Templeton Pegasus Limited own Pegasus Lake and engaged consulting firm WSP to test whether aeration would help improve water quality in the lake – specifically looking at whether it was possible to disrupt season stratification and reduce cyanobacterial blooms.

The trial took place between August 2022 and April 2023 and involved sectioning off part of the lake and installing aerators on the lakebed.

The study showed that aeration could reduce instances of stratification (the creating of distinct temperature layers within lake).  Aeration generated uniform water temperatures and prevented oxygen depletion in the bottom waters of the lake.

The aeration did not have any noticeable impacts on the overall nutrient levels in the lake which are also contributing to the cyanobacterial blooms. This was not unexpected as the aeration trial was selected to target the issue of stratification as a first step in addressing the water quality issues in the lake.

The lake has a surface of approximately 14 hectares and its primary function is a high quality amenity lake. The lake is also important for the management of stormwater and reducing the risk of flooding in severe storm events. The consent conditions say water quality needs to be suited for secondary contact which is boating or kayaking – not swimming.

The next steps being considered for the lake is to scale up the aeration system to address stratification throughout the lake and improve the health of the lake while acknowledging that it may not prevent algal blooms without nutrient reduction. The timing of this is up to Templeton Pegasus Limited as owner of the lake, noting the Council recommendation is that this is completed in 2024.

The lake currently does not comply with resource consent conditions and is closed due to the algal bloom.