Seaweed
"Seaweed" is a colloquial term and lacks a formal definition. A seaweed may belong to one of several groups of multicellular algae: the red algae, green algae, and brown algae. As these three groups do not have a common multicellular ancestor, the seaweed are in a polyphyletic group. In addition, some tuft-forming bluegreen algae (Cyanobacteria) are sometimes considered to be seaweed.
Sea foam
Foams can form following the degradation of harmful algal blooms (HABs). These are primarily composed of algal species, but can also consist of dinoflagellates and cyanobacteria. Biomass from algae in the bloom is integrated into sea foam in the sea surface microlayer. When the impacted sea foam breaks down, toxins from the algae are released into the air causing respiratory issues and occasionally initiating asthma attacks. "Phaeocystis globosa" is one algal species that is considered problematic, as observed in a study in the Netherlands. Its high biomass accumulation allows it to create large quantities of toxic foam that often wash onto beaches. "P. globosa" blooms are initiated in areas of high nutrient availability, often affiliated with coastal locations with a lot of stormwater runoff and eutrophication. Studies suggest that the development of foam is directly correlated to blooms caused by "P. globosa", despite that foam formation typically occurs approximately two weeks after the appearance of an algal bloom offshore. Organic material from "P. globosa" was observed decomposing while suspended at the sea surface, but was not observed lower in the water column. "P. globosa" is also considered a nuisance species because its large foam formations impair the public's ability to enjoy the beach.