On the trail of our rivers' mystery killer

Last updated 12:58 17/03/2014
DEADLY AGENT: Environmental science student Bethany Bridge, left, and Dr Susie Wood, research scientist at the Cawthron Institute with rocks covered in cyanobacteria from the Maitai River.

Relevant offers

Toxic algae found in the Maitai River is showing up around New Zealand. It's a serious problem that needs more attention, says Cawthron Institute scientist Susie Wood in part five of BILL MOORE'S series.

One minute your dog is enjoying a raucous run on the riverbank. The next, it's writhing in agony, frothing at the mouth. Soon, it's dead.

This is the experience of a number of dog-owners in Nelson and around New Zealand, and it's happening more often.

Cyanobacteria, sometimes called blue-green algae, is naturally occurring in New Zealand rivers and most of the time it isn't a problem. But when it blooms, as it has done regularly in Nelson's Maitai River over the last decade of summers, it can turn toxic and become a deadly killer.

Dogs are drawn to the musty smell of the cyanobacteria known as Phormidium - an odour many Kiwis associate with their rivers, although intensified - and if they chew on a toxic algal mat that's washed ashore they can meet a swift and horrible death.

The toxins kill by paralysing muscles, affecting respiration, and represent a danger to humans too.

That's why the Nelson City Council put out a warning this week, the latest in a long series. It urged that people avoid allowing their children to play in the river between Bridge St and Nile St, or let their dogs swim in that section, noting that swimming holes upstream from there were "currently not affected".

For Cawthron Institute biologist Susie Wood, a freshwater scientist who has become New Zealand's leading expert and a world authority on cyanobacteria, what's happening in the Maitai is a concerning symptom of a widening problem.

"We don't have very good historic records of algae in New Zealand rivers," she says. "My instincts, and from talking to quite a lot of people that have worked at regional councils for many years, is that it has got worse, and that's right across the country."

Cyanobacteria's deadly effects entered the national consciousness in 2005 when a series of dog deaths were linked to the Hutt River.

"The dogs were dying within 10 or 15 minutes," Dr Wood says. "We got samples down here [at Cawthron] and that was the first time we'd identified that specific toxin in New Zealand and the first time we'd confirmed that the cyanobacterial mats the dogs were eating produced the toxin.

"We were able to find the toxin in the stomach contents of the dogs, so we had really great confirmation. That really started the research."

Ad Feedback

Since then dogs have died after visiting the Maitai, Waimea and Takaka rivers in the Nelson region, and rivers in Canterbury, Otago, Southland, Hawke's Bay and Manawatu. Problems have also surfaced in the south of France, Cuba, and parts of the United States.

The main New Zealand study site has remained the Hutt River but with the help of a series of students the work has expanded. This year Cawthron has a British student, Bethany Bridge, working on cyanobacteria in the Maitai, regularly sampling a string of sites and looking at water quality, temperature, oxygen content, pH values and other variables.

In what seems paradoxical to a layman, cyanobacteria doesn't bloom in the polluted rivers linked to "dirty dairying". The most contaminated waterways are too dirty for it, Dr Wood says.

Nitrogen and phosphorus are critical to cyanobacteria growth. It grows well in rivers low in phosphorus, "so actually we're seeing quite a lot of these blooms in rivers around New Zealand that are quite clean", she says. Elevated nitrogen, but not extremely high concentrations, is also linked to blooms.

"When we look at the data from October, the Maitai fits those patterns really well."

The frequency of rainfall-fed flushes dictates the blooms.

"If the rainfall event is large enough it washes all of the mats away, and if that happens at a frequency of less than two weeks, we generally don't have blooms.

"But what that doesn't explain is why some sites in a river have problems and some don't, and why some rivers have problems and others don't. We've been working on that and we have ideas, but that's still where our research is sitting."

A third factor is sedimentation, something that increases with land use change. Forestry is the most likely cause for an observed increase in fine sediments in the lower reaches of the Maitai, she says.

"When you completely clear-fell like we do in New Zealand, the run-off is hugely increased - you just have to go out in a heavy rainfall event and see the sediment load that comes down."

The city section of the Maitai is heavily influenced by the Brook Stream, Dr Wood says, and there has been forest harvesting in the Brook catchment in the past year.

Some more research is needed but if the causes are what the scientists suspect, leaving uncut forest buffer zones of perhaps 100 metres beside rivers could make "a huge difference" to the amount of sediment washed in by rain.

Water from the Maitai reservoir used to maintain the river flow has "a very localised effect" on cyanobacteria in the stretch of river below the feeder pipe, probably due to a slight increase in nutrient concentrations and metals.

"I don't think it's the cause of our problems further down," she says.

Cyanobacteria is always present in the river but when conditions are right, it forms thick mats. They can be easily seen from the city's bridges as dark brown to black patches on the midstream rocks. When the mats reach sufficient size they detach and, if they are toxic - something which can only be established by chemical or genetic testing - they are at their most dangerous when they wash up on the banks.

"The message we give to the public is that you always consider it as toxic because we know it can change so quickly. It can go from not being toxic one week to being completely toxic the next week. It still looks the same."

Dr Wood says over the past few summers there have been tens of kilometres of New Zealand rivers covered with cyanobacteria mats producing the potentially lethal neurotoxin.

"In certain regions it poses a huge health risk, and I think we are very fortunate that no humans have become seriously sick."

She receives funding from several councils around New Zealand and works with students from Victoria and Canterbury universities.

Dr Wood welcomes the formation of Friends of the Maitai, whose members are offering to carry out data-gathering to help in the research.

"It would be great to get some national funding," she says. "It's always a challenge because there are so many threats to freshwater systems in New Zealand but I think it should be incorporated in a larger national programme, highlighted as something that's worth investigating. To my mind there's something changing in our rivers, and we should be worried about that."

At the local level, she is keen to see more work done on improving and protecting the Maitai, and is heartened by the city council's intention to spend $400,000 on a clean-up plan to be unveiled at the end of the month.

A former Commonwealth Games cyclist and top triathlete, Dr Wood says she runs and mountainbikes in the Maitai Valley a lot, and swims in the river. "I think most people in Nelson love the river. It's a fantastic resource, especially with all the accessible tracks and great swimming spots."

- Nelson


Special offers
Opinion poll

Does Nelson deserve to be classed as a city?



Don't know/Don't care

Vote Result

Related story: (See story)

Featured Promotions

Sponsored Content