Please note - this article is an ideal starting point for geography A-Level students. A more in-depth look at Atoll formation is included in Part 3 of this series.
Coral reefs are among the most diverse ecosystems on Earth, providing habitats for a multitude of marine species and acting as natural barriers against wave erosion. These ecosystems are primarily built by tiny organisms called coral polyps, which secrete calcium carbonate to form the rigid structures of the reefs. These structures has never ceased to amaze me during fantasic diving and snorkelling experiences where I have had the opportunity to see their ecosystems and sea life that thrive around them up close.
The above diagram refers to Continental and oceanic reefs. In A-Level Geography we focus on four main types of coral reefs: fringing reefs, barrier reefs, atolls, and patch reefs. Each type forms under specific conditions, influenced by factors like water depth, temperature, and wave action. This article explores the formation of these reef types, starting with fringing reefs, moving to barrier reefs, briefly touching on atolls, and concluding with patch reefs.
Source: Me and my smartphone!
Fringing Reefs
Fringing reefs are the most common type of reef and develop close to the shoreline, often directly attached to land masses. They are typically found in tropical and subtropical waters, where sea temperatures range from 23 to 29 degrees Celsius—a range conducive to coral growth (Sheppard et al., 2018). These reefs thrive in clear, shallow waters less than 30 metres deep, as corals rely on sunlight for photosynthesis carried out by symbiotic algae called zooxanthellae.
The formation of fringing reefs begins with the colonisation of a suitable substrate, such as volcanic rock or limestone, by coral larvae. Over time, the corals grow and expand horizontally and vertically, forming a reef flat that extends from the shore. As the reef matures, a shallow lagoon may develop between the reef and the land, though this is often minimal or absent in fringing reefs. These reefs are particularly sensitive to sedimentation and pollution due to their proximity to land.
Examples of fringing reefs include the reefs along the coast of the Red Sea and those in the Caribbean. Fringing reefs are often impacted by human activities such as coastal development and agricultural runoff, which can introduce sediment and nutrients harmful to coral health.
Barrier Reefs
Barrier reefs are characterised by their location farther from the shore, separated by a deeper, wider lagoon compared to fringing reefs. The formation of barrier reefs is thought to be a result of changes in sea level and the gradual subsidence of the land. As the landmass sinks or sea levels rise, the fringing reef continues to grow upward to maintain its position within the euphotic zone, where sunlight is sufficient for photosynthesis (Darwin, 1842).
Over time, the growing reef becomes increasingly detached from the land, forming a barrier reef. These reefs can extend for hundreds of kilometres and are among the largest reef systems in the world. The Great Barrier Reef off the coast of Australia is the most famous example, stretching over 2,300 kilometres and comprising over 2,900 individual reefs. The lagoon between the barrier reef and the mainland often supports diverse ecosystems, including seagrass beds and mangroves.
Barrier reefs are vital for protecting coastlines from wave action, reducing erosion, and serving as breeding grounds for numerous marine species. However, they are highly vulnerable to climate change, particularly coral bleaching caused by rising sea temperatures.
Atolls
Atolls are ring-shaped coral reefs that encircle a central lagoon. They usually form around volcanic islands in the open ocean and are the result of a sequence of geological and biological processes described by Charles Darwin in his "subsidence theory" (Darwin, 1842). According to this theory, atoll formation begins with a fringing reef surrounding a volcanic island. Over time, the island sinks or erodes, and the reef continues to grow upward to keep pace with rising sea levels or subsidence. Eventually, the volcanic island disappears below the ocean surface, leaving behind a ring of coral with a lagoon in the centre.
Atolls are predominantly found in the Pacific and Indian Oceans. The Maldives in the Indian Ocean and the Marshall Islands in the Pacific are notable examples. These isolated reefs are often hotspots for marine biodiversity but face significant threats from rising sea levels and ocean acidification.
More about atolls and the alternatives theories of their formation in Part 3 about Coral Reefs...
Patch Reefs
Patch reefs are smaller, isolated reefs that occur within lagoons or between larger reef systems. Unlike fringing and barrier reefs, patch reefs are not directly associated with shorelines. They typically form on sandy or rocky substrates in calm, shallow waters. Patch reefs vary widely in size and shape, ranging from a few metres to several hundred metres across.
The formation of patch reefs begins when coral larvae settle on a stable substrate, such as a rocky outcrop or seabed. These reefs grow slowly, as they are not exposed to the same water flow and nutrient supply as larger reef systems. However, patch reefs play an important role in local marine ecosystems by providing habitats for fish, invertebrates, and algae.
Patch reefs are often found in the lagoons of barrier reefs or atolls. For example, the Great Barrier Reef contains numerous patch reefs within its lagoon. Despite their small size, patch reefs contribute significantly to the overall biodiversity and ecological productivity of reef systems. They are particularly vulnerable to anchor damage from boats and sedimentation caused by human activities.
References
- Darwin, C. (1842). The Structure and Distribution of Coral Reefs. London: Smith, Elder & Co.
- Sheppard, C., Davy, S., & Pilling, G. (2018). The Biology of Coral Reefs. 2nd Edition. Oxford University Press.
- Hoegh-Guldberg, O. et al. (2017). "Coral Reefs Under Rapid Climate Change and Ocean Acidification." Science, 318(5857), pp. 1737–1742.
-Nagel, G. & Guiness, P. (2016). Cambridge Internatinal AS & A Level Geography, 2nd Edition. Hodder Education
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