If you've gone swimming in the ocean, then you probably know the feeling: that slimy, tickling sensation of seaweed brushing your legs and the bottoms of your feet. Although not the most pleasant sensation, it serves as a reminder of the abundant life in myriad forms that populates the world's oceans and marine environments. Marine plants serve numerous functions. Single-celled plants called phytoplankton are the basis of the marine food web. Larger plants provide habitats for fish and other marine organisms, and through photosynthesis, marine plants turn carbon dioxide into the oxygen that marine life needs to breathe.
Although, if you hold up a glass of saltwater, you'd never be able to see most of them with the naked eye, phytoplankton are the most essential organism for maintaining the life of the ocean. According to Robert Stewart's "Oceanography in the 21st Century," phytoplankton account for 95 percent of primary productivity in the ocean--that is, they carry out 95 percent of photosynthesis--and a full half of primary productivity in the world. Their function is vital for making the sun's light available to animals in the form of chemical energy. Phytoplankton include microscopic, single-celled organisms as well as algae. Phytoplankton are most common in the Northern Hemisphere near the coasts.
Kelp is a form of large, brown algae that grows off the coast of California. Kelp is distinctive because of its size and the effectiveness of its adaptations. According to the Missouri Botanical Garden, a single plant can grow up to 125 feet in length--taller than many trees. Although algae are distinguished from other plants by their lack of a root system, kelp possess anatomical characteristics that allow them to survive in the ocean while reaching such impressive sizes. A holdfast secures the plant to the sea floor while air-filled pockets, called floats, on the base of the leaves draw the leaves to the surface, where they can access the sunlight they need for photosynthesis. Kelp grows in forests and, like terrestrial forests, different life forms live among the different layers. For example, other algae live in the canopy, while the middle layers are home to snails and fish, while urchins and sea stars nestle against the holdfast.
Salt marshes form the boundaries between land and sea, where distinctions between the two become vague. Cordgrass is one of the most common plants found in the salt marsh, in large part because its adaptations allow it to thrive where other plants would perish. The blades are tough and the root system strong, allowing cordgrass to withstand the frequent flooding typical of the salt marsh. In addition, special glands help the plant to pump out salt, which lets it thrive in a high-saline environment and withstand heat and direct sunlight. Plants in the salt marsh provide habitats for animals and, when they die and decay, feed the microorganisms that form the basis of the marsh's food web.