Zonation on a rocky shore

Zonation Coursework

The seashore is a habitat that contains a wide range of microhabitats and ecological niches for different creatures. This is mainly due to the effects of the tides, that rise and fall twice each day.

Tides are the vertical movement of water in a periodical oscillation of the sea, due to the gravitational pull of the sun and moon. The tides are on a semi-diurnal cycle, so there are two high tides and two low tides each day.

Due to the orbit of the moon, the tides also have a monthly cycle. This creates neap (very low) and spring (very high) tides.

The seashore can be divided into several zones, which are illustrated on the diagram below:

EHWS = Extreme High Water Spring
(MHWS = Mean High Water Spring)
MHWN = Mean High Water Neap
(MTL = Mid Tide Level)
MLWN = Mean Low Water Neap
ELWS = Extreme Low Water Spring
(MLWS = Mean Low Water Spring)
CD = Chart datum

The Supralittoral Zone:
This is the highest zone on the shore, and lies above the EHWS mark, and therefore is never covered by seawater. However, it may be occasionally be spray wetted. Because of this, it is mainly inhabited by terrestrial species, such as lichen, that can live in areas of very high salinity.

The Littoral (Intertidal) Zone:
This zone is the area that is covered and uncovered by the tides, and therefore organisms that live here must be able to tolerate a large range of conditions. It can be further divided into the Littoral Fringe and the Eulittoral zone.

The Littoral Fringe (Splash Zone):
This part of the Littoral zone lies above the area that is completely submerged by the sea in normal conditions. However, it is frequently covered by splash from waves, and so is far more marine in character that the Supralittoral Zone. Lichens still dominate this zone, but some species of periwinkles and topshells may graze them.

The Eulittoral Zone:
This zone is the area of the beach that is regularly submerged by the tides, and can be divided into three more zones, the upper, middle and lower shores. It shows the greatest species diversity of any of the zones.

The Upper Shore:
This region of the shore lies between the EHWS and MHWN marks, and so is only immersed during spring tides. Because of this, organisms that live here must be adapted to survive long periods of desiccation. The two seaweeds that are the most common here, Fucus spiralis and Pelvetia canaliculata have adaptations to survive in this area.

The Middle Shore:
This region of the shore lies between the MHWN and MLWN marks, and will be submerged for half of every day, even during neap tides. The most common seaweed in this zone Fucus vesiculosus. Mussel beds will form and both limpets and periwinkles will graze the rocks. Sea anemones and crabs are residents of this zone.

The Lower Shore:
This region of the shore lies between the MLWN and ELWS marks, and will be submerged for most of each day, even during neap tides. The most important seaweed in this area is Fucus serratus, which will form large zones wherever suitable. It shows the greatest species diversity of any zone on the seashore.

The Sublittoral Zone:
This part of the shore lies below the ELWS mark, and is therefore never uncovered by the sea.

There are many types of organism found on the rocky shore. The two main photosynthetic organisms are the lichens and the macroalgae or seaweeds.
Lichen are the main organisms found in the splash zone and come in three distinct types; crustose, foliose and fruiticose.
Crustose lichens form a thin crust on the rock surface, and are impossible to remove without damage. Foliose lichens are leafy lichens that are not as firmly attached to the rocks. Fruiticose lichens extent vertically from the rock surface, and can sometimes be confused with mosses and small grasses. The leafy part of a lichen is known as the thallus.
Seaweeds are primarily divided by colour, into brown, red and green groups. Most marine seaweeds are brown seaweeds, with fewer red species, and even fewer green species. The three main parts of a seaweed are:
1. Frond (lamina, thallus, blade) (often broad and flat)
2. Stipe region (often long and cylindrical)
3. Basal attachment (holdfast)
The frond or thallus is the site of most of the photosynthetic activity in