Various stress factors can cause corals to lose their algae which destroys the habitat.
Coral Reefs are found in warm, nutrient-poor waters. The coral animal relies heavily on the photosynthetic activity of microscopic algae that it houses inside its tissues, but when stressed these algae can be ‘ejected’.
Coral Bleaching
When the symbiotic algae are ejected from the coral animal the reef will lose much of its colour – it will become ‘bleached’. Short term this can help the animal through a particularly stressful time, but if the situation continues for long enough the reef will die. Many factors are thought to combine to cause corals to eject their algae.
Warmer Water
Corals can only thrive within a narrow range of temperatures, and their seas are gradually warming up. Changing weather patterns are partly to blame for this, with unusual periods of clear skies in summer leading to local warming. The stress caused by high temperatures is one factor that can make corals bleach.
More Light
Algae need light to photosynthesise, but too much light damages their photosynthetic mechanisms. Weather changes can expose coral reefs to unacceptably high light levels, and increase in UV wavelengths is particularly damaging. This is a case of ‘too much of a good thing’ for the algae, and if they are not actually killed they will be ejected by the coral.
Sea Level
Because corals rely on the contributions made by their photosynthetic algae for efficient reef building they need to be close enough to the sea surface to allow sufficient light penetration. Reefs can only grow upwards very slowly, so any rapid rise in sea-level effectively lowers them. If this were to move them too far from the surface they would not be able to continue their reef-building.
Stresses
The three ‘stress factors’ outlined above can all be brought about by global warming, with changing weather patterns affecting both temperature and light levels locally. More general planet warming causes a rise in sea level, firstly by expansion of the water, and secondly by melting the polar ice-caps. Along with the warming there is an increase in atmospheric carbon dioxide levels, and this dissolves in the sea to give increasingly acid conditions. A small increase in acidity makes it difficult for corals to continue building reefs, and a large increase can even cause reefs to dissolve.
Global Warming
Two facts are undisputed – global temperatures are increasing, and so are levels of atmospheric carbon dioxide. Both phenomena stress corals. Additionally increasing marine pollution and reduction in abundance of plankton (caused sometimes by over-fishing) both add to this stress. Unless we act very quickly and effectively we will lose most of our coral reefs this century.
(See also an article about coral reefs and another more about the effects of fishing and the aquarium trade).
Other articles by John Blatchford
