With temperatures warming up, people using Hawke's Bay's lakes or stony rivers for swimming or walking dogs need to be aware of the risks of algae or bacteria growth.

Hawke's Bay Regional Council's water quality monitoring staff check water at 36 popular swimming spots around the region and also look out for algae growth. They are seeing higher levels of algal growth in waterways now.

Parts of the Tukituki River are prone to algal growth in summer because of lower water flows, higher water temperatures and nutrients in the water. Green strand algae is commonly seen but is harmless.

Black Phormidium bacteria growth on the rocks can turn toxic and becomes a hazard for humans and dogs. It can detach from the rocks and dry as mats on the sides of the rivers which can also be toxic.

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"Always treat the black growth on rocks in the water and the drying mats as toxic, stay out of the water if you see it, and swim or walk your dog elsewhere," says Anna Madarasz-Smith, HBRC's principal scientist marine and coasts.

Science has not yet identified when Phormidium turns toxic or why, so to be safe it should always be treated as harmful. Contact with Phormidium bacteria can make people sick, with flu-like symptoms.

Dogs are more vulnerable. They are particularly attracted to the musty odour of the mats and the toxins can be more dangerous. Dogs should be under control in river beds and not allowed to scavenge, and owners should take other water for dogs to drink.

Symptoms are vomiting, diarrhoea and lethargy, and heat stroke-like symptoms such as muscle tremors, breathing difficulties, paralysis and convulsions. Dogs should be taken immediately to the vet.

Algae and phormidium occur naturally, even in very clean fresh water. As regional council staff cannot check whole river systems each week, they appreciate information and photos from the public. Phone 0800 108 838 or email photos to info@hbrc.govt.nz (subject: Algae alert).

Check water quality and algae warnings on the national environmental monitoring site LAWA (Land Air Water Aotearoa) at lawa.org.nz, searching Can I Swim Here.