Kelp is a type of marine seaweed. It is a brown algae. There are many different kinds of kelp. Kelp grow in large forests, usually near the surface of the water. It grows in nutrient rich saltwater. Some kelps grow very fast (up to 30 cm a day). Kelp can reach a length of up to 60 metres. The blades are kept afloat by gas-filled bladders.