Didymo - Research Reports

• Didymo
• Cleaning Methods
• Incursion Chronology
• Research Reports
• Where is it?
• Regulatory Actions
• Media
• Stakeholder Updates
• Partners
• Protocols
• FAQs

The overall objective of the science and technical programme is to determine validated information regarding identification, detection, distribution, containment, impact and control/eradication of Didymosphenia geminata (didymo).

The desired outcome is 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 will support the actions of our partners to slow the spread and minimise impacts of didymo.

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).

Control

• Takaka Feasibility Study
• Control Trial Studies
• Control Literature Database
• Spring Fed Creek Control Study
• Survival Study
• Decontamination Study

Takaka Feasibility Report
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 (866 Kb) (33 pages)

 

Control Trial Studies
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 (2788 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 (1434 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 (1875 Kb) (101 pages)

 

Control Literature Database
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 (79 Kb) (10 pages)

 

Spring Fed Creek Control Study
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 (1332 Kb) (41 pages)

 

Survival Study
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 (1812 Kb) (121 pages)

 

Decontamination Study
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 (1124 Kb) (36 pages)

 

Distribution

• National Delimiting Survey: November - December 07
• Likely Environments Studies
• DNA Detection Studies
• Sampling Methods Study
• 2006-07 Summer National Delimiting Surveys
• August 2006 Delimiting Survey
• May 2006 Delimiting Survey
• January 2006 Delimiting Survey
• October 2005 Delimiting Survey
• December 2004 Delimiting Survey
• Waiau Arm and Lake Manapouri Benthic Surveys
• Waiau Arm Flow Cap Study
• Waiau Arm Margin Surveys

National Delimiting Survey: November - December 07
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 (408 Kb) (15 pages)

 

Likely Environments Studies
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 (1024 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 (6246 Kb) (98 pages)

 

DNA Detection Studies
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 (2268 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 (2967 Kb) (96 pages)

 

Sampling Methods Study
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 (761 Kb) (52 pages)

 

2006-07 Summer National Delimiting Surveys
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 (3092 Kb) (139 pages)

 

August 2006 National Delimiting Survey
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 (1597 Kb) (69 pages)

 

May 2006 National Delimiting Survey
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 (1053 Kb) (90 pages)

 

January 2006 National Delimiting Survey
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 (756 Kb) (38 pages)

 

October 2005 National Delimiting Survey
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 (2076 Kb) (116 pages)

 

December 2004 National Delimiting Survey
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 (494 Kb) (19 pages)

 

Waiau Arm and Lake Manapouri Benthic Surveys
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.)
Reports:   Benthic survey of the Waiau Arm to determine the distributional extent of Didymosphenia geminata. 2005. D. Sutherland et al. Download Report (463 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 (466 Kb) (16 pages)

 

Waiau Arm Flow Cap Study
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 (764 Kb) (19 pages)

 

Waiau Arm Margin Surveys
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.)
Reports:   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 (961 Kb) (35 pages)   Waiau Arm and Lake Manapouri Survey for Didymosphenia geminata: Ongoing Monitoring 1 June 2005. 2005 C. Kilroy Download Report (889 Kb) (3 pages)   Waiau Arm and Lake Manapouri Survey for Didymosphenia geminata: Ongoing Monitoring 4 July 2005. 2005 C. Kilroy Download Report (1123 Kb) (3 pages)   Waiau Arm and Lake Manapouri Survey for Didymosphenia geminata: Ongoing Monitoring 25 July 2005. 2005 C. Kilroy Download Report (865 Kb) (3 pages)   Waiau Arm and Lake Manapouri Survey for Didymosphenia geminata: Ongoing Monitoring 17 August 2005. 2005 C. Kilroy Download Report (287 Kb) (3 pages)

 

Impacts

• Ecology Studies (new report)
• Trout Impact Study
• Economic Impact Assessment
• Organism Impact Assessment
• Preliminary Impact Assessment

Ecology Studies
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 (1070 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 (1070 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 (5584 Kb) (140 pages)

 

Trout Impact Study
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 (874 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 (1039 Kb) (83 pages)

 

Economic Impact Assessment 2006
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 (149 Kb) (22 pages)

 

Organism Impact Assessment 2005
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 (2352 Kb) (92 pages)

 

Preliminary Impact Assessment 2004
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 (619 Kb) (40 pages)