Sea foam

Sea foam, ocean foam, beach foam, or spume is a type of foam created by the agitation of seawater, particularly when it contains higher concentrations of dissolved organic matter (including proteins, lignins, and lipids) derived from sources such as the offshore breakdown of algal blooms. These compounds can act as surfactants or foaming agents. As the seawater is churned by breaking waves in the surf zone adjacent to the shore, the surfactants under these turbulent conditions trap air, forming persistent bubbles that stick to each other through surface tension.

Sea foam is a global phenomenon, and it varies depending on location and the potential influence of the surrounding marine, freshwater, and/or terrestrial environments. Due to its low density and persistence, foam can be blown by strong on-shore winds inland, towards the beach. Human activities, such as production, transport or spillage of petroleum products or detergents, can also contribute to the formation of sea foam.

Sea foam is formed under conditions that are similar to the formation of sea spray. One of the main distinctions from sea spray formation is the presence of higher concentrations of dissolved organic matter from macrophytes and phytoplankton. The dissolved organic matter in the surface water, which can be derived from the natural environment or human-made sources, provides stability to the resulting sea foam.

The physical processes that contribute to sea foam formation are breaking surface waves, bubble entrainment, a process of bubbles being incorporated or captured within a liquid such as sea water and whitecap formation. Breaking of surface waves injects air from the atmosphere into the water column, leading to bubble creation. These bubbles get transported around the top few meters of the surface ocean due to their buoyancy. The smallest bubbles entrained in the water column dissolve entirely, leading to higher ratios of dissolved gases in the surface ocean. The bubbles that do not dissolve eventually make it back to the surface. As they rise, these bubbles accumulate hydrophobic substances. Presence of dissolved organic matter stabilizes the bubbles, aggregating together as sea foam. Some studies on sea foam report that breaking of algal cells in times of heavy swells makes sea foam production more likely.

Falling rain drops on the sea surface can also contribute to sea foam formation and destruction. There have been some non-mechanistic studies demonstrating increased sea foam formation due to high rainfall events. Turbulence in the surface mixed layer can affect the concentration of dissolved organic matter and aids in the formation of nutrient-dense foam.

The composition of sea foam is generally a mixture of decomposed organic materials, including zooplankton, phytoplankton, algae (including diatoms), bacteria, fungi, protozoans, and vascular plant detritus, though each occurrence of sea foam varies in its specific contents. In some areas, sea foam is found to be made up of primarily protein, dominant in both fresh and old foam, as well as lipids and carbohydrates. The high protein and low carbohydrate concentration suggest that sugars originally present in the surrounding mucilage created by algae or plant matter has been quickly consumed by bacteria. Additional research has shown that a small fraction of the dry weight in sea foam is organic carbon, which contains phenolics, sugars, amino sugars, and amino acids. In the Bay of Fundy, high mortality rates of an abundant tube-dwelling amphipod (Corophium volutator) by natural die-offs as well as predation by migrating seabirds contributed to amino sugars released in the surrounding environment and thus, in sea foam.

The organic matter in sea foam has been found to increase dramatically during phytoplankton blooms in the area. Some research has shown very high concentrations of microplankton in sea foam, with significantly higher numbers of autotrophic phytoplankton than heterotrophs Some foams are particularly rich in their diatom population which can make up the majority of the microalgal biomass in some cases. A diversity of bacteria is also present in sea foam; old foam tends to have a higher density of bacteria. One study found that 95% of sea foam bacteria were rod-shaped, while the surrounding surface water contained mostly coccoid-form bacteria and only 5% - 10% rod-shaped bacteria. There is also seasonal variability of sea foam composition; in some regions there is a seasonal occurrence of pollen in sea foam which can alter its chemistry. Though foam is not inherently toxic, it may contain high concentrations of contaminants. Foam bubbles can be coated with or contain these materials which can include petroleum compounds, pesticides, and herbicides.

Structurally, sea foam is thermodynamically unstable, though some sea foam can persist in the environment for several days at most. There are two types of sea foam categorized based on their stability: 1) Unstable or transient foams have very short lifetimes of only seconds. The bubbles formed in sea foam may burst releasing aerosols into the air, contributing to sea spray. 2) Metastable foams can have a lifetime of several hours to several days; their duration is sometimes attributed to small particles of silica, calcium, or iron which contribute to foam stability and longevity. Additionally, seawater that contains released dissolved organic material from phytoplankton and macrophytic algae that is then agitated in its environment is most likely to produce stable, longer-lasting foam when compared with seawater lacking one of those components. For example, filtered seawater when added to the fronds of the kelp, Ecklonia maxima, produced foam but it lacked the stability that unfiltered seawater provided. Additionally, kelp fronds that were maintained in flowing water therefore reducing their mucus coating, were unable to help foam form. Different types of salt are also found to have varying effects on bubble proximity within sea foam, therefore contributing to its stability.