kelp

Collective name for a group of large brown seaweeds. Kelp is also a term for the powdery ash of burned seaweeds, a source of iodine. (Typical families Fucaceae and Laminariaceae.)

The brown kelp (Macrocystis pyrifera), abundant in Antarctic and sub-Antarctic waters, is one of the fastest-growing living things, reaching lengths of up to 100 m/320 ft. It is farmed for the alginate industry, its rapid surface growth allowing cropping several times a year, but it is an unwanted pest in North Atlantic waters.