Coral Reefs

What makes a coral reef?

Conditions needed for reef formation

Although corals are found throughout the oceans of the world in polar and temperate waters, as well as in the tropics, it is only in the tropics that reefs are developed (i.e., hermatypic corals are only found in the tropics).  Coral reefs grow best in clear, shallow waters that remain above 18°C. Reefs are therefore only built in shallow tropical waters. Reef building is favored by clear water,  and can be adversely affected by sedimentation.  Corals have a ciliary-mucus mechanism which traps sediment and removes it, but that mechanism can be overwhelmed resulting in smothering of the coral.  In general, coral reef development is greater in areas that are subject to strong wave action which provides a constant source of fresh, oxygenated water and prevents sediment from settling on the colony.  Click here to learn more about coral reef formation.

Coral reef formation can be limited by:

• temperature: are only found in waters bounded by the 20°C isotherm, cannot develop below 18°C or above 30-35°C (get coral bleaching at high temperatures -- release of their zooxanthellae)
• depth: cannot develop in water deeper than 50-70 m and usually grow in 25 m or less; this is why reefs are restricted to the margin of the continents or islands; due to light restrictions as sufficient light must be available to the symbiotic zooxanthellae in the coral tissue for photosynthesis
• salinity: are intolerant of salinities deviating from that of normal seawater (32-35 ppt); thus are not found where inshore waters are subject to continuing influxes of freshwater from river discharge
• exposure to air: can secrete mucus to prevent desiccation, but are killed by long exposure to air

Types of Reefs

Fringing Reef - A fringing reef is either attached to, or closely borders adjacent land. There is no lagoon between the reef and the land. It typically has a flat upper surface, up to 1 km wide, and plunges steeply into deeper water at its seaward edge. Types of Reefs and Reef Formation

Barrier Reef - A barrier reef is separated from the land by a lagoon that may be of considerable length and width.Take a virtual dive on the Great Barrier Reef

Atolls - Atolls are roughly circular reefs enclosing a shallow lagoon. Some atolls have a central island. Atolls are formed as fringing reefs around a central island. As the island is eroded, the reef becomes a barrier reef and the central lagoon gets broader, until the island is eroded to sea level, forming the atoll. This theory was first proposed by Charles Darwin. Some atolls, however, are growing in 4000 to 5000 m of water, so the island must have subsided considerably. Subsidence of this degree requires a explanation on the scale of plate tectonics, which is the case. Consider a volcanic island that emerges above sea level near a mid-ocean ridge. After it becomes extinct it will slowly be eroded down to sea level. Plate tectonic movements take the volcanic island away from the ridge into deeper waters, forming a flat-topped submarine rise, known as a guyot.

The modern theory of coral atoll formation was originated by Charles Darwin as a result of his observations at Tuamotu Archipelago and Cocos Keeling Island made between November 1835 and April 1836, during the voyage of the "Beagle." His book, The Structure and Distribution of Coral Reefs, was first published in 1842 as Part I of the Geology of the Voyage of the 'Beagle.'

Darwin believed that the fringing reefs, barrier reefs, and atolls of volcanic islands represented a st/ccessional series through geologic time. He hypothesized that the transition from fringing to barrier reef to atoll could result from the upward growth of coral on the edge of a gradually sinking volcano. Gradual subsidence and continuous reef growth were fundamental to his theory. He believed that barrier reefs represented an intermediate stage between fringing reefs and atolls, and that the ring-like appearance of an atoll with a central lagoon resulted from the total submergence of the summit of a volcano.

The sequence of events in atoll formation can be summarized as follows: (1) an emergent oceanic volcano that is no longer active, is colonized by reef-building corals; (2) initial coral growth forms a fringing reef around the island. As the magma chamber of the volcano is depleted the island begins to sink, but coral growth continues, building upon past generations of corals; (3) as the volcano continues to subside, a barrier reef is formed with a lagoon between the island and the reef; (4) finally, the volcano completely disappears below sea level, leaving an atoll composed of low coral islets in a ring with a lagoon in the center. To learn more check out Explanation of Atoll Formation - Wikipedia.

The Biology of Coral

• Facts about Coral
• The Coral Animal
• Coral Biology
• Types of Coral
• Coral Reef Zonation

Threats to Coral Reefs

Coral reefs are ecologically important ecosystems that have a high biodiversity. Coral Reefs are home to over 25 percent of all marine life and are among the world's most fragile and endangered ecosystems. In the last few decades over 35 million acres of Coral Reefs have been obliterated. Reefs off of 93 countries have been damaged.

Disturbances affecting coral reefs include human caused and natural events. Recent accelerated coral reef decline seems to be related mostly to human impacts (overexploitation, overfishing, increased sedimentation and nutrient overloading. Natural disturbances which cause damage to coral reefs include violent storms, flooding, high and low temperatures. When corals are stressed by high temperature, ultraviolet light or other environmental changes, they lose their symbiotic algal cells, and appear white (the white skeleton is actually visible through the transparent tissue). Depending on the intensity and duration of the stress, the corals may recover or die. Threats to Coral

Estimated threat to coral reefs: High Medium Low