The algal bloom that tarnished Virginia Lake earlier this year refuses to go away.

Scientists expected the blue-green algae would die out in the chill winter, but not so.

Barry Gilliland, Horizons Regional Council's water quality leader, said latest sampling from the lake had shown a rapid increase in the algal cell count in the lake.

Mr Gilliland said he had no explanation as to the cause. He said the Wanganui District Council had been monitoring water temperatures and oxygen levels in the lake.

"That data might be able to tell us if the lake has 'turned over', meaning the top and bottom water may have been mixing.

If it has, then it could have released phosphorous, which the algae are taking advantage of and feeding off."

Mr Gilliland called the rising blue-green algae count as "odd" for this time of the year.

He said Lake Horowhenua, which had suffered from a similar sort of algal bloom to Virginia Lake, was showing no sign of the algae when latest counts were made late last month.

"It may be because Virginia Lake is more sheltered, and the weather may have been warmer too."

The latest samples, taken last week, showed definite green coloration and contained a fairly high density of small green particles clearly visible to the naked eye.

"The first thing that caught my attention on opening the sample bottle was a definite odour typical of blue-green algae," Mr Gilliland said.

"And looking at the sample under the microscope, I saw a number of colonies of the blue-green algae of the type we have consistently seen in the lake over the last few months."

He said the cell density level of 36,370 blue-green cells per millilitre at September 6, was a higher density that the previous sample taken on August 23 and more than twice the density above the August 16 sample.

Horizons testing indicated a big shift in algal density happened between August 6 and August 16.

Up to August 6 the cell estimate was less than 2000 per millilitre but it accelerated to nearly 18,000 per millilitre before hitting the latest peak.

"It's really all a bit up in the air at the moment. I don't know why we are seeing an increase in cell numbers at this time," Mr Gilliland said.

"As I have said before the simple factors that influence growth are available of nitrogen, phosphorus and sunlight. Growth is also affected by temperature. "Blue-green algae generally grow better at warmer summer temperatures, so growth during winter seems a little odd unless lake water temperatures have remained warmer than usual."

He said another complicating factor is the algas ability to regulate its depth in the lake and the fact that surface scum can drift around to different locations depending on wind direction.

"This means that our samples may not be completely representative of the whole lake at times," he said. Horizons will continue monitoring the Virginia Lake.