Two more popular rivers have joined the list of New Zealand waterways where toxic algae has taken hold.
The Otaki and Waingawa Rivers, north of Wellington, are now classed as extreme - which means more than half of the river bed is covered in toxic algae.
- Lake Ōmāpere turns green and smelly with algae bloom
- Toxic algae poisons South Island rivers
- Toxic algae outbreak: Wellington swimmers, dog walkers warned to avoid Hutt River
Warnings now exist for rivers in the Bay of Plenty, Waikato, Hawkes Bay, and Manawatu-Whanganui, in the north. Tasman, Nelson, Canterbury, Otago and Southland also have rivers with the algae in the south.
For the Otaki River, the amount of toxic algae has grown rapidly. 50 percent of the river is now covered in algae - two weeks ago it was 15 percent.
"It's definitely the worst we've seen in the Otaki River since records and monitoring began," said Mark Heath, a senior freshwater scientist.
"I think a lot of it's due to this really warm and dry weather, so when we get the water temperature lifting up, the low flows, it just seems to be this perfect breeding ground and environment for toxic algae to grow in."
Mr Heath says toxic algae have been around for billions of years - but the blooms have been getting worse for the last fifteen years.
"We're not 100% sure what the drivers are but under climate change scenarios, it's only predicted these toxic algal blooms are gonna get worse as our weather gets warmer, drier, and we have less rainfall," he said.
Also known as blue-green algae or cyanobacteria, it's distinguished from the more recognisable bright green algae by its musty smell and slimy appearance.
"These leathery, velvety mats that might be black or brown-green in colour, and they really attach to the rocks, and they peel off like a piece of leather," said Mr Heath.
When it's disturbed or dies, it floats to the surface, and can form mats at the water's edge - putting it within reach of babies, toddlers, or dogs.
"It only takes around about a fifty cent piece for a 20kg dog to have a lethal dose."
Contact with the algae can cause symptoms like skin irritation and muscle twitches, while swallowing it has the potential to cause nausea, vomiting, convulsions or loss of consciousness.
The bridge on State Highway 1 is a popular swimming spot - but swimmers and anglers have been told to stay away.
"It's a real shame but at the moment the risk is just not worth it," said Mr Heath.
Salt-water breaks down the neurotoxins in the algae, but what's needed is much-needed heavy rain to wash the algae out to sea.
For more information on toxic algae, and for advice on local waterways, visit:
http://www.gw.govt.nz/safeswim/
Newshub.
