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.
Wrack (seaweed)
It consists largely of species of "Fucus" — brown seaweeds with flat branched ribbon-like fronds, characterized in "F. serratus" by a saw-toothed margin and in "F. vesiculosus", another common species, by bearing air-bladders. Another component of sea wrack may be seagrasses such as "Zostera marina" a marine flowering plant with bright green long narrow grass-like leaves. "Posidonia australis", which occurs sub-tidally on the southern coasts of Australia, sheds its older ribbon-like leaf blades in winter, resulting in thick accumulations along more sheltered shorelines.Historically wrack was used for making manure, and for making "kelp", a form of potash.