Swimmers have been cleared to take a dip in the Maitai River as changes in toxic blue green algae levels mean there are no longer immediate health risks from swimming.
But locals are being warned to ''be vigilant'' with their dogs and small children and avoid any contact with the toxic algae, also known as cyanobacteria, which was still present in low levels and could be harmful if ingested.
Signs with photos will be placed along the Maitai to let the public know what the toxic algae looked like so they could avoid coming into contact with the algae.
''We're encouraging people to make sure they read these signs and be aware of what to look out for,'' said Cawthron Institute senior scientist Dr Susie Wood, who monitors levels of toxic algae in the river.
The Nelson City Council issued warnings in November against swimming in the river, or letting dogs drink from it as the levels of cyanobacteria were so high there was a high chance of a dog or human ingesting them.
Since then those levels have reduced considerably due to low rainfall and low river flow, and there is no longer an immediate risk to public health.
''Because those levels are now very low it is safe again to swim in the river, which is good timing for Christmas and the holidays.
''But we're still asking people to be careful with dogs and young children as there are still small patches of cyanobacteria in the river, and ingesting even a small amount can be harmful,'' said Dr Wood.
The levels of cyanobacteria could change again over summer so the public are asked to be keep an eye on warning issued by the council.
''The Maitai River is a dynamic environment - it's constantly changing and it's hard to predict wha may happen next.
'' It is possible that cyanobacteria may re-emerge this summer but for now the water is safe for swimming.
''We are constantly monitoring the river so if there is another emergence of cyanobacteria the council will notify people,'' said Dr Wood.
Status updates will be available on council's website and Facebook page.
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