Water Science: Salt, Lakes, and Fish

Each winter, VLAWMO works with the Birch Lake Improvement District (BLID) to monitor chloride levels in Birch Lake. While there are many varieties of road salt, one thing they have in common is chloride. With the widespread application of salt on roads and driveways each winter, this poses a dilemma for our lakes. While some chlorides are natural in the aquatic environment, excess quickly takes a toll on the lake's basic functions.

Dissolved Oxygen: Shallow lakes such as Birch are dynamic, changing drastically according to the conditions around them. In the winter, ice on shallow lakes can potentially freeze down to the lake bottom. If this happens, "pockets" can form under the ice, causing the dissolved oxygen within the pocket to be depleted. If this happens, fish stuck in the pocket aren't likely to survive, even if the lake has an aerator.

Lake Turnover: Beyond shallow lakes, even deeper lakes such as White Bear Lake are impacted by chlorides. Because chlorides increase water's density, contaminated water "sticks" to the bottom. Cold water is also more dense, so each fall warm surface water is cooled and brought to the lake bottom. Come Spring, this same water is brought back to the surface as the lake warms. Another term used to describe the different levels of chloride and temperature within a lake is "stratification". Aquatic life depend on the nutrient cycling that occurs along with lake turnover - but increased chlorides impair the lake's turnover if concentrations are high enough. See another video on water circulation to learn more about lake turnover.

While many lakes are monitored at ice-out, Birch Lake is monitored throughout the winter because it's shallow enough that if both a deep freeze and high salt content occur together, both oxygen and turnover would act to threaten the fish population. 

Resources are available from VLAWMO on how to properly apply salt de-icers if they're absolutely needed. Alternatives such as sand, grit, or acetate (non-chloride) de-icers are great ways to reduce or eliminate salt use and keep our lakes on a "low salt diet". 

 

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