Toxic algae has prompted a no-swim warning at Butchers Dam.

Observations by Otago Regional Council staff and water samples testing earlier this week has confirmed the presence of cyanobacteria.

Water quality scientist Helen Trotter said testing confirmed toxic algae was at the amber alert level.

“But with some warm weather forecast we expect the bloom could intensify quickly,” she said.

Algal blooms have been observed in Butchers Dam for the past few years, typically from mid-November and persisting for several weeks. 

Falls Dam on the Manuherekia River had also been identified as a potential hotspot.

Blooms were most common during summer when warm temperatures, low rainfall, more sunlight, and the right level of nutrients created conditions favourable to the growth of cyanobacteria.

Toxic algae can be present at low levels year-round but can reach high concentrations very quickly so new permanent warning signs have been installed at Butchers Dam and Falls Dam.

These will make visitors and residents aware of the potential risks from toxic algae at these locations. 

Those visiting Butchers Dam for recreation should stay out of the water until it is confirmed safe to swim here.

Algal blooms and pea soup-like appearance that can suggest toxic algae is present. PHOTO: SUPPLIED

New permanent warning sign at Butchers Dam. PHOTO: SUPPLIED

The trigger levels and corresponding management responses as shown on lawa.org.nz

Helen said each year the regional councils monitoring team collected weekly samples to test bacteria levels and check for toxic algae at popular swimming spots in rivers and lakes across Otago.

Not every waterway was tested and there would not always be a sign posted so it was important people knew what to look for, Helen said.

“It is important people remember that water quality and conditions can change rapidly, especially after rain. That’s why wherever you choose to swim it’s always advised to avoid swimming for at least two days after heavy or prolonged rain because this is when higher levels of contaminants can enter our waterways.’’

To check for the latest toxic algae sightings, go to www.orc.govt.nz/toxicalgaesightings

Otago Water Monitoring Programme

The Otago programme, which started in 2006, runs during the summer months from December 1 to March 31 as this is when most water activities occurred.

Water monitoring sites were reviewed annually, with ORC monitoring staff testing 36 sites weekly this year.

Freshwater sites were monitored for E. coli, marine sites for Enterococci, and lakes for cyanobacteria (toxic algae) to make sure water is safe for swimming.

Otago Regional Council Contact Recreation and Lake Cyanobacteria Monitoring Sites 2024-2025

The latest water quality results are posted to www.lawa.org.nz/swim (Land Air Water Aotearoa), a handy resource for New Zealanders throughout the country to check the quality of their favourite local waterway and swim spots.

It was also a helpful resource for people wanting to learn more about water quality and how to keep safe while out enjoying our waterways this summer.

LAWA (Land, Air, Water, Aotearoa)

LAWA was originally set up as a collaboration between New Zealand’s 16 regional councils and unitary authorities. 

LAWA is now a partnership between Te Uru Kahika (Regional and Unitary Councils Aotearoa), Cawthron Institute, the Ministry for the Environment, the Department of Conservation, Stats NZ and has been supported by the Tindall Foundation and Massey University.

A screenshot of the map on LAWA’s website showing favourite swimming spots throughout Otago.

People can use LAWA to check if a swim area is safe 

“Council monitoring programmes across New Zealand continue to grow, with almost 850 recreational swim sites now on LAWA’s Can I Swim Here web page,” Helen said.

People could search for their local swim spot ahead of a visit and plan their trip with the latest information. 

The map will show spots which are suitable, or not, for swimming using the handy traffic light system.

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