A simple stroll along a beach on a windy day can be mesmerising. It’s easy to spend hours watching waves crash and sea foam fizz across the sand, but this beautiful yet fleeting phenomenon can also give us clues about the health of the ocean.

Sea foam is produced by the turbulence caused by the force of the waves and the wind which, when combined with organic matter (mainly plankton), forms a mixture of water and air bubbles that clump together and rise to the surface as foam.

This colloidal dispersion – the name for non-soluble particles suspended in a gas or liquid – occurs because organic matter reduces surface tension, a physicochemical property whereby liquids behave as if they were covered by a thin elastic membrane.

In order to mix substances that cannot be dissolved – such as water and air or water and oil – surface tension must be reduced by surfactants (chemical compounds with both hydrophilic and hydrophobic parts) which act as a bridge. This creates an interface between the two components, allowing liquids to mix with gas and solids, or with other liquids, such as water and organic matter.

This property is essential for, for example, ensuring the bioavailability of certain organic compounds in agrochemicals, for biodegrading oil spills, and for remediating soil contaminated with fuel. On a more mundane level, it’s also the mechanism that helps us wash stubborn grease stains out of clothes.

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What sea foam tells us

Sunlight is essential for life on Earth, and consists of three main components: visible light, infrared (heat) and ultraviolet (UV) radiation. The main characteristics of light are its wavelength and frequency.

Visible light (with a wavelength of approximately 400 to 700 nanometres) is a small portion of the spectrum that the human eye can perceive. When light strikes an object, its surface absorbs certain wavelengths and reflects others. Only the reflected wavelengths can be seen by the eye, meaning only those colours are perceived by the brain.

The colour of the sea changes due to the selective absorption of light by the water, which is influenced by its chemical composition, marine life, and weather conditions. Water is, of course, transparent, but when there is a large volume of it, light absorption across the spectrum increases, resulting in a blue hue.

In sea foam, air bubbles scatter and reflect light in all directions without absorbing it, which explains why it appears white to the eye. It is, importantly, distinct from the colour “seafoam” in art or fashion, which is usually a muted blend of green and blue.

When it comes to sea health, white foam that disappears almost instantly is a clear sign of a healthy ecosystem. Conversely, dense foam that sticks around, a dark (typically brownish) colour, or an unpleasant odour are symptoms of either chemical pollution (from agricultural fertilisers, or industrial or urban runoff) or biological pollution.

One example of biological pollution is the recurring episodes of foam formation on the beaches of South Australia. During February and March in 2025 and 2026 (summer in the southern hemisphere), algal blooms covered swaths of the coast in foam, with terrible consequences for the health of animals and people alike.

If we look after the planet and avoid polluting it, it will continue to provide us with healthy environments and scenes of extraordinary beauty. But failure to do so won’t just harm nature and, by extension, our own health – it will also lead to the disappearance of things like the healthy, effervescent sea foam that we know, love and enjoy.

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Gumersindo Feijoo Costa no recibe salario, ni ejerce labores de consultoría, ni posee acciones, ni recibe financiación de ninguna compañía u organización que pueda obtener beneficio de este artículo, y ha declarado carecer de vínculos relevantes más allá del cargo académico citado.