Aerator Status

September 2015

The St. Mary Lake Aerators – Status Update

In the interests of transparency and accountability, the NSSWD Board of Trustees wishes to inform our ratepayers that the hypolimnetic aerators in St. Mary Lake will no longer be used and explain the rationale behind that decision.

The aerators were installed in the fall of 2008 and operated during the summers of 2009 through 2012. Midway through the summer of 2013, they were turned off in the face of evidence that they may have prolonged the toxic cyanobacterial bloom that occurred between 2011 and 2013.

In implementing aeration, the District followed the best advice available at the time from lake remediation experts and the Ministry of Environment. The principle behind hypolimnetic aeration is that phosphorus, a key nutrient for algae, will remain in the sediment if oxygen does not become depleted in the lower layer during the summer when the lake is stratified. However, by 2013, it was clear that the aerators had not accomplished the goal of improving water quality and limiting algal blooms by reducing internal phosphorus loading to surface waters from lake sediments.

As a result, the Board made the decision to study the aerators and St. Mary Lake to determine why they did not achieve the desired outcome. For that purpose, the District contributed substantial funds and staff time to the 2014-15 study of St. Mary Lake conducted by the Salt Spring Island Watershed Protection Authority (SSIWPA) Technical Advisory Committee. The results of that study clearly demonstrate that the magnitude of internal phosphorus loading is much smaller than suggested by previous studies, and also suggest the aerators negatively impacted water quality, possibly by disturbing bottom sediments and lowering their capacity to bind phosphorus, which in turn increased phosphorus loading and algal growth.

Although the aerators were unsuccessful as a lake remediation effort, the 2014-15 study has provided the District with a much greater understanding of the complex biogeochemical processes that take place in St. Mary Lake. The data collected were carefully analyzed by an experienced, well-qualified limnologist to help the Trustees make a science-based decision. As we move forward in a new era of water management, this valuable knowledge will continue to be used to improve management and stewardship of all our precious water resources.

Within the next month or two a decision will be made regarding the removal of the aerators. Although the District will no longer be using them they still have value and the District can apply any funds raised from their disposal to other capital projects.