Six main reef zones can be identified almost in the case of all coral reef formations that can be studied. Although variations in size, shape and ecosystem biodiversity may apply depending on the geographic areas in question and animal, coral and plant life species forming the food web, the existence of these zones is essential to the survival and long term thriving of coral colonies.
As we explore any coral reef, starting from the shoreline and moving out toward the open ocean, each drop or increase in elevation can tell us a great deal about the structure of the reef in question, as well as its development and the species of animals or plants that live in these areas.
The reef zones themselves are as follows:
- The first area that can be explored as soon as we leave the shoreline is the reef flat that stretches out from the shore to the reef crest. It contains two other significant zones – the back reef and lagoons – and its waters are generally shallow, warm and calm.
- The back reef is the area of the reef that extends to the reef crest and, like lagoons, is sheltered by it, so that its shallow, calm waters are ripe for the development of lush seagrass forests that herbivore fish and sea turtles feed on.
- Lagoons make up a body of water between the back reef and the shoreline, that is quite shallow and teeming with plant and animal life. They are sheltered by the forereef and crest, allowing for a wide variety of marine animals to thrive here as well.
- Next, the reef crest is the most barren zone of any coral reef formation, due to its exposure to strong waves, as well as air and sunlight during low tides. It has the main role of protecting the inner reef from stronger currents.
- The fore reef is the outermost layer of the coral reef formation. Here, strong and tall corals thrive, as the slope decreases and the waves are not as intense. Also, large fish and sea mammals are attracted by the rich biosphere of the fore reef, including barracudas and sea turtles.
- Finally, the deep reef features hard corals, an increase in sediments and a decreasing number of marine creatures, as it descends into the depths of the ocean.
Depending on the type of reef we’re looking at, lagoons may not be present – for instance, in the case of fringing reefs – and the reef flat or the forereef can sometimes extend seaward for several miles before the next change in elevation, offering significant information about the age and size of the reef formation.
When studying coral reef zones, one will find that different species of coral and plant life have evolved to survive in some zones, while they can’t be found in others. Many fish that thrive on the calm areas of lagoons and depend on the back reef’s lush seagrass would not be able to survive on the harsher environment of the upper fore reef and deep reef.
At the same time, many unique coral polyps and colonies have been found at depths where sunlight is scarce, and even less hospitable reef zones such as the reef crest house certain types of animals that have found refuge in certain areas below the waves.
The back reef is among the most important reef zones in any coral reef. Its role is often to provide shelter to a variety of sea creatures that form their habitats close to the protected areas of the lagoon and feed on the plant life growing on the areas of the reef crest that are not exposed to open sea currents and waves.
This side of the coral reef formation also plays an important role in containing lagoon reef zones, as well as maintaining their slow moving, warm currents to secure a safer environment for fragile coral reef marine species to thrive.
Back reefs are the area behind the coral reef crest that features an upward slope as seen from the shoreline. They are essentially part of the reef formation – in fact, they are the area that is located closest to the lagoon and shore.
In reef formations that are closer to the shoreline – such as fringing coral reefs – these areas still exist, even though they are smaller, since the reef doesn’t always feature a lagoon.
The shallow water and even depth in this area allows for increased sunlight and gentler currents to facilitate the thriving of plant and animal life, although factors such as higher temperature, occasional exposure due to lower tides and increased sunlight, also inhibit coral growth, so you might not see too many actual corals on the back reef.
Back reefs can range between a few meters to many kilometers, depending on the size of the coral reef formation they are a part of. Stretching out and away from the shoreline, they are still very well-protected from wave stress by the reef crest, making it ripe for the presence of feeding and nursery areas.
Dugongs and manatees, as well as sea turtles and a wide variety of herbivore fish thrive in the back reef area, due to its remarkable propensity for growing seagrasses. In most cases, seagrass and other types of underwater plants can grow with ease here, due to the area’s natural ability to maintain a warm, well-lit and quite stable environment.
Moreover, plants are able to grow more easily, since back reefs are not as densely populated with coral colonies as other areas of the reef, such as the fore reef zone and reef crest.
The back reef’s ability to grow plants is especially beneficial for sea turtles. These species, now labeled as endangered by most experts, are often seen as resting on back reefs, especially in areas such as the Caribbean or on Indo-Pacific reefs. Hawskbill turtles and green turtles are most often seen behind the reef crest, spending a great deal of time between it and the shore, and feeding on seagrass while resting on back reefs and swimming through lagoons.
The back reef is one of the most essential areas of any coral reef, and plays an important role not only in the support of a few marine species, but in the continuity and integrity of the coral reef food chain, almost in its entirety.
Coral Reef Crest
The reef crest is the part of the coral reef that usually lets us know where the reef is actually located. Reef crests come in all shapes and sizes, and although corals rarely actually grow here, there are many useful qualities associated with these reef zones.
Understanding the role played by reef crests in the structure and balanced continuity of coral reefs can be extremely important when trying to explain exactly how coral reefs work.
Any coral reef is formed by three main reef zones: reef crests, back reef and fore reef. While the back reef and fore reef – as the name would suggest – are found on the innermost and outermost side of the reef, and are usually completely submerged, reef crests are the uppermost areas of coral reefs, and are almost always visible above water at low tide.
Because of this, corals are almost never able to thrive on the reef crest, so you will rarely find any coral colonies here. Constantly uncovered, the crest of most coral reef formations is also bombarded with waves coming from the open sea, so that even plants and animals are rarely able to grow and find refuge in this hostile environment.
Nevertheless, the crest plays a vital role due to this fact, being assigned with the task of protecting the inner reef, the shoreline and any lagoons present from the strong ocean waves that would otherwise make the survival of small, fragile creatures, corals and plants almost impossible.
