Phosphorus levels in Lake Nepahwin cause for concern

John Gunn has lived near Lake Nepahwin in Sudbury's south end for three decades, often swimming in the lake throughout the summer. 

Like many Sudburians who frequent the lake, Gunn, who is the director of the Vale Living with Lakes Centre at Laurentian University, has noticed changes to the once clear waters. 

"Little by little the lake is getting greener and greener and there's far more algae, Gunn said. 

"I think it's pretty clear in the last five years that the lake is moving to a new state." 

Nepahwin's phosphorus levels have risen in recent years, hitting concentrations of 20 micrograms per litre in 2017 — a level Gunn calls a "magic guideline" that clear lakes should not cross. 

And now the city is looking at ways to improve monitoring, and in time, improve the health of the popular lake. 

Runoff, road salt

The increasing phosphorus levels prompted the city to commission a study of the lake and its watershed, the results of which were released this month. 

Of the more than 50 lakes the city monitors, Nepahwin is the only one with a "statistically significant" increase in phosphorus levels, according to Stephen Monet, the city's manager of environmental planning initiatives.

We don't want the lake to reach a point where kids can't enjoy swimming in the summer in the lake.— John Gunn, director, Vale Living with Lakes Centres

According to the study, urban runoff is the "most likely reason" for the increased phosphorus levels. 

While some negative effects may be from current practices, Gunn says some of the problems "are built up over decades." 

"We have a history of poor construction practices around, in the watershed, where we didn't control silt and runoff very well," Gunn said, resulting in phosphorus stores in the sediment of the lake, which are now being released — likely accelerated by climate change. 

Gunn says increased algae growth caused by high phosphorus levels is also compounded by another environmental problem — road salt.

Gunn says road salt affects zooplankton, which he describes as the "little lawnmowers that eat algae." As those organisms die, algae thrives. 

While the city has reduced its reliance on road salt in recent years, Gunn notes that the watershed covers a large area, including houses and box stores. 

"All of those people have to help out, in order that the beaches remain clean," Gunn said. 

"We don't want the lake to reach a point where kids can't enjoy swimming in the summer in the lake, and are having to fear for toxic algae blooms. So, that's the worst case scenario that could happen," Gunn said. 

'Cause for optimism'

With good environmental practices, Gunn hopes Lake Nepahwin will be able to return to better health over time, as old phosphorus deposits clear out, and fresh rainwater takes over. 

Monet, with the city, agrees. He says he doesn't "want to mislead or create false expectations like that this problem would solve itself tomorrow" — but he says he is hopeful. Partly because Lake Nepahwin doesn't have large pieces of land left to be developed, potentially further polluting the lake. 

"We don't have that with Lake Nepahwin, so that's another cause for optimism I guess, is that whatever has happened seemed to have happened already with regards to phosphorus loading due to human action," Monet said. 

The study from the city includes several recommendations, which Monet says the city is working on. Those include the continued monitoring of temperature, phosphorus and oxygen levels, along with reviewing storm water inputs and sanitary sewers, and identifying any opportunities for improvements.