Coral reefs

Coral reefs

Subject: Environment

Section: Ecosystem 

Context: Things aren’t looking too good for coral reefs. They’re suffering from bleaching, overfishing and are being cooked by warming oceans.

Coral Reefs:

• Coral reefs are one of the most biologically diverse marine ecosystems on the Earth.
• They are the underwater structures that are formed of coral polyps that are held together by calcium carbonate.
• Coral reefs are also regarded as the tropical rainforest of the sea and occupy just 0.1% of the ocean’s surface but are home to 25% of marine species.
• They are usually found in shallow areas at a depth less than 150 feet. However, some coral reefs extend even deeper, up to about 450 feet.
• Corals can be found in all the oceans but the biggest coral reefs are mostly found in the clear, shallow waters of the tropics and subtropics.
• The largest of these coral reef systems, The Great Barrier Reef in Australia, the largest coral reef is more than 1,500 miles long.

Conditions for Coral reef growth:

• Stable climatic conditions: Corals are highly susceptible to quick changes. They grow in regions where climate is significantly stable for a long period of time.
• Perpetually warm waters: Corals thrive in tropical waters [30°N and 30°S latitudes, temperature of water is around 20°C] where diurnal and annual temperature ranges are very narrow.
• Clear salt water: Clear salt water is suitable for coral growth, while both fresh water and highly saline water are harmful.
• Abundant Plankton: Adequate supply of oxygen and microscopic marine food, called plankton [phytoplankton], is essential for growth. As the plankton is more abundant on the seaward side, corals grow rapidly on the seaward side.
• Little or no pollution: Corals are highly fragile and are vulnerable to climate change and pollution and even a minute increase in marine pollution can be catastrophic.

Types of Coral Reefs:

• Fringing reefs consist of flat reef areas that directly skirt a non-reef island, often volcanic, or a mainland mass.
• Barrier reefs are also close to a non-reef landmass but lie several kilometres offshore, separated from the landmass by a lagoon or channel often about 50 metres (160 feet) deep. Some barrier reefs are more or less circular, surrounding an island, but larger barrier reefs, such as those along the Red Sea coast and Australia’s Great Barrier Reef, are complex linear features consisting of chains of reef patches, some of them elongated into ribbon reefs.
• Atolls are like circular barrier reefs but without their central landmass.

Benefits:

• Marine diversity: Coral reefs have many species of marine organisms that depend on them for food, shelter, and reproduction. Healthy corals support thriving marine life, but unhealthy corals can lead to loss of the marine line which in turn will disrupt the food chain.
• Tourism: Healthy coral reefs are attractive tourism destinations, and bleaching can affect the local economy negatively.
• Fishing: Fishing communities are depended on corals for seasonal migrations of fish species. Unhealthy corals will lead to a decrease in fishing activities impacting economic activities.
• Coastline protection: Coral reefs protect coastlines by absorbing constant wave energy from the ocean, thereby protecting people living near the coast from increased storm damage, erosion, and flooding
  • Coral reefs act like low-crested breakwaters and absorb 97% of wave energy.

Coral Bleaching

• The coral and the zooxanthellae share a symbiotic relationship and 90% of the nutrients that are produced by the algae are transferred to the coral hosts.
• But this relationship gets affected under severe environmental stress which causes the loss of symbiotic algae (zooxanthellae).
• As a result, the white calcium-carbonate exoskeleton is visible through its transparent tissue leading to a condition known as Coral Bleaching.
• The corals become vulnerable in the absence of the algae and begin to die if the temperature of the sea remains high for weeks.

Causes of coral bleaching

• Warm Sea Temperature: Coral species prefer to live in waters close to the warm temperatures which they can tolerate but a slight increase in ocean temperature can harm corals.
• Extreme low tides: Extreme events of low tides exposes the corals to solar and ultraviolet radiations which can induce coral bleaching.
• Ocean Acidification: Oceans are the carbon sinks, but more carbon dioxide increases the acidity of the ocean. This increase in the acidity of ocean water inhibits the coral’s ability to create calcareous skeletons, which is essential for their survival.
• Diseases: Species of bacteria like vibrio shiloiinhibits the photosynthesis of zooxanthellae. These bacteria become more potent at higher sea temperatures.
• Ocean Pollution: The increasing nutrient concentrations leads to excessive phytoplankton growth, and attracts more and more marine life, which may cause strain on the reefs.
• Sedimentation: High rates of land erosion causes silt and other sediments to leach into ocean waters which causes sedimentation and water turbidity. The siltation tends to smother corals and turbidity reduces light availability potentially reducing coral photosynthesis and growth.
• Anthropogenic threat: Over-fishing, pollution from agricultural and industrial runoff,coral mining, development of industrial areas near coral ecosystems have adverse impacts on coral reefs.
• Predators: Acanthasterplanci, also known as Crown-of-Thorns Starfish, eats corals during the night. They may destroy the entire coral reefs if found in huge numbers.