Seaweed is any of a number of species of brown, green, or red algae, found in greatest profusion in shallow waters on rocky coasts. Kelps are the largest forms. Many species are important for the manufacture of fertilizers or food, or as a valuable source of chemicals such as iodine. Kingdom Protoctista.
Kelps are the largest forms. The giant kelp (Macrocystis sp.) exceeds 45 meters (150 feet) in length. Phylum: phaeophyta.