Toxic algae spark climate fears
The number of dogs killed after eating toxic algae from the banks of rivers and lakes used to be tracked by Nelson’s Cawthron Institute.
But as that number 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 sick from what the local public health unit suspects is shellfish contaminated with cyanotoxins – originating from that same algae.
Last week, swimmers and dog owners were asked to avoid areas of the Hutt River after high levels of toxic algae were found.
‘‘These are some of the most lethal natural compounds. We should take them seriously,’’ Cawthron senior freshwater scientist Susie Wood said.
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 bluegreen algae – is the culprit behind dozens of council and district health board warnings across the country each summer, urging people to stay out of their favourite swimming spots and keep their pets away too.
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. 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.
Two cases of cyanotoxin poisoning have been reported to the Canterbury District Health Board in the past two years.
Since January 2012, there have been 39 confirmed or suspected cases of paralytic shellfish poisoning in the Bay of Plenty and Rotorua-Lakes District area, most of them around Tauranga. 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.
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 it was ‘‘highly likely’’ to be there.
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. The toxins could pose more of a risk in freshwater crayfish/kōura and shellfish, such as mussels and oysters.
Wood said the cyanotoxins that accumulated in seafood could cause irreversible liver damage.
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 mustysmelling but deadly deposits algal blooms left on riverbanks, and Wood said she was ‘‘really scared’’ a toddler might eat it someday.
Cyanobacteria were likely to become a bigger problem in the near future. ‘‘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.