Algae (pictured above)
Algae are often misunderstood and thought of as merely the green slime that seems to grow on any solid object left in the water and sun for a long enough period. In reality, algae embrace a wide range of single-celled and multicelled organisms. The plants known as seaweeds, for example, are multicellular algae. There is some argument about whether algae are actually plants because so many varieties are much simpler and lack the organ development of land plants. However, like land plants, they rely on photosynthesis for survival. For this reason, algae of all kinds are found near the ocean surface, where there is a steady supply of light. Another typical requirement is having something to attach themselves to, but some versions of single-celled algae and even seaweed don't need such a base. Seaweeds that are based on something use what is called a holdfast, rather than roots. A patch of seaweed on a rock, for example, is merely anchored to that rock and is not drawing nutrients out of it as a plant with roots might try to do. Most forms of single-celled algae and seaweeds are red, brown or green. Some varieties of algae form symbiotic relationships with other sealife, such as corals or sponges. Sponges, for example, offer the algae some protection while the algae provide the sponge with food and oxygen.
Seagrass gets its name because it bears a superficial resemblance to land grasses. However, they are angiosperms, or flowering plants, and therefore close cousins of land plants. Where seaweeds can attach themselves to rocks, seagrasses have real roots and need a silty, muddy or sandy bottom to grow. They also need sunlight to power their photosynthesis, and sheltered waters. These requirements dictate that seagrasses are found mostly in coastal areas and never in the open ocean.
Kelp is a form of brown, multicellular algae, but is different enough from other forms of algae that it should be addressed separately. It grows only in temperate or arctic conditions, preferring the cold water. The fronds of kelp anchor to the sea floor and use gas-filled organs to extend themselves up to the surface, and it is not uncommon to see fronds of kelp reaching up from depths of 60 or 70 feet. Kelp can form into very dense groupings, which are called kelp forests. Forests of kelp are an attractive and interesting environment for divers, and are one of the most dynamic and interesting marine ecosystems.