The California coast is famous for its beds of giant kelp (Macrocystis pyrifera), the largest seaweed on the planet. It forms dense forests just offshore, and in Monterey Bay it outcompetes bull kelp for sunlight in many places, but the latter dominates in more exposed areas. Inshore of these giant species, other smaller kelps thrive. The kelp forests provide a unique habitat. Sea otters, which live among the kelp forests and eat sea urchins, are thought to be important in controlling the urchins, which graze on the kelp. Seagrasses of the genus Phyllospadix are also found in Monterey Bay. Unusually for seagrasses, they can attach to rock and grow in the surf zone or in intertidal pools on rocky coasts. Each year over 140,000 tons of giant kelp are harvested in California for the extraction of alginates, which are used in the textile, food, and medical industries.
In exceptional circumstances, giant kelp can be eighty meters long. The forests are at their thickest in late summer and decline during the dark winter months.