Seaweed
Seaweeds are algae and are classified into brown, red, and also green algae which is based on their pigment composition. Seaweeds are most often mistaken with other photosynthetic organisms. Seaweeds are described as plants, but biologists typically do not consider them true Plantae.
They should not be confused with plants, such as seagrasses. In addition, a few species of cyanobacteria bear a resemblance to seaweed algae. Many phycologists prefer the term “marine macroalgae” over “seaweeds”. The ecology of seaweeds is dominated by two specific environmental requirements. These are the presence of sea-water (or at least brackish water) and the presence of light sufficient to drive photosynthesis.
A very common requirement is also to have a firm point of attachment. As a result, seaweeds are most commonly found in the littoral zone and within that zone more frequently on rocky shores than on sand or shingle. The ecological niches utilised by seaweeds are wide ranging.
At the highest level are those that inhabit the zone that is only wetted by sea spray through top the deepest living that are attached to the sea-bed under several metres of water. In some parts of the world, the area colonized by littoral seaweeds can extend for several miles away from the shore. The limiting factor in such cases is the availability of sufficient sun-light to support photosynthesis. The deepest living sea-weeds are the various kelps.
In addition to the familiar sea-shore seaweeds, a number of species have adapted to a fully planktonic niche and are free-floating, often with the assistance of gas filled sacs. Sargassum is one of the better know examples of this type of seaweed. A number of species have adapted to the specilised environment of tidal rock pools. In this niche seaweeds are able to withstand rapidly changing temperature and salinity and even occasional drying.