What causes the seas to foam?

A: Sea foam needs two ingredients: something to increase the surface tension of the water, like a bubble bath, and something to froth it up, like water running into a tub.

In the ocean or sea, the bubble bath is usually dissolved organic material, and strong surface winds or the breaking of waves on the beach stir up the water with air to make bubbles. The organic material comes from a number of sources, usually a concentration of biomass such as the phytoplankton bloom that causes a red tide or a fish kill. A bloom is an increase in the numbers of some species or complex of species that then die or are eaten, releasing organic material. The material can also come from sewer spills and other terrestrial runoff.

In addition, there are a number of mechanisms that concentrate the foam, which is commonly blown up on the beach, for example. Seafoam is seldom seen in the open ocean as most areas do not support high concentrations of plankton, though there are spots where the right conditions for an organic bloom occur.