The chemistry behind sea foam

[Global] While birdwatching down in the salt marshes of the Parker River National Wildlife Refuge this past weekend, something completely unrelated to birds caught my eye. It was a beautiful bright windy day. So windy that even the water in the shallow salt marsh pannes (pools that form on salt marshes — places where birds like to hang out and eat) had white caps. What caught my eye was the thin layer of fluffy, bright white foam that clung to the downwind edges of the pannes. I’ve seen sea foam at the beach but never in a salt panne, the conditions must have been just right.

For sea foam to form you need a few basic ingredients. Water is important, it doesn’t necessarily need to be ocean water-foam also forms on lakes and along the bases of some trees after a good rain. You also need something to churn up the water, introducing air into the mixture; a brisk wind, waves or a lashing rain storm work. Finally, for naturally-occurring sea foam to form, you need some kind of decomposing organic material. The carbohydrates, proteins and fats found in planktonic algae and seaweed for example. Large algal blooms are a common source of sea foam.

The chemistry now gets a little complicated. These fats and proteins act as surfactants (one of the ingredients in soap), molecules that have two ends — one end sticks to water (hydrophilic), the other end is repelled by water (hydrophobic). When you get enough of these molecules in one place bubbles start to form because the surfactant molecules start to line up with one end sticking to the surface of the water while the other end is held out of the water. These molecules can also organize themselves back to back, with a thin film of water held between the two lines of surfactant. Bubbles form because, from a physics standpoint, spheres are the easiest shape to form. I like to imagine the situation as something like buffalo circling to protect their young from a pack of wolves. Like the buffalo, surfactant molecules are trying to protect their hydrophobic ends from the water.

The way surfactants function in soap is similar. They break down the interface between water and oils and/or dirt. Their hydrophobic ends surround the oil and dirt molecules and hold them in suspension. Instead of sticking to each other, the molecules of oil and dirt are lifted and carried away inside of surfactant bubbles.

Even though sea foam can have natural causes it isn’t necessarily harmless. According to NOAA (the National Oceanic and Atmospheric Administration), “Most sea foam is not harmful to humans and is often an indication of a productive ocean ecosystem. But when large harmful algal blooms decay near shore, there are potential for impacts to human health and the environment. Along Gulf coast beaches during blooms of Karenia brevis, for example, popping sea foam bubbles are one way that algal toxins become airborne. The resulting aerosol can irritate the eyes of beach goers and poses a health risk for those with asthma or other respiratory conditions. Scientists studying the cause of a seabird die-offs found a soap-like foam from a decaying Akashiwo sanguinea algae bloom had removed the waterproofing on feathers, making it harder for birds to fly. This led to the onset of fatal hypothermia in many birds.”

When we see sea foam we tend to think it is caused by pollution. It certainly can be. For example, the discharge of various types of chemical waste and sewage can cause the production of sea foam. In general, but not always, natural foam is light in texture, not slimy and with no unpleasant odors. Natural foam is usually white but can have a reddish or brownish tinge depending upon the type of plankton it comes from or the presence of tannins in the water.

The sea foam around the salt pannes was most likely naturally-occurring, perhaps due to the presence of all those migratory waterfowl I had traveled to Plum Island to watch fouling the waters.


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