Coral Reefs & Coral Bleaching | Geography Optional for UPSC (Notes) PDF Download

Introduction

• Corals are essentially calcified structures created by the skeletons of tiny marine creatures known as polyps. These polyps extract calcium salts from the surrounding seawater and use them to form rigid skeletons, which serve as a protective barrier for their soft bodies. The resulting structures are what we commonly refer to as corals.
• These corals exist in colonies that are attached to the rocky ocean floor. As new generations of polyps develop, they build their skeletons on top of the remains of deceased polyps. Over time, the tubular skeletons grow both upwards and outwards, creating a solid, calcareous rock mass known collectively as corals.
• The formation of a shallow rock layer due to these accumulations is known as a reef. Over time, these reefs can develop into islands. Corals can be found in various shapes and colors, which is dependent on the type of salts or other components that make up their composition.
• The continued growth of corals can result in them appearing in various forms on the ocean's surface over an extended period. Additionally, small marine plants called algae also contribute to coral growth by depositing calcium carbonate.

Ideal Conditions for Coral Growth

• Coral growth is best suited for tropical regions, specifically between 30°N and 30°S latitudes. The optimal depth for corals to thrive is between 45 and 55 meters below the sea surface, as this depth provides ample sunlight exposure.
• Water temperatures around 20°C are ideal for coral growth. Clear saltwater is necessary for corals to prosper, while both freshwater and highly saline water can be detrimental to the growth of coral polyps.
• A sufficient supply of oxygen and microscopic marine food, known as plankton, is crucial for the growth and survival of corals. Since there is a higher abundance of food on the seaward side, corals tend to grow more rapidly in that area.

Types of Coral Reefs

Coral reefs can be categorized based on their large-scale morphology, which includes their size, shape, and relationship to nearby landmasses. Although there can be some overlap among the different types of reefs, they are generally distinguishable from one another. The three main types of coral reefs are fringing reefs, barrier reefs, and atolls.

• In the context of coral reef classification, a lagoon refers to a relatively wide body of water situated between the shore and the main reef area. This body of water usually contains some deeper sections.
• Fringing reefs are the most common of the three major types of coral reefs. These reefs are coral platforms that are attached to a continental coast or an island, and they are sometimes separated from the shore by a narrow, shallow lagoon or channel. Fringing reefs typically extend as a narrow belt, ranging from 0.5 km to 2.5 km in width. They grow from the deep ocean floor, with their seaward side sloping steeply into the deep sea. Coral polyps do not extend outward due to the sudden and significant increase in depth. The surface of a fringing reef is typically rough, as it is covered with coral remains, forming a boulder zone or reef flat.

Barrier Reef

• A barrier reef is the largest among the three types of coral reefs and spans hundreds of kilometers in length and several kilometers in width. It forms an irregular, discontinuous ring around a coastline or an island, situated almost parallel to the shore. One of the distinguishing features of a barrier reef is its relatively far location from the coast, accompanied by a wider and deeper lagoon. Occasionally, channels that cut across the barrier reef connect the lagoon to the open sea.
• In terms of structure, a barrier reef is quite thick, reaching depths of up to 180 meters below the surface. The seaward side of the reef exhibits a steep slope that descends into the deep ocean. The surface of barrier reefs typically consists of coral debris, boulders, and sand.
• The most well-known example of a barrier reef is the Great Barrier Reef, located off the northeastern coast of Australia. This massive reef stretches over 1,900 kilometers in length and spans up to 160 kilometers in width.

Atolls

• Atolls are ring-shaped coral reefs that either partially or entirely encircle a lagoon. The lagoon's surface is typically level, while the seaward side of the reef has a steep slope that descends into the deep sea. Lagoons can have depths ranging from 80 to 150 meters and may connect with the sea through channels that cut across the reef.
• Atolls are often formed in areas far from deep-sea platforms, where underwater features like submerged islands or volcanic cones provide suitable conditions for coral growth. They are more commonly found in the Pacific Ocean than any other ocean, with notable examples including the Fiji atoll and the Funafuti atoll in the Ellice Islands. The Lakshadweep islands also host a significant number of atolls.
• In the South Pacific, many atolls are located in the middle of the ocean. These reef formations can be found in places like French Polynesia, the Caroline and Marshall Islands, Micronesia, and the Cook Islands. The Indian Ocean also contains a substantial amount of atolls, with examples in the Maldives and Chagos island groups, the Seychelles, and the Cocos Island group.

Formation of Lakshadweep Islands (Atoll Formation)

• The classification system for coral reefs, initially proposed by Charles Darwin, continues to be widely used today. According to Darwin's theory, fringing reefs start to develop near the coastlines of newly formed islands when ecological conditions become favorable for the growth of hard corals. 
• As the island slowly sinks into the sea, the coral continues to grow at a rate that allows it to stay at the sea surface but moves further away from the shore, turning into a barrier reef. Eventually, the island submerges completely beneath the sea, leaving behind a ring of coral that encircles a central lagoon, thus forming an atoll.

