Phormidium is a naturally occurring algae and thrives in Otago rivers during summer.

Otago Regional Council Scientist Hugo Borges said given the warmer temperatures and the resulting stable flows, Phormidium and other potentially toxic algae has the opportunity to form mats.

Phormidium forms thick dark brown mats on rocks in the riverbed and can sometimes look like black tar. In the flow of water, Phormidium mats detach from the riverbed and wash up on riverbanks.

“It is important that people using waterways are aware of the risks associated with its possible presence, and know how to identify it wherever they are in Otago,” Mr Borges said.

“The Phormidium mats can detach from the riverbed and accumulate along the water’s edge. The musty smell is particularly attractive to dogs and, if eaten, Phormidium can result in severe poisoning or death.”

Anyone concerned their pet may have consumed toxic algae should contact their vet immediately, he said.

More information on potentially toxic algae is available on the LAWA factsheet: https://www.lawa.org.nz/learn/factsheets/potentially-toxic-algae/