River control trial shows promise
Neil Blair (NIWA) watches as a non-toxic tracer
dye is released in a preliminary trial to test
distribution and finalise the water-quality
monitoring schedule.
Photo: Kevin Trainor, Southern
Technical Services Ltd.
The first river trial of an algaecide for didymo has shown promising results.
The trial, carried out in February this year, measured the impact of the chelated copper product, Gemex™ on didymo in Princhester Creek, in the Te Anau basin. The impacts of the treatment on invertebrates and fish were monitored following the single treatment, with tests also carried out on algae, water quality and sediments.
Speaking at the didymo science seminar, Sue Clearwater of the National Institute of Water and Atmospheric Research (NIWA) outlined the highly anticipated results of the trial, which were detailed in a paper prepared by herself and colleagues Phillip Jellyman, Barry Biggs, Neil Blair, Chris Hickey and John Clayton.
She explained that the river trial was the culmination of an intensive process to identify and test if there were potential control agents that were effective, feasible and low risk. Ten possible biocides were initially selected and screened in artificial channels at the Monowai River in Southland, along with trials of different contact times. This list was whittled down to four products which were effective against didymo. The biocides were trialled at various concentrations, with an exposure time of 60 minutes in all cases. At the same time, the effects of these biocides were tested against non-target species at NIWA's facility in Hamilton.
As these trials were completed, the choice was narrowed down to a chelated copper compound (Gemex™), which appeared to have a low impact on non-target species, while making a significant dent in didymo colonies at a concentration of 20 mg Cu/L (as chelated copper).
The next step was the river trial at Princhester Creek and pre-treatment monitoring started in January 2007. On completion of a satisfactory exploratory trial run, with the release of a non-toxic red dye into the stream to test distribution and finalise the water-quality monitoring regime, the 'live' trial applying Gemex™ could start in early February.
Sue Clearwater said the portable setup for releasing the product into the river had been developed in preparation for a rapid response, and the diffuser hose with flow meter and pump for precision dosing was ideal for the trial.
Eight monitoring sites were set up in Princhester Creek, including one control site upstream. The tracer dye was again used, and a precise dose of Gemex™ was released for one hour. As expected, the concentration of copper began to decrease further downstream, as the compound was adsorbed by the mats of didymo, and the product dispersed and diluted.
Nonetheless, the impact on the didymo was significant, with good levels of suppression shown up to 3 kilometres downstream of the release site. At a site 0.3 kilometres below the release, where there was an early-stage didymo infestation, no live didymo cells at all could be found 42 days following the treatment.
Further downstream the combination of dilution, thicker mats of didymo and possible 're-seeding' from an infested tributary meant the treatment was progressively less effective, but there was some effect on didymo nonetheless. Live didymo cell density in the mats was reduced after treatment and other algae species were beginning to colonise them. As expected, the dead mats of didymo did not degrade after treatment, and physical disturbance, such as a flood, would have been needed to dislodge them.
The impact of the copper on non-target species appeared to be minimal, Sue said. "The only exception was some juvenile rainbow trout at two monitoring sites 2–3 kilometres downstream. However, in future, it may be possible to mitigate the effect on young trout by manipulating water hardness with lime."
The treatment did not appear to significantly affect native galaxiid fish species; a temporary decrease in numbers was recorded at one site.
Results of the trial suggested Gemex™ has the potential to eliminate didymo from a waterway with minimal impact on non-target species if the infestation was detected and treated in the very early stages.
She said that copper does not biomagnify in aquatic food webs and there was minimal evidence of it in sediments tested in Princhester Creek.
"Gemex™ shouldn't be seen as a long-term solution, however, because long-term use would result in copper accumulation in sediments in downstream environments such as lakes and estuaries."
Chris Bicknell, Didymo Response Manager MAF Biosecurity New Zealand, says that while it appears Gemex™ has some potential as an emergency control measure, it is not a 'silver bullet' for removing didymo.
"We believe chelated copper is likely to be useful only in waterways with low levels of didymo and rivers below a certain size/flow. More research needs to be done about where and when it can be used and any possible effects it may have on non-target species such as molluscs, eels, koura (freshwater crayfish) and other algae.
"The best way to control the spread of didymo is for all freshwater users to check, clean and dry their equipment between waterways. It makes better sense to prevent it getting into waterways in the first place than trying to remove it once it's there."
A full report on the control trial study should be available in September at:
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Page last updated: 30 April 2008