Fears climate change could tip toxic algae to deadly levels - Stuff.co.nz

The number of dogs killed after eating toxic algae from the banks of our rivers and lakes used to be tracked by Nelson's Cawthron Institute.

But as that figure crept into the hundreds, researchers lost count. Now, it's not just dogs getting sick.

In the past decade, nearly 40 people in the Bay of Plenty and Lakes districts have become ill from what the local public health unit suspects is shellfish contaminated with cyanotoxins – originating from that same algae.

"These are some of the most lethal natural compounds. We should take them seriously," Cawthron senior freshwater scientist Susie Wood said.

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With temperatures rising and waters warming, she feared the problem would only get worse, and eventually, it could be a person that dies.

Cyanobacteria – also known as blue-green algae – is the culprit behind dozens of council and district health board warnings across the country each summer, urging people and pets to keep out of their favourite swimming spots.

Lakes were typically dominated by free-floating planktonic blooms, while rivers were more prone to benthic blooms – which grow in mats attached to rocks at the bottom.

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Lake Forsyth on Canterbury's Banks Peninsula frequently has toxic algae blooms (file photo)

But they are all cyanobacteria, Wood said, and most contain a cocktail of dangerous substances called cyanotoxins.

Cyanobacteria are already making some New Zealanders sick.

There have been two cases in the past two years of cyanotoxin poisoning reported to the Canterbury District Health Board.

In 2020, a person presented with tingling hands and feet and stomach cramps after exposure to algae, and in 2021, a child developed a skin rash after playing in water.

However, Toi Te Ora, the Bay of Plenty and Rotorua-Lakes District's public health unit, has not had any cases of cyanotoxin poisoning from algal blooms reported in recent years.

Medical Officer of Health Jim Miller said that did not mean cyanobacteria were not having an impact.

"There have been a number of cases of paralytic shellfish poisoning (PSP) which are linked to saxitoxins produced by cyanobacteria," he said.

Since January 2012, there have been 39 confirmed or suspected cases of PSP in the area, with the bulk of those around Tauranga.

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Dr Susie Wood inspecting a phytoplankton sample (file photo).

Most other health boards around the country had no incidents to report, but Wood said many cases were likely sliding under the radar.

"[Cyanotoxin poisoning] can have quite generic symptoms - like diarrhoea and stomach cramps - which are also associated with other things like food poisoning.

"I think a lot of it would go unreported. But some of the more serious, long-term problems it can cause are not really known about in New Zealand."

In the Canadian province of New Brunswick, a cyanotoxin known as BMAA was being investigated as a potential cause of a mysterious brain disease, which has killed at least eight people.

Onen study indicated there were high concentrations of BMAA in lobster, the product of a major economic activity in the province.

While the science on BMAA causing brain disease was not yet settled, Wood said they still had not tested for it in local algae strains, although she thought it was "highly likely" to be there.

Dean Kozanic/Stuff

In the Bay of Plenty and Lakes Districts, there have been 39 confirmed or suspected cases of paralytic shellfish poisoning in the past decade (file photo).

In New Zealand, Wood said there was some risk of cyanotoxins building up in kaimoana (seafood).

A study of Lake Forsyth on Canterbury's Banks Peninsula revealed cyanotoxins accumulated in the tuna - or freshwater eels – that called its waters home. Another study in Lake Rotorua found a similar impact on trout.

"If someone was eating the flesh of those organisms regularly, it could be a risk."

For fish, she said most of the toxins were compounded in the liver, but could pose more of a risk in freshwater crayfish/kōura and shellfish, like mussels and oysters.

Wood said the cyanotoxins that accumulated in seafood could cause irreversible liver damage in humans, and could even promote liver cancer.

Greenpeace

In February, Greenpeace put three billboards into the algae-filled Selwyn River in Canterbury calling for the Government to cap fertiliser use as part of its freshwater reforms.

But the greatest risk in New Zealand was to recreational water users, with the water's edge posing a particular danger.

Dogs were attracted to the musty-smelling but deadly deposits algal blooms left on riverbanks, and Wood said she was "really scared" a toddler might eat it someday.

"You do see people all around the country still kayaking or water skiing in areas with algal blooms, although swimmers tend to avoid them when there are warnings in place.

"The biggest risk is when you consume the water, [and] it is highly unlikely recreational users would ingest enough to seriously harm them."

In spots with serious algal blooms, Wood said it was possible accidentally swallowing a few hundred millilitres while swimming could make someone sick.

Mark Dwyer/Stuff

Dr Susie Wood says Kiwis should still enjoy NZ's rivers and lakes, but need to know what to look out for (file photo).

Cyanobacteria were likely to become a bigger problem in the near future, Wood said.

"Algal blooms are a symptom of human impact on the landscape."

Cyanobacteria thrived in waterways rich in nutrients like nitrates, which often originated from synthetic fertilisers used on farms. They also loved warmer water, and declining water quality, the presence of non-native fish, and storm events were also helping them along.

The flow-on effects from cleaning up our waterways would be important in managing the risks long-term, she said."

"We will see more [algal blooms] over the next 10 years because of climate change.

"Unfortunately, cyanobacteria really thrive in those warm conditions. It will get worse."

However, Wood said none of this should put Kiwis off using waterways. "Swimming and fishing are part of who we are ... just be careful."

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Watch out for algal mats stuck to rocks on riverbeds, distriict health boards say (file photo).

Councils had monitoring systems in place, and people could find out about local swimming spots on LAWA's Safe to Swim website.

"Obviously there's no way we can monitor everything, [so] it's a matter of the public knowing what to look out for."

DHBs around the country warned people should be wary of water with green or brown particles floating in it, but particularly of dark brown or black mats along riverbeds.

They recommended avoiding contact with water anywhere a health warning was in place.

Although councils often placed warning signs, they might not be at all access points, so all low-flowing rivers should be treated with caution.

Swimmers might experience itchy and irritated eyes or skin, and hay fever-like allergic reactions, while anyone who ingested contaminated water might experience headaches, fever, diarrhoea, abdominal pain, nausea or vomiting.

Anyone with symptoms was urged to visit their doctor immediately, and let them know they had contact with algal mats or water with algal mats present.

Pets that showed signs of illness after coming into contact with algal mats should be taken to a vet immediately.

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