Rock pools can be the best or worst of microhabitats depending on their position on the shore. Rocky coasts can be harsh environments for animals and plants. Pools can provide a useful refuge, enabling some lower shore, or slow-moving animals to survive further up the rocks. However, life in rock pools is not all roses, because a whole new set of environmental rules apply. These can test life to the limits.

Here, we look at two typical pools which might be found on a rocky coast in the UK, one just above the low tide level and one just above the high tide level. We will see what lives in each of the pools and find out why.

Life in the high level pool
If you have ever visited a rocky coast and looked at rock pools, you may have noticed that those at the top of the beach are not the most interesting. They contain only a few species and are nearly always full of a green weed, Enteromorpha which forms a refuge for small animals such as sandhoppers. Shore crabs may also be found lurking in the weed, along with small prawns, and swarming on the surface of the water you may see hundreds of tiny black insects. These are springtails and are among the small number of marine insect species. In the cracks and crevices surrounding the pool, sea slaters are found, and on the rocks, a few barnacles may be present.

Life in the low level pool
Life in the low level pool is much more varied than in pools higher up. Anyone heading to the beach to look for seashore life will head straight for the pools nearest the sea as they are more likely to contain animals which live in the sea and have become stranded by the outgoing tide. These pools are often larger and deeper and are dominated by brown seaweeds such as Fucus serratus, or serrated wrack, and the kelps Ascophyllum nodosum or Laminaria. Large edible crabs hide in crevices, and larger prawns and even the occasional lobster may be found. Fish are much more common. Juvenile pollock and wrasse may be found along with gobies and blennies. Starfish, brittlestars, urchins, sponges, hydroids, anemones, chitons and sea squirts can all be found in low level pools.

Biodiversity
The diversity of life in the low level pools is much greater, because pools near the low tide mark are only cut off from the sea for a short time. If the pool is deep and large it will differ only slightly from the surrounding rocky sea bed. In contrast, the high level pool has little biodiversity because it is cut off from the sea for hours and occasionally days. The few species that can survive these conditions are well adapted to the extreme environment.

Sometimes it's hard being a pool dweller...
Whether a pool is exposed to the air for a short or long time, it has to function on its own resources until it is refreshed by the next tide. Just as people living on islands rely on boats or planes to bring in fresh supplies, the inhabitants of pools need fresh sea water to provide food and oxygen. The longer a pool stays exposed, the more the resources dwindle and the more likely the animals and plants are to experience environmental stress.

Pool shape and size
The shape of a pool is very important. A large shallow pool has a large surface area to volume ratio - meaning a lot of the pool is exposed at the surface. A smaller deep pool has a small surface area to volume ratio. This becomes important when we consider that other problems can be made worse if the pool's dimensions are unfavourable.

Temperature
The temperature of rock pools is affected by air temperature and direct sunlight on the pool. Although sunshine heats the water, the surrounding bedrock acts as a buffer, conducting heat away and making the temperature change less extreme. The maximum fluctuation for a normal day is 10-11 degrees centigrade, even in tropical pools.

Pools shaded by cliff faces may have roughly the same temperature as the sea, even on a mid-summer day. Shallow pools with large surface areas are likely to be more affected by temperature. The major impact of temperature changes for life is a change in rates of photosynthesis and respiration. For every 10 degrees centigrade (to an upper limit) photosynthesis increases by one and a half times and respiration by two to three times. This causes a build up of by-products, such as carbon dioxide and oxygen, within the water.

Oxygen
Photosynthesis in plants, especially algae, cause oxygen concentrations to peak in the afternoon. At night, available oxygen is depleted rapidly and animals such as prawns and sandhoppers migrate to the surface. Crabs, blennies and other mobile animals leave pools at night when oxygen levels are lowest. A pool with a large surface area to volume ratio will allow more oxygen to diffuse into the water from the air.

Carbon dioxide and pH
The other result of photosynthesis during the day is that carbon dioxide is taken up by the plants. As the level of carbon dioxide drops, the pH rises, making the water more alkaline. This change is reversed at night when photosynthesis ceases but plants and animals continuing to respire - using up oxygen and releasing carbon dioxide. High levels of dissolved carbon dioxide in the water make the pool more acidic.

Salinity
The sun and wind evaporate water from the surface of pools. In pools isolated from the sea, this causes an relative increase in salinity (salt-concentration). Heavy rainfall can lower salinity rapidly, but this is not a significant problem because the less dense freshwater forms a layer capping the saline water below. The mixing is not uniform and animals and plants in the pool's depths remain unaffected. Many species including prawns and crabs are able to survive a wide salinity range. Some live in estuaries, where salinity changes are extreme - much more than the fluctuations in rock pools.

Organic matter
During severe storms, huge amounts of weed may be ripped free and tossed on to the upper shore. Pools at high levels may become filled with fragments of rotting weed which can kill off some of the species living there. However, scavenging animals such as shrimps or sandhoppers may thrive in these conditions. This problem really only affects the highest pools which may miss consecutive high tides.

It's tough at the top
It's obvious, then, that higher shore pools are tougher places to live. Many factors influence the species present in pools, but the longer a pool is exposed, the more time adverse conditions have to develop. For most species, conditions are more favourable if the pool is closer to sea level. In low level pools, the sea is never far away and many species can live there successfully.

A variety of plants including several species of marine algae can be found in low level pools. They give the animals living there a wider variety of shelter and food sources. In higher pools, the dominant weed is Enteromorpha and the shelter and food is limited to that one species. It's similar to comparing a wheatfield to a field of wild grasses and flowers. A much smaller variety of insects can live in a monoculture, such as a wheatfield, but a field of wild grasses and flowers might contain all sorts of different species. Life in rock pools at the top of the shore is harsh, and there are only a few species tough enough to survive there.