Seaweed Zonation
Brown Seaweeds of Rocky Shores are Adapted to their own Zone
The genus Fucus contains three species which illustrate the phenomenon of zonation on the sea shore.
The Habitat
The tide covers the shore twice a day in most places on the planet, with plants and animals of the lower zone covered by water for longer periods than those of the upper. At the two extremes there are small regions of the shore which are exposed to air (or covered by water) much less frequently. Wave action can vary with geographic location and slope, and the direction the shore faces can also be important.
Exposure to Air
Almost all organisms found on the rocky shore are of marine origin, with their close relatives all living in the sea. Their basic adaptations are for aquatic life so the time spent in the air causes them problems – the most common is probably desiccation. Air temperature fluctuates much more widely than sea temperature so that coping with rapid temperature changes is also a challenge.
Wave Action
The three species of seaweed considered here are permanently attached to the rock. Winds at sea cause waves, and when these approach the shore their energy is released. Different rocky shores affect the characteristics of this energy transfer, and if the waves are too powerful the holdfasts break and the seaweed is simply washed away.
Competition and Grazing
The lower (shore) limit for many organisms is determined by which species they come into contact with there. A better adapted organism will be more successful and will therefore dominate. In a similar way encountering predators (for animals) or grazers (for plants) will have obvious consequences. The physiological problems met with in the air usually limit the upward distribution of plants and animals on the shore, but competition and grazing determine how far down the shore plants (and animals) can be found.
Three species of Fucus (a genus of brown seaweed) are widespread on rocky shores bordering the North Atlantic. They each have their own characteristic place on the shore.
- Fucus spiralis (Spiral Wrack) is very tough and is only found in a narrow band near the upper limit of many rocky shores. Above this level the fronds dry out too much, and below it there is competition from Fucus vesiculosus (Bladder Wrack).
- In a similar Bladder Wrack is limited by desiccation at its upper limit, and by competition with Fucus serratus (Serrated Wrack) lower down.
- As we move further down the shore (and into the sea) the Serrated Wrack is both heavily grazed and out-competed by other seaweeds.
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