Earth Science: Water – Coral Reefs: Underwater Forests
Rich in life and incredibly diverse, coral reefs are the rain forests of the oceans. Coral colonies provide shelter for a myriad of fish and invertebrate species, whose diverse colors speckle the clear tropical water.
Tropical coral reefs are mainly formed by corals of the genus Madrepora. Coral polyps take calcium and carbon dioxide from seawater and create a calcium carbonate outer skeleton. The skeletons of dead colonies are reoccupied by new generations of coral polyps. Together with the excretions of crust-forming calcareous algae, these new polyps form the skeleton of the reef body, which can result in vast coral reefs.
The Great Barrier Reef off the northeastern coast of Australia stretches over 8,500 feet (2,600 m) of coastline. Coral polyps share a symbiotic existence with the zooxanthellae algae embedded in their skin. Through photosynthesis, the algae turn carbon dioxide and water into oxygen and sugar that is used by the polyps, which in return provide nutrition and protection for the algae.
The carbon dioxide that is also taken in by the algae triggers the corals to produce more calcite. Certain madrepores occur outside the tropics up to a depth of about 20,000 feet (over 6,000 m). Instead of taking advantage of a symbiosis with zooxanthellae, these corals get their nutrients directly from the water around them.
Coral reefs are found between the Tropic of Cancer and the Tropic of Capricorn. They occur in areas where water temperatures are on average 74.3°F (23.5°C) and never below 68°F (20°C). Reef-building madrepores cannot cope with large temperature fluctuations, nor with water that is too cold or too warm as it causes them to die or degenerate. Therefore, the warming of oceans due to climate change is a serious threat to tropical coral reefs, resulting in death or “coral bleaching.”
Healthy reefs act as breakwaters for coastal protection; if the reefs die, many of the flat islands in tropical regions will be exposed to flooding. The sea level rise due to melting ice caps will result in the loss of several islands, whose inhabitants are already noticing the consequences of frequent flooding.
GREAT BARRIER REEF
The Great Barrier Reef off the northeastern coast of Australia is the largest coral reef on Earth, as well as the largest construction made by living organisms: a labyrinth of an estimated 2,500 individual reefs, lagoons, and 600 islands.
It is a unique habitat for over 4,000 species of sea snails, mollusks, crustaceans, sponges, sea stars, and sea urchins, as well as about 1,400 species of fish. As a paradise popular with sailors and divers, a major part of the reef is now a national park and UNESCO World Heritage Site, protecting it for the future.
ISSUES TO SOLVE
RESEARCHERS believe that around three-quarters of all reefs have been destroyed or are at risk of being destroyed by changing environ mental conditions, tourism, motorboats, over-fishing, and sewage dumping.