Sea foam on San Francisco’s Ocean Beach. Photo: vision63.
Sometimes, the wind and the waves whip the ocean into a lather. And that word—lather—is a pretty accurate description of sea foam. Sea foam is made of dissolved organic matter, a substance that is so important in the world ocean that it gets its own acronym, DOM. DOM consists primarily of the broken-down bodies of phytoplankton, including microalgae and bacteria. Algal blooms, when they start to die off, create lots of DOM. In sea foam, the DOM acts like soap, creating small bubbles that float on the water.
Dissolved organic matter is full of proteins and lipids (plus lots of carbon, which we’ll get to later). The DOM molecules can act as surfactants, similar to soap and other detergents. The molecules have a hydrophilic end that sticks to water and repels oil, and a hydrophobic end that sticks to oil and repels water. The DOM decreases water’s surface tension and promotes the creation of bubbles as the water is stirred by wind and waves.
Big storms can create huge amounts of sea foam. In 2007, the area north of Sydney, Australia was dubbed the Cappuccino Coast, as foam engulfed 30 miles of shoreline. All this foam can obscure things like rocks and sea snakes, so foam frolickers should frolic with caution.
The best part about sea foam, in my opinion, is not these big foam events, but the fact that sea foam calls attention to dissolved organic matter. We rarely see it (it is dissolved, after all), and we rarely think about it, but DOM plays a massively important role on Earth. It is a key part of the marine food web, though it is hard to eat, because the particles are so tiny. Bacteria are some of the few organisms can eat DOM.
Also, the DOM in the ocean is one of Earth’s largest carbon reservoirs. DOM is produced in the upper ocean, where the phytoplankton and zooplankton live—DOM is made of the spilled contents of their bodies and their cells. The DOM that is not consumed at the surface gradually drifts downward in the water column; it can be found in the deepest parts of the ocean, albeit at lower concentrations than at the surface. As we continue to pump carbon dioxide into the air, some of this carbon ends up as DOM, and it travels slowly throughout the ocean. Next time you see sea foam, think of the dissolved particles of organic matter and the important role they play in the ocean.