Sea foam may look sinister but it is mostly harmless and natural

Phenomenon, often seen around Britain’s coast at this time of year, is caused by a combination of algae and weather

A t this time of year a sinister-looking substance can often be sighted around Britain’s coast: a frothy foam piled up along the shoreline or appearing in long ribbons offshore. People sometimes assume this foam is the result of pollution or sewage dumping. In fact it is a common natural phenomenon produced by a combination of algae and weather.

Sea algae start to grow in April as conditions warm. The most common sort, phaeocystis, is not toxic and forms part of the marine food chain. When the algal bloom dies it leaves a brown scum of organic material with surfactant properties, which, like soap, lowers the surface tension of the water.

These natural surfactants create foam when the water is disturbed. Breaking waves churn up the water and produce yellowish-brown foam along the shoreline. This may be so abundant that fragments blow about like thistledown.

Wind blowing over the sea creates rotating horizontal cylinders of water, like submerged rolling pins. These rotating currents, known as Langmuir circulation, push water downward at one point and up in another. Sea foam gathers in long parallel lines in the calm sections, known as windrows, foam lines, or drift lines.

Sea foam may look unnatural, as well as unsightly, and it sometimes smells foul. But it is generally natural and harmless.