MUSKOKA LAKES — An invasion of potentially toxic blue-green algae shocked property owners on Three Mile Lake in September 2005.
Now residents are getting answers into the cause of the bloom, also known as cyanobacteria or pond scum.
Results of a two-year scientific study point to a number of factors, including an increase in phosphorus and unusually warm, dry conditions, which came together to create optimal conditions for growth of the blue-green algae, according to Andrew Paterson, a senior scientist with the Ministry of the Environment. Paterson, who works at the Dorset Environmental Science Centre, made a final report on the ministry study to the District of Muskoka’s planning and economic development committee on March 25.
The study looked at the lake’s levels of phosphorus from many different sources including shoreline development, the watershed and internal sediments in the lake.
Phosphorus contributes to the growth of blooms, said Paterson.
“We know that phosphorus is important. It is linked to the increase in blue-green algae in this particular lake,” he said.
The study determined the most significant load of phosphorus, 50 per cent, found at Hammell’s Bay came from the lake sediments.
The second most came from the watershed. There is still a significant amount, 20 per cent, which can be attributed to shoreline development, he said.
The development of surface scum of the algae is linked to weather, Paterson said, and the fall of 2005 was one of the driest and warmest on record in the area.
“I believe that one of the major contributors to the formation of the scum … was the dry and hot conditions.” He added algae are not uncommon for the lake, but the formation of scum is rare.
Paterson said the weather in 2005 may have been a one in 70-year event, but the climate could be changing.
“Under the scenario of climate warming for the region, what will the frequency (of algae outbreaks) increase to?” asked Paterson.
At the time of the algae outbreak, there was a public warning advising property owners not to use the water for drinking, bathing, cooking, dishwashing, clothes washing, and water sports.
Wayne Snow, president of the Three Mile Lake Association, said he remembered looking out at the lake and seeing the green scum.
Property owners are still concerned the scum will return and the possible health risks this would pose, said Snow. Armed with the results of the study the residents are on guard for another outbreak.
“I think it will happen again,” said Snow.
Algae blooms aren’t necessarily toxic and testing showed the Three Mile Lake blooms were not toxic, said Paterson.
“In the end, maybe the threat level wasn’t as high as perceived,” he said.
On the other hand, Paterson said residents should always err on the side of caution as some species of algae are toxic. Residents should always report scum.
Snow thinks residents are getting the message and are working on what is reasonable to control. The property owners are thinking more about septic tank control and shoreline development.
“That’s the key message. That is what we can focus on,” said Snow.
While the largest portion of phosphorus is coming from lake sediments, Paterson suggested the only thing residents can control is the shoreline development and septic systems. Residents can increase shoreline vegetation, reduce use of detergents with phosphorus and ensure septic systems are properly maintained, he said.