Seaweed is a type of marine algae that grows through photosynthesis. Because seaweed requires sunlight for photosynthesis, it is found at depths where light can penetrate through seawater. Although many types of seaweed are found at depths of 328 feet, other types can live at depths of approximately 1,000 feet.
A type of seaweed, called bull kelp, can grow to be a length of about 100 feet. However, other types can grow to a length of about 200 feet. The different types of seaweed are brown, green and red. Seaweed can be annuals or perennials, Red seaweed can live between 7 to 15 years. Although seaweed may look like a plant, it is a type of algae that belongs to the kingdom Protista.
Some basic structures associated with seaweed are holdfasts, stipes and air blades. Holdfasts anchor seaweed to rocks while stipes resemble the stem of terrestrial plants. Some types of seaweed have air blades that help it float in water.