| Why Coral Reefs Only Found in the Tropics Peruvian fishermen were the first to discover that every so often the water off the western coast of South America would get considerably warmer. Because this phenomenon began around Christmas time, the fishermen gave it the name El Niño, after the Christ child. El Niño is caused by a weakening or reversal in the direction of the Pacific trade winds. This portends major climatic consequences for most of the world, including severe and frequent hurricanes, droughts in normally rainy areas and floods in normally dry areas. For the South American fishermen El Niño means a serious decline in their catch. The reason for this is that warmer water means an end to the rising currents that carry nutrients to the surface. A scarcity of nutrients means a scarcity of plankton, the primary food source for all marine life, and when plankton is scarce so are fish and every other living organism. Why then, is the world's most diverse ecosystem, the coral reef, found only in warm tropical waters where plankton are always scarce? The community of plants and animals called the coral reef centers on and around a mass of inorganic limestone. Tiny animals called reef building coral polyps are the parties responsible for the construction of these, often immense, rock-like structures. In order to understand why polyps must confine their activities to nutrient poor tropical water, we need to know something about the nature of these unique animals. Coral polyps begin their life as free-floating larvae, the result of the mass spawning of mature polyps. The larvae gradually settle to the bottom. If they chance upon a clean and suitable area of the ocean floor, they will attach themselves to the substrate and simultaneously manufacture an outer skeleton becoming a mature polyp. The material used for both the attachment and the production of the skeleton is limestone. The polyp then reproduces itself asexually by splitting itself into two halves. Each half grows back into identical polyps. This process of asexual reproduction continues until there is a large colony of polyps all stuck together, which we call coral. Different coral colonies living together make up the coral reef. Over the years coral grows outward and upward and the reef gets larger. When the coral dies, the limestone skeletons remain and are usually colonized by new polyps or other life forms. Coral polyps manufacture most of the limestone that makes up the reef. Limestone is a chemical compound called calcium carbonate, the basic ingredients of which are calcium and carbon dioxide. The polyp takes calcium that is dissolved in seawater out of solution and combines it with the carbon dioxide that it produces (like all other animals) as a byproduct of respiration. Chemical reactions are favored by some conditions and inhibited
by others. Just as a damp environment speeds up the rusting of iron,
the production of calcium carbonate by the coral polyp can only
proceed at a reasonable rate in an environment of warm water, high
salinity and low carbon dioxide concentration. These factors are
typical of warm, shallow tropical water. (Even under optimum conditions,
however, this process is extremely slow with many corals growing
only one or two centimeters per year.) Therefore it is the limitations applied by the laws of chemistry that dictate where coral reefs can exist and where they cannot. The limitation of a warm water environment, however, results in a serious problem for the coral polyp, that is, there really will not be very much to eat. To find out how the reef building coral polyp adapts to and overcomes this critical difficulty see the perfect partnership page. |