiently to negatively affect trout growth. If anything, moderate-high levels of didymo were associated with higher trout growth potential in the didymo affected rivers studied.

Results should be interpreted with caution as they are based on only two sampling occasions during autumn and winter. Further research, taking account of the variability in drift density over time and space (especially by season), is needed before more definitive conclusions can be drawn.
Report: Invertebrate Drift and Trout Growth Potential in Didymo (Didymosphenia geminata) Affected Reaches of the Mararoa and Oreti Rivers: April and August 2006. Shearer et al. 2007 Download Report Link to PDF document (1040 KB) (83 pages)
Description: This assessment conducted by Deloitte in conjunction with New Zealand Institute of Economic Research estimates that didymo has caused $127.8 million in negative impacts from 2006-2011, and will lead to between $210.6 and $854.8 million from 2011 to 2020. A present day cost estimate for programmes currently underway to manage other freshwater pest plants is $5.3 million per annum.
The economic impact assessment updates the original 2006 NZIER assessment, and will be used in conjunction with agreed performance indicators and annual Check, Clean, Dry audience research results to inform a review of the Didymo and Freshwater Pest Long-term management programme
Report: Didymo and other freshwater pests: Economic Impact Assessment. August 2011. Deloitte Download Report Link to PDF document (329 KB) (36 pages)
Description: This assessment conducted by the New Zealand Institute for Economic Research estimates potential present value impacts of Didymosphenia on New Zealand to be between $58 million and 285 million over the eight years 2004/05 to 2011/12. The economic impact assessment will be used, in conjunction with the response options analysis developed from the results of the control trials, to determine the costs and benefits of long term management options for didymo.
Report: Didymosphenia geminata Economic Impact Assessment. 2006. New Zealand Institute of Economic Research Download Report Link to PDF document (150 KB) (22 pages)
Description: Based on a series of workshops to determine perceived impacts of Didymosphenia and also on the Likely Environments Study, this assessment by All Oceans Ecology examined the risks that didymo poses to New Zealand's core values (economic, environmental, social, cultural). Impacts to all the core values were perceived to be high to extreme.
Report: Organism Impact Assessment (OIA) for Potential Impacts of Didymosphenia geminata. 2005. M. Campbell Download Report Link to PDF document (2353 KB) (92 pages)
Description: This report by NIWA was originally commissioned by Environment Southland upon the discovery of Didymosphenia geminata and was subsequently purchased in Nov 2004 by MAF Biosecurity New Zealand when didymo was declared an Unwanted Organism and the national incursion response was launched. The report summarised the current state of knowledge on didymo as of Oct 2004.
Report: A new alien diatom, Didymosphenia geminata (Lyngbye) Schmidt: its biology, distribution, effects and potential risks for New Zealand fresh waters. 2004. C. Kilroy Download Report Link to PDF document (620 KB) (40 pages)