Why is there green foam on the lake?

I’ve been canoeing a lot this summer, so I’ve also seen a lot of notices about blue-green algal blooms, but this past weekend took it to a whole other level with piles of green foam on the beach and beautifully, but disturbingly coloured banks. While these blooms are relatively common regardless of conditions, warming climates and run off from inhabited areas both contribute to the frequency and intensity of the blooms.

Since the blooms typically happen during warmer summer weather, as temperatures increase due to climate change, it is anticipated that the blooms may become more common just based on temperature. In addition, warmer water may not mix as well which would disturb the algae less and allow it to grow faster. Warmer water is also easier for algae to float to the surface in (warmer water is less dense). The blooms also absorb sunlight which warms the water further (EPA, n.d.).

Yan et al (2017) also looked at another potential relationship: do the blooms themselves contribute to climate change? They found that large amounts of both methane and carbon dioxide are produced as the blue-green algal blooms decompose. These are both important greenhouse gasses. Further, the decomposing blooms release the nutrients nitrogen and phosphorous. These contribute to something called eutrophication.

A eutrophic lake is one that has a lot of nutrients available for producers like blue-green algae. Other things that contribute to eutrophication is run off from agricultural areas (eg. fertilizer carried into the water), release of sewage into water, and things like washing your car in your driveway.

An important note (to some): Blue-green algae is the common name for cyanobacteria. This is a type of prokaryotic organism (like bacteria) that are a little different from the cells that you and the tree down the street are made of. They are also different from true algae so some scientists disagree with calling cyanobacteria blue-green algae. However, it is the name commonly used in the warnings posted at lakes so I’ve kept it.

So why do we need to be warned about high levels of blue-green algae? It produces toxins that can cause irritation for most humans but can actually kill animals like dogs if they drink the water.