A blue-green algae that has bloomed near Omokoroa in Tauranga Harbour is being investigated by Bay of Plenty Regional Council scientists.
The algae looks like "fine, green cotton wool" and is growing in 20 hectares of sea grass in the upper reaches of Mangawhai Bay, on the eastern side of the Omokoroa Peninsula.
Regional council coastal scientist Rob Win said they do not yet know if it is harmful to people or what has caused the bloom. He suggested people steer clear and avoid contact as a precautionary measure.
"It's a type of blue-green algae or cyanobacteria that looks a lot like a species called Lyngbya. We've seen it amongst sea grass beds in the harbour before but not on this scale."
Mr Win said the bloom was likely to have occurred in response to unusually warm water temperatures in the harbour over summer and the high number of swans around Omokoroa.
"There's lots of swan excrement in the water there at the moment that may have fuelled algae growth," Mr Win said.
Mr Win said that the algae accumulates gas bubbles (from high rates of photosynthesis) around its filaments, causing algae clumps to rise to the surface and form large conspicuous floating mats.
"We're keeping an eye on it. In the meantime if anyone experiences irritation after contact with the algae, they should seek medical advice," Mr Win said.