Coral Reef Growth is One of Nature's Wonders

Posted by Quality Marine Staff on July 11, 2011

Part 1: Formation of Coral Reefs

Introduction

Coral reefs are home to some of the most diverse ecosystems on Earth. Unfortunately, recent years have seen massive bleaching and die-offs within these amazing reef ecosystems, and the loss is much maligned amongst scientists and the public at-large. Recent studies have even shown that public opinion in the United States overwhelmingly supports the conservation of these ecosystems, as they have come to be a poster-child for the marine environment.

Often, we see photographs of beautiful, tropical coral lagoons. In reality, coral reefs exist in many different forms and locations. There are fringing, barrier, and atoll reefs; there are back reefs and lagoons, along with forereefs, and deep reef walls; there are even cold water reefs, seamounts, and deep reefs. It is in the formation of these disparate types of reefs that the truly amazing nature of coral ecology emerges.

The development of coral reefs involves numerous influences, of both biological and geological natures. Most of the time, coral reefs are pictured as mature environments that are fully-grown. In fact, coral reefs are constantly changing, if slowly, and will perpetually be in the throes of creation. The fascinating life forms we find on and around the reef area are there as the result of eons of leisurely accretion of old coral skeletons, debris, and coral sand created by coral-eating fish and destructive storms. Moreover, reefs exist only because tectonic movements lead to uplift that brings submerged hard substrate into the photic, or sunlight, zone. During the life of a coral reef, these changes will cause it to smoothly transition through several distinct stages.

Let us begin to explore what a coral reef is, and how it is created.

If you ask a random person to define the word reef, you are likely to hear a vague description involving tropical corals and fish. If you happened to ask a sailor or fisher, you might hear that a reef is something to which boats should give a wide berth. Ask a scientist, you get a complex explanation about uplift, sediment deposition, wave action, and epibenthic fauna. What all these answers are really telling you is that the word reef loosely encompasses almost every area of exposed rock in the ocean.

From a nautical perspective, a reef is a rocky outcropping upon which a ship can run agroundneedless to say, sailors avoid this type of reef. Fishers, on the other hand, have a mixed perspective. For example, as they are sailors they must avoid crashing into the reef and tearing or losing nets on the rocks. Yet, submerged reefs are often highly productive fishing zonesespecially in cold water.

A coral reef arises where exposed rocky outcroppings reach up into the photic zone. They are special because they grow biologically, as well as geologically. That is, coral reefs are essentially animal colonies that create massive geological structures. Exclusively found in shallow waters, and not usually deeper than 165 feet, coral reefs can only grow in water warmer than 68 degrees Fahrenheit. For these reasons, coral reefs are rarely found beyond 30 degrees of the equator. Reef formation also depends on other aspects of water quality. They need very clear, sediment free water with highly stable salinity levels. Corals must also live in low-nutrient water or they will not be able to outgrow macroalgal competitors. As such, they rarely form near the mouths of large rivers, and usually do well when regularly cleansed by turbulent waters.

Coral reefs develop a complex community of algae, coral, sponges, invertebrates, and fish, all specifically tailored to the prevailing conditions. Hermatypic (reef-building) corals, which are mainly Scleractinians, and encrusting coralline algae are the main contributors to the growth of the reef. The symbiotic cooperation of reef-builders is what makes this type of reef truly astounding. Crevices between upward growing corals collect sediment and other debris, including dead skeletons from algae, coral, and other invertebrates. Coralline algae and bryozoans then lock all the loose sedimental-debris into highly porous aragonite rock.

When volcanic mountain chains rise from the seabed, their islands frequently create excellent coral habitat by meeting the strict requirements of reef-buildings organisms. In tropical regions, where it rains almost every day, these porous landforms are scoured free of sediment and nutrients very quickly. What remains is locked away in lush jungle growth. Large rivers develop less often, and open exposure to ocean currents quickly acts to transport freshwater and sediments away. Conditions around volcanic islands are essentially ideal for coral growth.

Though coral reefs form on continental fringes as well, it is useful to consider the island paradigm because it nicely encompasses the major components of coral reef formation:If you ask a random person to define the word reef, you are likely to hear a vague description involving tropical corals and fish. If you happened to ask a sailor or fisher, you might hear that a reef is something to which boats should give a wide berth. Ask a scientist, you get a complex explanation about uplift, sediment deposition, wave action, and epibenthic fauna. What all these answers are really telling you is that the word reef loosely encompasses almost every area of exposed rock in the ocean.

What is a Coral Reef?

Rising Seamount/Developing Reef

As a tectonic plate gradually moves over a hotspot in the earths crust, a plume of molten magma begins to pressure the plate from below. This pressure causes the crust to deform and rise into a conical mountain. Though the process is extremely slow, the mountain may eventually reach monumental proportions. For instance, from its submarine base to its lofty summit, the Hawiian volcano Mauna Kea is actually more 3,700 feet taller than Mount Everest.

