Brown plague: Didymo

Five years since didymo was officially discovered in New Zealand, the invasive alga has spread rapidly through the South Island's rivers, writes Shane Gilchrist.

If you want to see red, check Biosecurity New Zealand's latest didymo risk map, which shows the level of threat to South Island rivers from the alga.

The map is colour-coded, with hot hues denoting high likelihood of infestation.

Sadly, blue and green are less prominent.

The map parallels the state of many of our rivers.

Blue and green, the natural tones of many of our upcountry waterways, have been usurped by the beige and brown algal blooms of a freshwater plant that likes cool water, a stable and moderate flow, and high UV levels.

From Southland to Marlborough, the depiction of braids east of the Main Divide is not unlike the human circulatory system: in this case, these arteries and capillaries are in danger of becoming clogged.

Can we fix it? Well, not really.

According to Biosecurity New Zealand, which operates under the Ministry of Agriculture and Forestry (Maf), eradicating any microscopic organism from an aquatic environment is virtually impossible.

Cathy Kilroy, a scientist with the National Institute of Water and Atmospheric Research (Niwa), agrees with this rather pessimistic appraisal: "Once didymo is found in a river, it would be pretty hard to stop it spreading downstream - unless you get in there and kill everything."

She should know.

Since October 2004, when she made the historic announcement she had identified didymo in Southland's Waiau and Mararoa rivers, Ms Kilroy has been studying the biology and ecology of the alga.

"I was working on a Meridian Energy project with Fish and Game New Zealand ... I had heard of didymo and had seen it overseas but it wasn't on my radar at all. I didn't think it would be a problem here."

Eleven months after her discovery - the first time didymo had been found in the southern hemisphere, though retrospective reports indicate it was visible in the Mararoa River as early as 2001 - the alga turned up several hundred kilometres north, in the Buller River, lending credence to the belief humans have been a key contributor to its distribution.

"It stayed in the Waiau catchment for almost a year before we found it in any other river," Ms Kilroy says, referring to the Buller River discovery in November 2005.

"Very quickly after that it was found in the Hawea River, then the Waitaki. It popped up in a few different rivers.

"If it had spread from the Waiau River, just gradually moved out to other rivers, then you'd think [its spread could be attributed to factors other than human] but to pop up in the Buller and the Clutha catchment ..."

The unintentional transport of microscopic didymo cells via recreational or industrial equipment, including fishing gear, vehicles and boats, prompted the implementation of the "Check, Clean, Dry" campaign soon after Ms Kilroy's discovery.

Yet while angling carries a high risk of spreading didymo, activities such as tramping and hunting are also potentially problematic.

An American angler is believed to be responsible for introducing the alga to New Zealand, with DNA tests revealing the strain to be from the United States.

The cost to the economy, including the loss of commercial eel fisheries, damage to water supplies, biodiversity values and tourism, is estimated at up to $285 million over the next six years.

In Otago, didymo is now well spread.

Large blooms have been found throughout the Clutha catchment, including the Hawea River, and also in the Waitaki River (officially within Environment Canterbury's boundaries) and the Lower Kakanui River.

In January last year, it was discovered in the Route Burn; by Christmas, it was in the remote Nevis River.

In April this year, it was found downstream of the confluence of the east and west branches of the Manuherikia River, a waterway in which it was first discovered in late 2007.

Besides its aesthetic impact - it's not called "rock snot" for nothing - the weed also poses a threat to the nationally threatened lowland longjaw galaxias in the Kauri and Kakanui Rivers in North Otago (by smothering habitat).

Didymo has also caused problems in irrigation schemes, clogging intake screens and sprinklers.

In simple terms, a key factor in the severity of didymo is the speed of the flow of water next to a riverbed.

A recent Otago Regional Council study of the Fraser River, near Alexandra, showed that didymo cover was high when flows were low and stable.

However, when flows increased, much of the weed was removed and replaced by native algae.

When flows became more stable, the didymo returned.

In rivers with long, stable flows, such as the Hawea, Clutha, Waitaki and Waiau, all of which are controlled by hydro-electricity schemes, didymo blooms are large.