Distribution of Coral Reefs
The majority of reef-building corals can be found in tropical and subtropical waters, usually between 30° north and 30° south latitudes. The area with the highest concentration of coral reefs and the most diverse coral species is the Indonesian and Philippine archipelago. Other significant regions with coral reef concentrations include Australia's Great Barrier Reef, the Red Sea, and the Caribbean, although the Caribbean has a much lower diversity compared to the major Indo-Pacific regions.
The main coral reef regions in the world are as follows:

• Caribbean and Western Atlantic
• Eastern Pacific
• Central and Western Pacific
• Indian Ocean
• Arabian Gulf
• Red Sea

Coral Reefs Bleaching

• Coral reefs around the world have experienced significant degradation in recent decades due to various disturbances, both natural and human-induced. Human activities such as overexploitation, overfishing, increased sedimentation, and nutrient overloading have contributed to the accelerated decline of coral reefs. Natural disturbances, on the other hand, include violent storms, flooding, extreme temperatures, and El Nino Southern Oscillation (ENSO) events.
• Coral bleaching is a common stress response of corals to these disturbances. It occurs when the symbiotic relationship between the coral host and the marine algae, which provide the coral with much of its color, is disrupted. As a result, the coral tissue becomes transparent, revealing the bright white coral skeleton. This phenomenon is called coral bleaching.
• When corals bleach, they begin to starve, as most of them rely on the algae for their sustenance. If environmental conditions return to normal, corals can regain their algae, regain their normal color, and potentially survive. However, the stress caused by bleaching usually results in reduced coral growth and reproduction, as well as increased vulnerability to diseases.
• If the stress causing the bleaching persists, bleached corals are likely to die. In cases where coral death rates are high following bleaching events, it can take many years or even decades for the affected coral reefs to fully recover.

Causes of Coral Bleaching
Coral bleaching is a stress response that can be triggered by various factors, either alone or in combination. As a result, it is challenging to pinpoint the exact causes of bleaching events. The following stressors have been associated with coral reef bleaching events:

• Temperature: Coral species thrive within a relatively narrow temperature range, making them sensitive to both low and high sea temperatures. Bleaching events can occur due to sudden temperature drops from intense upwelling episodes, seasonal cold-air outbreaks, and other causes.
• Solar Irradiance: Bleaching often occurs during summer months when temperatures and solar irradiance are at their highest. This is especially true for shallow-living corals and those exposed on colony summits.
• Subaerial Exposure: Coral reefs can experience bleaching when they are suddenly exposed to the atmosphere during events such as extreme low tides, sea-level drops related to El Niño-Southern Oscillation (ENSO), or tectonic uplift.
• Freshwater Dilution: Rapid dilution of reef waters from storm-generated precipitation and runoff can also cause coral reef bleaching.

Other potential causes of coral bleaching include increased concentrations of inorganic nutrients, sedimentation, oxygen starvation due to a rise in zooplankton levels from overfishing, ocean acidification, changes in salinity, sea-level changes caused by global warming, and cyanide fishing, among others.

Spatial And Temporal range of Coral Reef Bleaching

• Coral reef bleaching, which leads to mass coral mortality, has been observed in all major reef provinces since the 1870s. 
• However, the frequency and scale of these events have significantly increased since the late 1970s, possibly due to an increase in the number of observers and a heightened interest in reporting such incidents. 
• From 1979 to 1990, there were over 60 coral reef bleaching events out of 105 mass coral mortality incidents reported, compared to only three bleaching events among 63 mass coral mortality incidents recorded during the previous 103 years.

Conclusion

Coral reefs are vital marine ecosystems that provide habitat and sustenance to a wide variety of marine species. They are categorized into three main types: fringing reefs, barrier reefs, and atolls. Coral reefs predominantly thrive in tropical and subtropical regions, with the highest concentration and diversity found in the Indonesian and Philippine archipelago. However, these essential ecosystems are facing significant threats due to human activities and natural disturbances, leading to widespread coral bleaching and degradation. Efforts to understand and mitigate the causes of coral bleaching are crucial for the preservation and recovery of these invaluable ecosystems.

Frequently Asked Questions (FAQs) of Coral Reefs & Coral Bleaching

What are corals and how do they form reefs?

Corals are calcified structures created by the skeletons of tiny marine creatures called polyps. These polyps extract calcium salts from seawater and form rigid skeletons as a protective barrier for their soft bodies. Coral reefs are formed when these colonies of coral polyps grow and develop over time, accumulating in layers and creating a shallow rock layer called a reef.

What are the ideal conditions for coral growth?

Coral growth is best suited for tropical regions, specifically between 30°N and 30°S latitudes. The optimal depth for corals to thrive is between 45 and 55 meters below the sea surface, which provides ample sunlight exposure. Water temperatures around 20°C, clear saltwater, a sufficient supply of oxygen, and a steady supply of microscopic marine food (plankton) are also crucial for coral growth.

What are the three main types of coral reefs?

The three main types of coral reefs are fringing reefs, barrier reefs, and atolls. Fringing reefs are coral platforms attached to a continental coast or an island, sometimes separated from the shore by a narrow, shallow lagoon. Barrier reefs are the largest of the three types, forming irregular, discontinuous rings around coastlines or islands, with a wider and deeper lagoon. Atolls are ring-shaped coral reefs that either partially or entirely encircle a lagoon.

What causes coral bleaching, and how does it affect coral reefs?

Coral bleaching is a stress response that occurs when the symbiotic relationship between the coral host and the marine algae is disrupted, causing the coral tissue to become transparent and revealing the bright white coral skeleton. Bleaching can be triggered by various factors such as temperature changes, solar irradiance, subaerial exposure, and freshwater dilution, among others. Bleached corals can potentially recover if environmental conditions return to normal; however, the stress caused by bleaching usually results in reduced coral growth, reproduction, and increased vulnerability to diseases. If the stress persists, bleached corals are likely to die, leading to long recovery periods for the affected coral reefs.

Has the frequency and scale of coral bleaching events increased in recent years?

Yes, the frequency and scale of coral bleaching events have significantly increased since the late 1970s, possibly due to an increase in the number of observers and a heightened interest in reporting such incidents. For example, from 1979 to 1990, over 60 coral reef bleaching events were reported, compared to only three bleaching events among 63 mass coral mortality incidents recorded during the previous 103 years.