Algae and lake scums

Lakes contain various algae and scums throughout the year, so how do you know which are natural and which can cause harm to humans or to the lake? Today, I’ll describe some of the most common substances that lake users will come across.

ironbacteriaFirst of all, have you ever seen a stream, wetland, pond or ditch with a rust-colored substance all over the  bottom and sides? Commonly, if you see this substance you may also see an oily film on the surface of the water. Although this looks like pollution, it is actually not. In areas that have iron-rich water, ground water seepage, and low flow, naturally occurring bacteria called Iron Bacteria oxidize iron for energy. The by-product is ferric iron, which becomes iron oxide when it is exposed to air and water. Iron Bacteria are not known to pose any environmental or human health risk.

What causes the foam that forms along the shoreline on a wavy day? Most foam, if it is fishy smelling, is natural and is made up of natural organic compounds. As aquatic organisms decompose (fish and algae), they release these organic compounds, which mix with the air during wave action and form foam along the shoreline. If you see foam that smells perfumed or soapy, it could be from detergent or soap. Bathing in the lake is fun, but it is better for the lake if you avoid it. foam If you do bathe, make sure you use a product that is phosphorus-free, breaks down naturally, and doesn’t leave a film or foam on the top of the water. This kind of soap can usually be found in camping or outdoor supply stores. Soap film on the surface of the water hurts the aquatic insects (water striders) and spiders that walk on top of the water by breaking the surface tension. The insects and spiders then fall in the water and drown.

What causes algal blooms and why are there different looking algae throughout the summer? First of all, algae are a natural part of a lake’s ecosystem. They are food for many aquatic organisms and they produce oxygen in the water. Algae become a nuisance when they form dense mats and smell as they decompose. The decomposition process also uses up oxygen, which other aquatic organisms such as fish need for survival.

Different types of algae grow best in different conditions, and lakes usually go through a cycle of algae blooms throughout the season. In late spring, diatoms are the most plentiful. They are tiny algae and turn the water brownish, but usually don’t form smelly mats on the lake surface. Later on in the dog days of summer, blue-green algae (Cyanobacteria) dominate and can become a nuisance. When blue-green algae cause the water to look like blue paint, there can be a chance that toxins are present. These toxins can be harmful to pets and children if ingested, so it is best to avoid swimming in these conditions. Blue-green algae are not always toxic, and so far we are unable to predict when they will occur. For more information on toxic blue-green algae, visit:

Phosphorus is food for algae, so usually the more phosphorus is in the lake, the more algae there will be. You can decrease the amount of phosphorus being added to the lake by making sure your septic system is properly maintained, not fertilizing your lawn, and constructing a buffer of native plants along the shoreline to filter and absorb runoff.