Virginia Lake algae will need ongoing management

A thick layer of algae appeared on Rotokawau Virginia Lake in April. Photo/ Bevan Conley

Excessive nutrient runoff from the development of land around Rotokawau Virginia Lake means algal bloom will likely need management, Whanganui District Council says.

At the end of summer a thick, dark green algae bloom appeared on the lake, a popular Whanganui tourist spot, raising concerns about the impression it left on visitors.

The council said the algae, which needs sunlight, nitrogen and phosphorous to grow, was probably helped by the warm summer.

On top of that ultrasonic units used to fight the growth are currently not working.

The bloom resulted in a report by council senior wastewater engineer Arno Benadie which revealed the first major bloom occurred in 2007 - followed by another in 2010 - and that excess nutrients in the lake meant it would likely be an ongoing problem.

"Prior to this there had been increasing indications of blooms in areas where water tended to be more stagnant but the severity of the 2007 bloom had not been seen previously," the report said.

"Stormwater soakage areas which would previously drain into the ground and go through a natural filtration process before reaching the lake are being lost to a built environment resulting in more nutrients such as nitrogen and phosphorous... getting into the water."

Testing revealed this year's bloom was the non-toxic Microcystis wesenbergii.

A range of mitigation measures have been put in since 2007 including a wave catcher to circulate oxygenated water to the bottom of the lake, an overflow system to create churn and the ultrasonic emitters.

"The results have been really, really good," Benadie told the council's property and community services committee.

"Since 2011 we must've been through four summers with no algal bloom at all which is an extraordinary result for Virginia Lake.

"We are completely confident we will have them fixed up and working by the start of spring and summer."

Benadie said chemical fixes were not a desirable option.

"Country wide people would resist adding more chemicals to the problem. Every time you add something it just adds more to the problem."

The nutrient hot spots are where there is a high amount vegetation hanging over the lake.