Signs that your lake might have elevated levels of bacteria

Swim Advisory Photo by Shutterstock/Tom Grundy

Summer’s here and it’s swimming season. There’s nothing quite like diving into the cool waters of a lake, but sometimes it’s best not to think about what’s sharing those waters with you—it might give germophobes an aneurysm. This certainly isn’t to say you shouldn’t go swimming, but if you’re emerging from the water with red-rimmed eyes, an irritated throat, nose, or ears, or an upset stomach, it’s possible these are symptoms of elevated levels of bacteria. To be safe, you should call your local health unit to test the water.

On Monday, July 8, the Simcoe Muskoka District Health Unit (SMDHU) lifted a swim advisory that had been put in place at Gravenhurst’s Muskoka Bay Park Beach. The advisory had been in effect for just under a week after the SMDHU had found water samples with elevated levels of bacteria. “What we test for in our water samples is for E. coli,” says Christina Wieder, the manager of SMDHU’s Safe Water program. “There are provincial standards for E. coli and what levels are considered to be safe for swimming and when we should be issuing postings.”

To test the water, the SMDHU takes a minimum of five water samples—this may increase depending on the size of the beach—approximately once a week. After the samples have been tested at their lab, they take the geometric mean, “which is similar to an average,” Wieder says, “but a geometric mean takes out the high peaks and the really low numbers.” They compare that number to the provincial standard to see whether a swim advisory is necessary. If the number is above the provincial standard, the health unit will take a resample of the water to validate results.

While the SMDHU doesn’t identify the exact reason for elevated E. coli levels in places like Muskoka Bay Park Beach, Wieder says there are a number of factors that could cause a spike.

“Bacteria levels can increase in the summer, and that can be due to heavy rainfall, cloudy water, large numbers of swimmers, large numbers of birds. If a sample was taken in shallow water or we had really high waves. If there were other events like a municipal sewage release, that could also affect the E. coli levels in the water.”

Wieder says rain, wind, and waterfowl are the most prominent factors. “Rain will wash contaminants into the rivers, the streams, the lakes, and while small amounts of rain are unlikely to have an effect, we advise you to avoid swimming 24 to 48 hours after heavy rainfall,” she says. “The other thing is wind can also have an effect on it as well because wind can build up significant waves.” The waves then stir up bacteria in the water.

“The other part is wild waterfowl—geese and gulls,” Wieder says. “We highly recommend not feeding them and having an environment where they want to come back to because geese and gulls, through their feces, do have a significant impact on E. coli levels.”

If there is no swim advisory in effect on your lake, but you are still exhibiting symptoms of elevated bacteria levels, Wieder says one of the signs you should look for is murky water. “If you can’t see your feet when you’re standing waist deep in the water then bacteria levels may also be higher.”

If there is a swim advisory in effect, then it’s there for a reason. “We’re not saying you can’t swim, but what we’re saying is if you choose to swim during this advisory, you want to avoid dunking your head or ingesting the water,” Wieder says. “And if you are compromised, you take that into consideration when you choose to swim at this beach.”

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