In due time, a seamount rising from the abyss reaches the photic zone and photosynthetic organisms displace less competitive inhabitants. At this time, the rudimentary base of a coral reef can begin to form. Coral reefs begin to grow at depths of around 165 feet. It may take tens of thousands years for the seamount to finally breach the surface and create an island. So, though the island is brand new, the coral reef is long-since fully developed and thriving.

Growing Island/Fringing Reef

Approaching and emerging at the surface, the growing island will inevitably lift reefs permanently above sea level. Obviously, these sections die and contribute to the mass of the island thereafter. The remaining submerged coral is perpetually brought closer and shallower, with wind and waves dramatically shaping it along the way. The result is usually a broad reef flat which extends all the way from the reef crest to the coastlinefringing it.

The new stressors reforming the reef change its shape forever. Wave action very near the surface, at the reef crest, can be violent. If so there may be an algal ridge at the very peak. Otherwise, coral growth at the crest can be the some of the most luxurious on the reef. Behind the crest, on the flat, the coral reef cannot grow into an area which suffers prolonged exposure during low tide. The extreme low tide line is thus the upper limit of coral growththe reason why the reef flats are flat! Good examples of fringing reefs can be found around the Hawaiian Islands.

Subsiding Island/Barrier Reef

Eventually every island sinks back into the sea. As the tectonic plate upon which it resides moves it away from the mantle hotspot, upward growth ceases. From there, the island is inexorably worn away. Landslides cleave massive wedges from mountainsides. Wind, rain, and waves tear the stone apart. The island finally retreats from coral reef it helped form, leaving it free-standing. The long-term future of the reef depends on whether or not it grows faster than the island sinks. If so, the reef will remain viable indefinitely; if not, the reef eventually sinks out of its photic zone and ceases to grow hermatypic life.

As the distance between the reef crests and the islands coast increases, barrier reefs and lagoons develop. The reef crests form obstructions that bear the full force of the waves and currents, while the lagoons sluggish waters collect soft sediments and sand. Lagoons are sometimes dotted with patch reefs, or coral knolls, as well as beds of seagrass.

Barrier reefs can grow to gargantuan sizes. The worlds largest coral reef is Australias Great Barrier Reef, stretching 1,200 miles in length and up to 200 miles in width! Amazingly, because corals reef are such tightly integrated communities, they can almost be thought of as singular organisms. The Great Barrier Reef is a testament of the power of a coral reef ecosystem to create geological features at a global scale.

Sunken Island/Coral Atoll

When the last vestiges of the island have ceased to be, and the ocean has reclaimed the spot, a coral atoll remains. Where an island once grew and died only reefs and lagoons are now found. Coral atolls typically form in the western Pacific, while occurring less frequently in the Atlantic. They may exist far away from any land masses, in which case they live without being subjected to runoff. The ecology is similar to the previous stage, but even more luxurious because of the pristine water quality. However, the reef is losing the support of the island base. It must grow upward faster than it sinks, or eventually drop below the photic zone and lose all its reef-building organisms.

Atolls take the form of amorphous rings, and may have small islands and sand cays (keys) atop the reef crests. On the outside of the atoll, the reef wall creates a virtually sheer drop to the ocean floor, which is sometimes many thousands of feet deep. Inside the ring, the lagoon floor will continue to settle as it rests on the subsiding mountain top, and can be up to 200 feet deep.

Premier examples of coral atolls can be found throughout the Indo-pacific region, but particularly stunning examples occur in the island nations of Fiji and French Polynesia.

Tablemount/Guyot

The final stage of the island/reef mechanism is the tablemount, or guyot. Basically, a tablemount is a seamount with a flat top, with the flatness being caused by the fact that the mount was once above sealevel. Provided that the top of the guyot is sufficiently shallow, it could still support an existing reef. However, eventually the mount drops out of the photic zone, and the coral reef is no more.

In Part 2, the discussion picks up with a description of reef sections and zonation. The particulars of the communities in these zones will also be reviewed.

References:

Block, Heidi. Coral Reef Zonation. Encyclopedia of Earth: January, 2008. Url: http://www.eoearth.org/article/Coral_reef_zonatio...

Geology. Highest Mountain in the World. 2009. url: http://geology.com/records/highest-mountain-in-th...

Huber, Michael E. and Peter Castro Marine Biology 5th Ed. Boston : McGraw Hill, 2005.

Jrank Science Encyclopedia. Coral and Coral Reef - Coral Reef Development and Zonation. 2009. url: http://science.jrank.org/pages/1789/Coral-Coral-R...

King, Matthew. 2009.

Thurman, Harold V. and Alan P. Trujillo. Introductory Oceanography, Tenth Edition. Upper Saddle River; Pearson Prentice Hall. 2004.