Kelp forests grow along rocky coastlines in depths of 18 to 90 feet (6 to 30 meters). Kelps are types of brown algae, which hold on to the rocky bottom with root-like structures called holdfasts. From these holdfasts, long streamers of kelp grow up toward the surface, with gas bladders at each leaf to keep the plant upright, such as on the Giant Perennial Kelp. Bull Kelp has a hollow stem and a single large gas bladder near the water's surface from which streamers grow.
Information provided by: http://www.onr.navy.mil