Rock pools

When last did you take the time to just quietly sit at a rock pool and enjoy God's creation right there before your eyes? Yes, we did these things as children... play in the pools, try to catch a little fish with our fingers and pushing and feeling the sea anemones. But even then, did we really look and see with more than a passing interest in what is really in front of us - the amazing creation right there in front of us? Do we see the rock pools through which God's glory and omnipotence shine like the sun?  

Rock pools usually are isolated and closed ecosystems with their own producers (algae which produce food by photosynthesis), herbivores (e.g. sea urchins, limpets), filter feeders (e.g. sponges, mussels and barnacles), carnivores (e.g. octopus, starfish, whelks and fishes) and decomposers (e.g. sea cucumbers).  Each of these sea creatures has its own special build and habits fitting to its own unique role in the ecosystem it lives in. We could write more than one fascinating blog if we were to focus on each of them. But for now I want to focus on the difficult circumstances these rock pool inhabitants live under.

The biggest problem for the fauna and flora living on rocks, is the changing tides. For a few hours per day they are submerged under water, only to be exposed to the heat and dehydration of sun and wind a few hours later. Most of the fauna only feed while under water. The further they are from the water, the more difficult their circumstances get. This is why you will find the more delicate species near the water and the more hardy ones further away.  You will also find they made different adjustments to survive. This is why we can divide the zones between tide lines along our rocky beaches, in five definite zones.  These zones are divided according to the amount of submerging and also according to which species are found most in the particular zone.

Plants and animals living in the rock pools have different circumstances than those stuck to the rocks. In spite of the fact that pools are continually wet, they can be stressful environments because of alternating isolation and flooding by the tide that causes fluctuations in temperature, salinity and dissolved oxygen.  Pools further away from the sea experience different conditions from pools closer to the sea, which are flooded most of the time.  Conditions also differ between day and night.  And the animals have to adapt daily to this changing environment. See the following as examples:

A large, shallow pool's temperature fluctuations will be bigger than that of a deep pool with the same volume of water. Water temperature is not only influenced by air temperature and the sun's rays, but also by the temperature of the surrounding rocks. Rock pools' temperatures during the day will thus be higher than the sea's and at night colder. A rock pool's temperature can fluctuate with 10-11°C (day and night temperatures).  The algae and animals have to be able to handle this. Remember, they cannot sweat or cool off by evaporation. A raised temperature can also lead to raised photosynthesis and respiration by algae which in turn can lead to a build up in carbon dioxide and oxygen. 

The raised oxygen levels during the day can be four times higher than normal levels, this can be toxic for the inhabitants of the pool. At night the oxygen gets absorbed so much quicker by the algae and animals than in daytime, which causes oxygen levels to be too low for inhabitants to survive.  Animals even die. This is when crab-like creatures and rock-fish move to different pools. The bigger shallow pools' oxygen levels do not drop as quickly as the deeper pools. Because of photosynthesis carbon dioxide levels in the water keep changing. If the CO2 levels are low, the pH levels will rise and the water will be more alkaline during daytime. The opposite will happen during nighttime, so the water will then be more acidic.  No wonder that life in a rock pool is described as one of the harshest habitats on earth.  During the day water can be warm, toxic and alkaline and during night it will turn to cold, oxygen depleted and acidic.

If you live in an isolated rock pool, you have one more problem: because of evaporation the water level in your pool will quickly drop and this will raise the salinity of the water. When it rains, the salinity will be so diluted that you'll be living in freshwater. Incredible. God created animals and plants which can withstand these extreme opposites. And we have not even taken into consideration the daily fight in finding food and not be food. 

Still this is not the end of our rock pool species' problems. Humans are still the biggest threat to our rocky beaches. Apart from the development of little towns and the building of man-made structures, pollution levels have increased drastically. Just have a look at the amount of plastic, fishing lines and other trash floating around in the rock pools, and worse even - has sank to the bottom of the pools. Poaching by syndicates and fishermen who do not keep to their quotas are the reason some of our rocky coast species like abalone and crayfish are almost extinct.  Alien species like the Mediterranean sea mussel (Mytilus galloprovincialis) are a threat to our coasts. These mussels were imported from Europe in the 1970`s and have supplanted our own indigenous black mussel by spreading at a speed of 115km per year.  Today it is the dominant inter-tidal mussel throughout the region.

Next time you stop at a rock pool - look at it through new and informed eyes, be prepared to be amazed by the beauty of the different species and their endurance; enjoy the diversity God has created to the honour of his holy Name.