The Phosphorus Question: Is Lake Manitouwabing Healthy?
Happily the short answer to the Phosphorus Question is yes. But it’s vulnerable.
Phosphorus is an essential nutrient for all biological life in freshwater. So the amount of phosphorus in its water is a good measure of overall lake health. Lakes with a phosphorus range below 10ppb/l, classified as oligotrophic, have minimal aquatic plants and are a good habitat for trout; Lake Joseph in Muskoka is a good example. Lakes with a phosphorus range above 20ppb/l are classified as eutrophic. These lakes have large areas of aquatic plants, and can have blue green algal blooms, which can potentially release microcystin, a toxin harmful to humans and wild life.
Lake Manitouwabing is classified as a mesotrophic lake. This means its phosphorus level ranges from 10 to 20 parts per billion per litre (ppb/l), which is good for aquatic life and an array of fish species. In 2018, the average concentration of phosphorus in Lake Manitouwabing was 11ppb/l, a good range for healthy life. A more recent study of the lake in 2021, however, found a range of 18.6 ppb/l. This indicates the lake is approaching eutrophic levels. These levels could lead to blue green algal blooms and the release of microcystin. That’s why our lake is vulnerable.
Can we do anything about it? To answer this question, it’s important to consider the two main sources of the phosphorus in our lake.
First, phosphorus in our freshwaters is released when organic material decomposes. This is a natural and wholly expected process. Water samples around the lake reveal that most of the phosphorus in it comes from decomposing matter from the many wetlands around the lake, and the consistent levels of phosphorus in these samples show that these wetlands are healthy.
Second of all, human activity is a second source of phosphorus. The good news is, we can change our activities to reduce this source.
Shallow-rooted grass, for instance, only partially absorbs phosphorus-laden rain water if there is a lack of deep-rooted vegetation along shorelines and too much grass.
One important solution is to create a 10-metre wide buffer along the shoreline, planted with deep rooted perennials, shrubs and trees that can absorb rainwater. Not only will these plants absorb any phosphorus available, they will provide a habitat for pollinators, reptiles, songbirds and small mammals, discourage Canada Geese and maintain our unique shoreline aesthetic.
Another solution is to down-size your lawn to a size sufficient for running around and social activities. You won’t need to spend as much time caring for it and have more “hammock time.”
It’s also important to save the environment and your money by reducing or not using lawn fertilizers, which send excess phosphorus and nitrogen into the lake.
Septic systems are probably the biggest source of excess phosphorus in our lakes. Out-dated or poorly functioning systems leak phosphorus and overflowing tanks are a major contaminant. Deep-rooted vegetation and heavy vehicles or storage containers on septic beds can disrupt the seepage from the perforated pipes into the gravel layers, reducing efficacy of the bed.
Give your septic system filters an annual check-up to make sure they are working properly and pump the tanks out periodically to prevent overflow. How often is dependent upon how many people use the system, the number of bathrooms and the age / construction of the system. Older systems should be checked and reviewed in their entirety to make sure they can handle modern demands of multiple bathrooms and dishwashers. Many existing septic systems are now approaching the end of their effective life and may not meet current construction regulations. Have an older system inspected in case it’s no longer functioning properly and arrange to rebuild it to meet current regulatory standards.
Finally, shoreline erosion from motor boats powered by high horsepower motors has recently become a source of excess phosphorus in the lake. Such boats can even cause damage moving through narrow passages at moderate speeds. The wakes from these powerful engines stir up bottom sediments that can smother aquatic vegetation. The turbid waters they create are impossible for sunlight to penetrate, thereby depriving plants of the light they need for photosynthesis. As a result, plants at the bottom die, and the decaying bottom vegetation releases phosphorus that puts our lake in a vulnerable position.
Boat operators can do better if they just learn what size of wake they create when moving at moderate speeds to minimize the size of their wakes.
Lake Manitouwabing is a great place; we need to remain vigilant about phosphorus to keep it that way.
This article was researched, written and edited by members of the MLCA, based on several years of water sampling by the volunteers of the Lake Partner Program and the Ontario Ministry of the Environment. In 2023 the Lake Partner Program volunteers will sample the lake to collect data on phosphorus and E. coli to report back to the MLCA the health of our lake.
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