The potentially toxic algal bloom affecting Lake Waihola is getting worse and could be there for the summer.
''The lake is very bright green,'' Otago Regional Council environmental information and science director John Threlfall said.
The regional council issued warnings last week for users to be aware of the alga, which could cause allergic reactions in people or animals if it touched skin or was ingested.
It occurred naturally in a variety of water conditions and belonged to the cyanobacteria group of algae. It had the potential to produce a series of toxins that were passed to the water.
Further sampling at the lake this week had shown the bloom had worsened. It was also still present at Tomahawk Lagoon.
The Cawthron Institute in Nelson was analysing samples taken from both blooms to determine their toxicity. The results should be available in about a week.
However, despite not knowing exactly how toxic the blue-green alga was, given the level of its growth, it was believed it could still cause a reaction if it touched skin, he said.
Lake Waihola was affected by the alga last summer but not to the same extent. About 10 years ago, it was also affected extensively.
''It is concerning for locals and we don't know how long it will last,'' Mr Threlfall said.
It was possible the bloom could continue in the lake for the rest of the summer, although a change in weather might reduce its impact for a while, he said.
''If it gets windy and cooler, it might do something, but it will still be around.''
Public health officials warned people swimming in areas with algal bloom could develop allergic reactions but it depended on the concentration of the toxins.
Fishermen and boat users should also exercise caution and dog owners should ensure their animals did not enter the water.
