Blue green algae bloom in the lagoon in July, 2020
After 6 weeks of operation, lagoon was sufficiently clear that the bottom of the lagoon was visible
Hazardous algal blooms (HABs) or blue-green algae (BGA) blooms are increasing in Canada's recreational and water supply lakes. Simple avoidance of affected lakes for drinking or recreational activities is impractical and yet the occurrence of BGAs is increasing in response to climate change. Stewardship, including nutrient reduction, is important, but we need to try something new which will have immediate results, and be sustainable over the long term. Only active management will preserve some of Ontario's most important recreational resources subject to HABs from frequent closures and warnings. The toxins from HABs may have direct and indirect human health effects. Ultrasound has been demonstrated to safely control HABs in small ponds and reservoirs in Canada, the USA and internationally . Our 2018 project has demonstrated that ultrasound can effectively treat larger bodies of water (e.g potentially up to 10,000 hectares).
Blue-green algae are cyanobacteria which occur naturally in aquatic environments. BGA flourish in slow-moving or still waters with high nutrient levels and sufficient sunlight and especially with warm temperatures. Some BGA produce microcystin toxins which are the most commonly produced toxin of the cyanobacterial toxins. Microcystin toxins are tasteless, colourless and odourless but are toxic to both humans and animals. Typical exposure routes are through ingestion or inhalation of aerosols or steam from the contaminated water. Consequently, BGA is primarily of concern during the summer periods when body contact recreational activities (swimming, boating) are common or the water is being used as a drinking water supply or irrigation.
The long term approach to managing algae is through nutrient control and reduction. This approach, however, provides no immediate remedy and may not even be achievable in systems with shallow waters, nutrient rich sediments, restricted circulation and continued nutrient loading from its watershed. In-situ nutrient management options could include precipitation of phosphorus with flocculants such as aluminum sulfate or polyacrylamide polymers and the isolation of nutrient rich bottom sediments with inert and phosphorus adsorbing caps such as Phoslock®. Common algae control methods include the use of copper sulfate (can be toxic to other species especially invertebrates and fish), sodium percarbonate (peroxide, a sterilant with health and safety concerns), pond dyes for shading (not appropriate for recreational areas) and herbicides which can leave chemical residues and result in undesirable side effects including restricted human access. Ultraviolet treatment can be effective on closed systems but has high power demands and is not appropriate for open water conditions. In short, existing alternatives each have issues and limitations and consequently, the management of BGA requires an innovative approach.
The use of ultrasound to control BGA avoids the use of chemicals and provides rapid relief from the concerns associated with cyanobacterial toxins and provides a safe and appealing aquatic environment for users. Importantly, ultrasound, at the frequencies and power levels used to control blue-green algae, has been demonstrated to have no negative impacts on macrophytes, fish, other aquatic organisms and, most importantly, poses no risk to human users.
The structure of BGA makes it susceptible to critical resonance vibration induced by the ultrasound source. The blue-green alga contains clusters of many tiny cylindrical vesicles about 75 x 300 nm (1 nm is 1 billionth of a metre or one millionth of a millimetre). The wall of the gas vesicle, which is permeable to gases but not to water, is about 2 nm thick. Gas vesicles in cyanobacteria are used to make them buoyant which is what they use to move through the water column to access food. As the algae creates carbohydrate mass (the green colour observed in the water) during sunlight hours, they gain enough weight to be heavier than water and sink. This allows the algae to find the necessary nutrients near the bottom or at deeper depths. As the carbohydrate ballast is consumed, they slowly rise to the surface due to the air contained in the vesicles. Ultrasound damages the wall of the gas vesicle, interrupting this cycle and restricting the growth of the algae. In essence, ultrasound restricts the ability of the algae to find food resulting in reduced growth and reproductive rates and eventually, death.