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While the foam that washes up on shore may look like the foam in your kitchen sink, lake foam is usually a naturally occurring phenomenon and not caused by pollution. Foam is created when the surface tension of water (the attraction of water molecules for each other, which gives a drop of water its round shape) is broken down and air is mixed in, causing bubbles. Surface tension can be reduced by natural organic compounds as well as pollutants, such as soaps and detergents.
Natural organic compounds are released into the water by live and decomposing algae, plants materials (Dissolved Organic Carbon - DOC) and fish. As wind stirs the water, air is mixed in with the water because of its reduced surface tension, and foam is produced. Foam will often collect in large quantities on windward shores which is where we usually see them collect on Lake of the Woods. Natural foam will usually have a dirty brown color, and will smell earthy or fishy. In addition to causing foam, DOC provides energy and performs many additional functions important to aquatic ecosystems. The foam caused by these circumstances is not toxic.
Foam from detergents and soaps (synthetics) is now uncommon as most are now biodegradable (able to be broken down by bacteria) and unable to produce the long-lasting foam found on shorelines. Foam from detergents will be white and sweet smelling or scented. More commonly, human activities like the introduction of nitrogen or phosphorus fertilizers can cause increases in algae or aquatic plant growth. These can cause plant and algae blooms and an increase in foam production along with the removal of oxygen as these plants decompose. This is why limiting the use of these substances around Lake of the Woods is also important.