Author: Laura Dewis

Understanding the intertidal zone

Updated Thursday, 27th September 2007
An article about the ecology of the intertidal zone, including weblinks and reading list. Part of the BBC/OU's programme website for Rough Science 1.

This page was published over 14 years ago. Please be aware that due to the passage of time, the information provided on this page may be out of date or otherwise inaccurate, and any views or opinions expressed may no longer be relevant. Some technical elements such as audio-visual and interactive media may no longer work. For more detail, see our Archive and Deletion Policy.

For ecologists, the most interesting sections of coastline are the areas between the highest and lowest tides, known as intertidal zones. They're wonderful places to explore, preferably when the tide's out! Although they represent only a very small proportion of the Earth's surface, they're important because they're regions in which two very different types of environment meet, and in which a complex web of interactions takes place.

Ecology and ecosystems

Animals and plants are not isolated units. Their activities depend, among other things, on their environment, the conditions in which they live and the populations of other organisms with which they interact. The study of the inter-actions between organisms and their environment is called ecology and the complex system of inter-actions the ecosystem.

The marine ecosystem, which can be divided into many parts, is the largest on Earth. It’s often where different ecosystems meet that the richest variety of life is found. Here, species from each ecosystem come into contact with each other and with other species that exploit the best of both worlds.


The level of the sea rises and falls. This usually happens twice a day in the UK but the pattern varies around the world. On the island of Capraia there’s very little tide because of the size and enclosed nature of the Mediterranean sea. To complicate matters, the tide does not move an equal amount every day. Like the moon, it goes through a monthly cycle. On some days the tidal range is small, with little more than half the intertidal zone underwater (neap tides). On other days the tide completely covers and then exposes the whole intertidal zone (spring tides). In Britain, a spring tide is followed by a daily decrease in tidal range, until the neap tide seven days later. This is then followed by a daily increase in tidal range for a further seven days, until there’s another spring tide.

tidal range

The variation in tidal range

A stressful environment

Rocky shores, often ignored by holidaymakers in favour of sandy beaches, support an enormous range of life. Rocky-shore dwellers have an increasingly harder time the further they are from the sea, since they rely on seawater and most are unable to move with the tides. Seaweed and other plants and animals need to attach themselves to rocks to avoid being washed away by the movement of the water. This means that they’re stranded above the waterline for hours at a time.


By contrast, mobile animals, like crabs, can migrate up and down the shore with the tide and can therefore avoid the problem of being left ‘high and dry’.

The sea is in fact a very stable environment: the salinity (saltiness), temperature and pH (acidity) are constant, and there’s a plentiful supply of raw materials for plant growth (nutrients, water and carbon dioxide). It also provides a supportive medium for swimming or crawling. Aquatic organisms that spend a lot of time out of the water are more likely to dry out, and, since activities like feeding, respiration and mobility normally take place underwater, the time available for these activities is significantly reduced. Even rock pools can’t offer a stable environment, since their salinity varies so much due to evaporation and rainfall.

So, why do some species choose to live on the wave-battered shore? The main reason is to exploit an environment that many other marine organisms can’t tolerate. By living on the shoreline, seaweeds benefit from the light that would be blocked out by deeper waters. Some animals, on the other hand, may avoid predators and competition for food by occupying a position higher up the shore.


Even though it’s only a few metres wide, the change in environmental conditions as you move from the bottom of the shore to the top is quite marked, and it’s responsible for shaping the intertidal community. Only those species that have evolved to cope with living out of the water can occupy positions at the top of the shore. Those that easily dry out, or are sensitive to changes in salinity and temperature, are forced to occupy positions closer to the sea. The result is a zoned community, with different organisms forming distinct bands across the shore.

So why don’t all the organisms live close to the sea, where conditions are most favourable? Well, here they might be competing for resources such as food, light and space. Species that are out-competed, out-grazed or simply eaten at the lower shore, shift their distribution further up the shore to comparative safety. The factors that control zonation are many and varied. They include reproductive strategies, growth, and methods of dispersal. Buy a decent shore guide, get your wellies on, and go looking for these patterns yourself next time you’re by the sea.

Harvesting the sea

We eat plenty of food from the intertidal zone: for example, fish, lobsters, prawns, mussels, cockles, limpets, winkles and whelks. To harvest the sea, all you need is some basic equipment and a knowledge of the shoreline.

But what about seaweed as a food source? It’s extremely nutritious, since it absorbs nutrients from the sea and concentrates them in its tissues. Apart from a few small pockets in South Wales and North Devon, seaweed is not part of many British diets. It is, however, a very common food in the East; the Chinese have been harvesting it for 5 000 years.


Seaweed attaches itself to a rock by its holdfast. Limpets and whelks, on the other hand, are snails that use suction by contracting their foot muscle. Barnacles are crustaceans that attach themselves to surfaces using a biological cement.

In the West, most of the seaweed that’s collected or cultured today is used in industrial processes. Red algae are used in the manufacture of agar, a gel-like material. Others, like Irish Moss (Chondrus crispus), are used as a source of carageenans, which can be turned into blancmange and jellies (like the delicious castaway seaweed and tree strawberry jelly we had on Capraia).

During the Second World War, the inhabitants of Jersey used seaweed to supplement their meagre diets. Alginates extracted from many seaweeds, such as the kelps, are used in dairy products, puddings, fruit syrups and even beer, whilst they act as a foam stabiliser. In the pharmaceutical industry, seaweeds are used in the production of a variety of things, from toothpaste to surgical threads. In fact, the composition of our castaway toothpaste was pretty close to that of most high street brands.

A kelp industry thrived around the northern coast of France and western Britain for many years. The weeds were dried and burnt to produce an alkaline ash that was used in the production of soap and glass.

The industry received a boost in 1811 when it was discovered that many seaweeds contained iodine, a fact we made use of to produce our castaway photographic film. A whole new industry based on extracting the iodine grew up. In fact, the ancient Chinese had long been using kelp as a cure for goitre, a disease caused by iodine deficiency. Seaweeds are also said to be useful in treating stomach ulcers, blood disorders, dysentery and even broken bones.


The Marine Conservation Society
Plymouth Marine Laboratory – Remote Sensing Group
Fisheries Research Services

The BBC and The Open University are not responsible for the content of external websites


Block 4 of S103, Discovering Science, The Open University

Campbell A C, Seashores and Shallow Seas of Britain and Europe, Hamlyn

Hawkins S J and Jones H D, Marine Conservation Society Marine Field Course Guide 1: Rocky shores, Immel

McDonald K, Food from the Seashore, Pelham Books

Packham C, Rocky Shorelands, Collins

Raffaelli D and Hawkins S, Intertidal Ecology, Chapman & Hall

Swain P A, Chemistry and Kelp, School Science Review, 81 (294), pp. 89–94, 1999


Become an OU student



Ratings & Comments

Share this free course

Copyright information

For further information, take a look at our frequently asked questions which may give you the support you need.

Have a question?