Ascophyllum nodosum exposed to the sun in Nova Scotia, Canada, Terra.
Seaweed is a macroscopic, multicellular, marine algae that lives near the seabed. The term includes some members of the red, brown and green algae. Seaweeds can also be classified by use (as food, fuel, medicine, fertilizer, filtration, etc.). The study of seaweed is known as Phycology.
Ecology[]
Two specific environmental requirements dominate seaweed ecology. These are the presence of seawater (or at least brackish water) and the presence of light sufficient to drive photosynthesis. Another common requirement is a firm attachment point. As a result, seaweeds most commonly inhabit the part of a sea that is close to the shore and within that zone more frequently on rocky shores than on sand or shingle. Seaweeds occupy a wide range of ecological niches. The highest elevation is only wetted by the tops of sea spray, the lowest is several meters deep. In some areas, seaweeds can extend kilometers out to sea. The limiting factor in such cases is sunlight availability. The deepest living seaweeds are some species of red algae.
A number of species have adapted to a fully planktonic niche and are free-floating, depending on gas-filled sacs to maintain an acceptable depth.