I am standing on the cliff top overlooking the small cove. Its western end is protected by towering rock sheets that, over a few million years, have been raised to an almost vertical position. Parts of the sheets have been torn open more recently by the forces of the Atlantic Ocean and now a small sea cave extends into the rock. The eastern end of the cove seems to have been tortured considerably more by the waters of the Atlantic. Low lying rock platforms are overlooked by narrow rock towers and twisted cliffs. Caves, tunnels, blow holes and deep pools have been sculpted into these cliffs. The petrified remains of sand volcanoes and ripples that flowing water left on the bottom of an ancient river bed reveal the ancient origins of the rock.
A path and an artificial ramp, built for the filming of the Hollywood classic Ryan’s Daughter, lead to the bottom of the cove. Here it becomes clear that the cliffs that encircle the head of the cove are not made of solid rock but consist of clay and rocks of various sizes. Lumps of clay at the bottom of the cove and sagging parts of the cliff top state the obvious: These soft cliffs are being eroded away with every spell of rain and every gust of wind that comes in from the north Atlantic. Just below the cliffs the eastern half of the cove is covered with a boulder beach. Hundreds of hefty rocks, rounded and smoothed by water, cover the underlying rock platforms that stretch out into the bay and become visible where the boulder beach ends. On the other side of the cove the boulders are missing and sculpted and terraced rock dominates the scene. In the far eastern corner the Atlantic has managed to drill a canal into the rock that disappears under the soft cliffs und underlying rock only to reappear a few meters inland where a sinkhole has formed.
The rock platforms in the center of the cove are covered in seaweeds and dotted with rock pools of different size and depth. Massive rocks dislodged from the cliffs at the entrance of the bay and thrown onto the shore are a brutal reminder of the violent storms that hit this coast on a regular basis. It is a place that never fails to amaze me. First it’s only a kaleidoscope of colours, shapes and textures, beautiful in its own right. A closer look reveals a microcosmos of life and death, struggle and survival. It is Ireland’s most fascinating habitat: The rocky shore.
I have been visiting this particular cove for almost a decade and have been watching its inhabitants going about their daily business. All the images in this and the following posts have been made on this little stretch of rock, between the tides
If there is one truly wild place left in Ireland it is its coast. Ireland boasts over 3000 kilometers of coastline, the majority of which is made of rock, mainly shale, sandstone and limestone. This rock comes in a variety of shapes. The most eye-catching of those are the sheer cliffs that rise vertically out of the sea, can reach heights of several hundred meters and in places have been sculpted into pillars, arches and caves. As a habitat these rock faces are home to a variety of seabirds. Over the summer months these colonies with many thousand individuals transform the rock into a busy bird metropolis. In autumn the birds leave and the cliffs return to their quiet and barren state. Other varieties of the rocky shore however are keeping busy all year round. Where the rock is gently sloping into the sea or low lying rock platforms are regularly engulfed and released by the tides life is teeming under and above the water. These kind of rocky shores are one of the most crowded habitats imaginable. Especially the intertidal zone hosts a staggering variety of plants and animals with a population density comparable to the tropical rainforest.
This is an ever changing and rough environment full of challenges. The first challenge the rocky shore inhabitants face is the power of the waves when they meet the shore. The force of this wave power depends on a number of factors like the depth of offshore water, fetch (the expanse of open water before it meets a shoreline), aspect (the direction the shore faces), and position (the layout of coastal features). The worst case scenario in Ireland is a west or north facing shore without any protecting headland. The vast extend and the deep waters of the Atlantic Ocean combined with regular low pressure systems can create massive waves that can hit the shore with a power of up to 30 tons/square meter. On the other end of the scale are the rocky shores of the east coast, ideally situated inside a bay which is protected by headlands or a reef. The variations between these two extremes of an exposed and a sheltered rocky shore are of course numerous. The prevailing fauna and flora has adapted to these variations and has come up with tailormade and very interesting solutions to avoid being crushed to death on the rocks or being swept away by waves.
An even greater danger than crashing waves however comes with the tides. Twice each day the inhabitants of any rocky shore are being exposed to sun and wind and then being covered again by seawater. The movement of the seawater is of course mainly caused by the moon with the further away sun lending a helping hand. The gravitational forces of the moon and sun pull on our planet’s oceans and move these vast amounts of water. Roughly twice each month, when the moon is full or new and aligned with the sun, spring tides occour. At spring tide’s high water is higher and low water is lower than the remainder of the month and subsequently a wider area of the shore is exposed at low tide and covered at high tide. The spring tides are at their greatest at the equinoxes in March and September and at their weakest at the solstices in June and December. The ‘spring’ in spring tide has nothing to do with the season but has its origin in the old Anglo-Saxon language where the word springam meant ‘to rise’. The rest of the month the moon and sun sit at approximately right angles and their gravitational forces oppose each other causing smaller tides known as neap tides.
Most rocky shore dwellers are built for life under water, they breathe through gills and the concentration of their internal fluids is the same as that of the seawater surrounding them. Water can pass freely in and out and all is good as long as the plant or animal is immersed in water. The moment the plant or animal is emersed it loses water through evaporation and is in danger of dying from dehydration and, if it is not able to extract oxygen from the air, suffocation. The latter obviously doesn’t apply to plants. Those however face another problem: The plants of the intertidal zone, the seaweeds, can only photosynthesize when they are covered with water. Yet another problem that comes with immersion and emersion are temperature fluctuations. Under water the temperature is more or less constant no matter the time of day or year. Out of the water however temperatures can reach boiling point on a dark rock surface on a clear summer day or can go below freezing in winter.
One solution to the problem of dehydration, and the one that most rocky shore inhabitants capable of locomotion use, is to hide under seaweed, in rock crevices or rock pools, anywhere some moisture and shade remain after the tide has gone out and wait for the water to return. The safety of a rock pool is however deceptive. Variations in water salinity can be substantial. Average sea water salinity along Ireland’s shores is 3.5%. In rock pools this can rise to around 9% when wind and high air temperatures increase evaporation or drop to around 0.5% when rain or run-off from nearby fields add freshwater to the pool. Osmoconformer, animals like the acorn barnacle or common mussel that can control their internal fluids, are able to cope with these fluctuations. For stenohaline animals like anemones that have no control over their internal fluids, this can be lethal. An anemone stranded in a pool with decreasing salinity will involuntarily take on water and eventually burst.
Another solution to the dehydration problem is having a shell. A shell protects from evaporation and the space between the body and the shell, known as mantle cavity, can be used to store water which keeps the body moist and is a source of oxygen. Seaweeds unfortunately have no protection mechanisms against temperature fluctuations and evaporation and simply have to cope with the fact. Some do this better than others, the channel wrack for example can tolerate 95% of water loss and fully recover after 20 minutes immersion.
TO BE CONTINUED…
Images & Text Copyright by Carsten Krieger