Toxic algae, also known as cyanobacteria, can rapidly “bloom” from a thin film into thick mats that can cover the beds of rivers and lakes, and thrives in periods of low flow and warm temperatures.
The alga releases toxins when its cells are broken, providing a greater risk to dogs who are often interested in the musky smell of the mats.
People may also experience rashes if they come into contact with cyanobacteria or become nauseous if they swallow contaminated water.
The city’s high-risk locations include the Maitai Camp, sections of the Maitai River around the Aratuna Bridge and Avon Terrace, at Isel Park, and at Māori Pa Rd along the Wakapuaka River.
But toxic alga doesn’t tend to grow in deeper pools, where sunlight limits growth, meaning that popular Nelson swimming holes are likely to be relatively safe this summer.
Nelson City Council has installed new signs at popular recreation sites at risk from mats building up on the riverbed.
With a traffic light system, the signs can easily be altered for when the risk of cyanobacteria presence increases from low to medium or high, providing dog walkers and others with clarity on the current warning level.
The new signs have currently only been installed at two Maitai River sites – at the Domett St footbridge and at the campground – but other signage is already in place at all designated recreation bathing sites on the Maitai and Wakapuaka Rivers, and in Isel Park.
“We also have information pamphlets, weekly monitoring to check cyanobacteria coverage at key sites, reporting on the council web, and regular contact with Health Protection Officers and Nelson vets if further actions are required,” said Mandy Bishop, the council’s group manager environmental management.
“When required we will also issue media releases and alerts via LAWA (Land, Air, Water Aotearoa), Our Nelson information stories, and social media.”
She added that the council has worked with the Cawthron Institute and Nelson vets to develop a protocol for assessing sick dogs.
Education events are also planned for dog owners to learn more about cyanobacteria.
By Max Frethey
Local Democracy Reporter
