A new technique tipped to prevent the release of sediment and phosphorous will soon be tested in Lake Rotoehu.
The de-stratification project will take water from the deepest part of the lake and pump it to the surface to eliminate pockets of low oxygen water. which contributes to the release of phosphorous and nitrogen – key nutrients which lead to poor water quality and algal blooms.
The Rotorua Te Arawa Lakes Strategy Group approved the aeration project and if successful on Rotoehu, it could be used on larger lakes such as Rotorua.
“This release [of nutrients] occurs at times when a lake has low oxygen levels in its bottom waters. For shallow lakes such as Rotorua and Rotoehu, short periods of lake stratification (separation between the surface and bottom waters due to temperature differential) occurs during summer and autumn,” Bay of Plenty Regional Council lake operations manager Andy Bruere said.
Aeration is the application of a capping agent such as alum or dredging the sediments. The process of restoration could be accelerated by addressing both catchment and nutrient inputs, as well as nutrient recycling from lake sediments.
Research by the University of Waikato and the Regional Council has shown there was a high level of success in capping agents in small lakes or bays but dredging had much higher cost estimates for New Zealand due to geothermal contamination with mercury and arsenic.
The $524,000 project has the support of the community, especially Maori and the Tautara Matawharua Maori Trust Farm, who provided on-going access close to a new floating wetland on the lake.