Churning sea water

Found on the beach: sea foam. At first sight, there’s not much to the creamy-white frothy stuff that gets blown across the Brighton Beach shingle now and then, and, yet, look it up and you soon find yourself at the intersection of ocean chemistry, biology, and weather. 

When waves churn up water containing dissolved organic compounds - mostly proteins, lipids, dead plankton, fragments of seaweed, and microbial by-products - these act as surfactants, lowering surface tension and allowing bubbles to form and persist. The rougher the sea, the more vigorously the water column is mixed, drawing these organics from deeper layers to the surface. Brighton’s English Channel water, especially after storms that rip up seaweed beds, is particularly good at producing short-lived, bubbly foam.

A single litre of seawater can contain millions of phytoplankton cells, and when some species die off in blooms, the cellular breakdown releases long-chain molecules that are remarkably similar to the stabilisers used in food foams (like the head on a beer). That’s why foam can look so creamy despite being nothing more than air, water, and microscopic biological debris.

Globally, sea foam becomes more intriguing - and sometimes alarming. The most notorious examples are the ‘foam tsunamis’ of Australia’s east coast, where intense storm swell can drive metres-deep, cappuccino-coloured foam through seaside towns. In 2020 at Yamba and 2007 at Sydney’s beaches, whole cars disappeared under it. The foam itself was harmless; the force of the waves beneath it was not.

In California, the breakdown of Phaeocystis algal blooms has produced foam rich in proteins that can become irritant, stinging exposed skin. Conversely, along South Africa’s KwaZulu-Natal coast, winter sardine runs produce a slick of fish oils that help create thick ivory-coloured foam valued by surfers because it flattens chop on the surface.

The strangest example comes from the North Sea, where researchers found sea foam rich in microplastics - tiny fragments that stick to the bubbles and are blown far inland, making foam one of the vectors by which coastal microplastic pollution travels beyond the shoreline. The foam acts like a sticky film, picking up plastic shards, tyre particles and airborne dust.

Foam can also carry nutrients and spores. After the giant Sargassum blooms in the Caribbean, decaying mats generate surfactants that fuel disproportionately large foam lines on beaches; these sometimes carry bacterial loads high enough to make cleanup crews wear masks. In contrast, in Iceland and parts of the Baltic, sea foam streaks can be associated with whitish pumice ash, swept into the surf after volcanic eruptions.

Even the colour can be telling. Most foam is white because the bubbles scatter light uniformly, but brown foam signals a higher concentration of organic matter; greenish foam often appears during phytoplankton bloom collapse; and pinkish foam has occasionally been recorded after massive blooms of pigmented dinoflagellates - the same organisms responsible for some red tides.

Brighton’s foam, by comparison, is modest, fleeting and almost always benign. It’s simply the English Channel exhaling - a reminder of the constant churning, mixing, and invisible biological life just offshore, thrown up in small bright heaps that the wind leaves on the stones for a few minutes before carrying them away.

More information can be found at National Ocean Service, Science Direct, Marine Insight and How Stuff Works. The photograph of foam on the beach at Yamba was found at the BBC (Nature’s Weirdest Events).