Didymo control research showing promise
Dr Barry Biggs.
By Mike Bradstock
Reports of encouraging progress in the search for a viable method to control didymo attracted strong interest from stakeholders at a didymo forum held in Christchurch on 5 December.
Participants were especially interested in a presentation by Dr Barry Biggs of NIWA describing ongoing research into the problem and the organism that causes it. He explained that didymo can remain attached at very high water velocities.
"Didymo can also grow in extremely low-nutrient waters, but is fairly slow to form large biomasses, so that rivers which are prone to flooding and carry a sediment load should get scoured free of didymo regularly," Dr Biggs said. "But, unfortunately, many of these rivers also have stable side-braids away from their main flow, which would be great habitat for it.
"Didymo grows best in stable lake-fed rivers – in other words, the same kind of water preferred by trout. Unfortunately, in this habitat the biomass can build up to greater than 200 grams per square metre – over four times the maximum biomass recommended for algae on the bed of rivers for a healthy freshwater environment," he said.
Such dense growths change the habitat significantly for fish, and the invertebrates they feed on, and even make the water in its proximity so alkaline that it can potentially attack the delicate gills of fish. Dr Biggs said it was thought that, as they grew many freshwater insects were affected by the changed water quality or became unable to emerge as adults from the mats of didymo.
He also sounded a warning that didymo was likely to be just the first of many unwanted aquatic organisms to which New Zealand was at risk. It was very difficult to predict what organisms could easily be spread, he said, and the traffic was not all 'one way'. New Zealand mud snails had recently become a big problem in rivers and streams of the Pacific Northwest of the United States.
Dr Biggs outlined the rapid pace of research since didymo was first discovered in New Zealand in October 2004. Since then, New Zealand researchers have become world experts on didymo, and in May 2006 Dr Biggs was invited to give the keynote address at the first international didymo symposium, sponsored by the North American Fisheries Society.
The present didymo science programme is operated by a closely coordinated network of scientists from Crown research institutes, universities and local authorities. Different subgroups focus on distribution, impacts, containment and control. Some very rapid progress has been made, with a very sensitive detection method developed by Dr Craig Cary of Waikato University. This test, although yet to be validated for use in the field, can detect didymo in concentrations as low as 1 cell per litre.
Even more encouraging is work to date by Dr Biggs's team towards controlling the organism. They set out to design a new didymo-specific algaecide – a chemical that can be used to poison the alga without killing other aquatic life. The result is a new chelated copper compound which has been found to be very effective in trials at Monowai, near Manapouri. An experimental dosage regime yielded a 99 percent kill of didymo, but resulted in only a very low mortality of invertebrates and no death of fish.
The next step is to determine the effectiveness and impacts in a real river environment, including the best dosage regimes, application and control strategies – how much, how often and how far apart the algaecide needs to be applied to be effective along the full length of a river, and in which waterways it will work best. Laboratory tests so far suggest the best results may be obtained with three doses of the algaecide, each applied daily for a period of one hour. At the time of the seminar, application had just been made to ERMA for the first in-river tests, and approval for these came through in mid-December. The trials are due to get underway in Southland this month.
But this work is all about control, not eradication, Dr Biggs said: "It will never be possible to eradicate every last didymo cell by this method." Therefore, it also leaves other questions to be answered, like how fast didymo will grow back. "We have to find out whether the treatment will be effective for long enough to be ecologically useful," Dr Biggs said. "That is, will it last long enough to make a difference in terms of restoring environmental quality, populations of aquatic life and other user values?"
Even if a practical control tool is forthcoming, waterway users must still Clean, Check, Dry between waterways to help prevent the spread not only of didymo, but also of other aquatic pests.
Correction: Didymo article
In Biosecurity 72:17 we reported that the Tekapo hydro canal produces 2 tonnes of didymo per day. This was an error. Didymo has not been detected in the hydro canals. However, background ecology risk studies do predict that over 3 tonnes of didymo material (wet weight) per day on average could potentially accumulate on the Tekapo canal outlet screens if didymo were to become established in the canal.
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Page last updated: 30 April 2008