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

At 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.