Another significant trait associated with reef crests located near most Indo-Pacific islands and coasts is the presence of a seaweed margin known as the algal ridge. The algal ridge is often exposed at low tide, and it is the area of the crest where the wave action is strongest. Calcareous red algae are most common here, and the ridge may also be a refuge for certain types of more resilient corals in areas where the waves aren’t as strong and the ridge is not usually exposed to open air during low tide.
Although the reef crest is normally not a good place for animals – and certainly not for most types of corals – some coral colonies are actually able to live here, as long as the wave action is less severe.
Elkhorn corals are usually quite prevalent on the reef crests of coral reef formations located in the Caribbean. At the same time, there are a number of short branching corals that can also survive, and are found on less exposed areas of reef crests even more often.
Even though they may be the highest point of the reef, reef crests are teeming with life just below the surface where the effects of the tide and waves are not as strong and the irregular texture of the reef can offer protection against predators and stronger waves. Only small animals can thrive here, however, such as small crabs and shrimp.
The protective and often vital qualities of the reef crest are essential for any fish, sea creatures and plants that live on the reef flat, and as time goes by and the coral structure continues to grow, this trait becomes even more important, especially in a more dynamic environment.
Coral Reef Flat
Understanding the main reef zones is essential to having a broader view of how corals actually work, and the reef flat is among the most significant reef zones, as well as the largest.
This is the entirety of the flat area that separates the shore from the coral reef crest. It includes both the back reef and the lagoon area (if present), and is often uncovered during low tide periods, extending the shoreline in some cases right to the reef itself.
Smooth, sandy areas that extend from the shore to the reef crest, reef flats commonly start with a low downward slope, and in most cases are just a few feet deep. Also known as the inner reef, this area sustains most of the damage incurred from sediments, due to being adjacent to the land. Because of this, coral colonies rarely survive here, and you can only find a few soft corals in more protected parts of the inner reef.
On the other hand, seagrass and seaweed grow in abundance on the reef flat, particularly near the reef slope, where coral growths are also more significant. At this point, an ascending slope begins, also known as the back reef, and can extend from a few feet to several miles, depending on how large and old the coral reef actually is.
The inner reef’s lowest depths are usually known to form a lagoon zone that allows for significant species diversity, due to the much warmer and tamer currents, compared to those hitting the reef crest and fore reef zone.
Mangroves and various plants found in the area between land and the main areas of the inner reef protect the shoreline by trapping sediments. They often turn these sediments into ideal habitats for small marine animals and fish. At the same time, seagrass beds can also be found in this reef zone, providing vital nourishment for a variety of species.
The inner reef is home to many types of underwater fauna, including hundreds of species of colorful fish, as well as sea turtles, as well as many other fascinating species of animals and plants alike, including sea cucumbers, sponges, anemones and conch.
It’s important to note that the zone may differ greatly in some areas, depending on the types of reef formations present and their distance from the actual shoreline. The reef flat can, in some cases, be much more difficult to navigate by larger fish and sea mammals, particularly due to it being separated from the open sea and offering far less space than the fore reef.
Nevertheless, due to the common presence of lagoons, as well as the larger depth and smoothness of these areas, inner reef zones can, in some cases, allow larger fish to thrive as well as herbivore ones that feed on the abundant seagrass, making it easy for the ecosystem to fully develop the food chain that makes it possible for the wide range of coral reef species and marine life to survive most successfully.
Whether you consider the multiple complex reef formations of the Great Barrier Reef or the volcanic islands of the Pacific, the reef flat reef zone plays an essential role in both the continuing growth and expansion of corals themselves and the marine life species thriving in the areas around them.
Coral Reef Lagoons
Lagoons are among the most beautiful types of reef zones in existence. Commonly found both in continental coastal areas and surrounding volcanic or barrier islands, these bodies of water are essential for maintaining the balance associated with a coral reef’s biosphere.
Also known as sounds, bays or estuaries, lagoons are often sheltered by small islands or islets, as well as small or large coral reef formations that sometimes stretch out for miles offshore, allowing the lagoon to grow to a significant size.
A lagoon is essentially a body of water protected by islands or reefs that are more common on coasts, and commonly surrounded either by barrier or atoll corals.
Due to this fact, the water is relatively shallow and calm, having access to the open sea through smaller channel that filter out larger waves and make it possible for water circulation to continue at a slow but consistent rate, while maintaining water temperatures quite stable (around 75-80 degrees in most cases).
The dimensions of a lagoon can range from very small ones found near fringing or atoll reef that are located quite close to the shoreline of a small island, to elongated or irregularly shaped, larger bodies of water that can be found between the shoreline of a larger island or continent and a barrier reef formation situated parallel to the land mass and stretching out in both directions.
These types of lagoon formations can be seen, for example, on the East Coast of the United States, where they extend intermittently for about 1,500 km.
The coastal lagoon is the most common type found in nature, widely distributed throughout the world, and forming about 13% of all continental coastlines. It is generally found near coasts protected from the open sea by barrier islands.
Water movement here is facilitated through discharging rivers and tidal forces, the latter playing a less significant role than in the case of a coral lagoon.
A coral lagoon can be found in the tropics and is restricted to tropical open seas that allow for the specific conditions required for coral growth. The presence of the lagoon is often essential for the thriving of all coral reef zones, particularly due to the calm, stable water and rich biosphere that it can support.
Most of these reef zones are found in the Caribbean and around volcanic islands located in the Pacific Ocean. Surrounded by either atoll or barrier reefs and, in some cases, also associated with the formation of larger fringing reefs that are located farther away from the shore and allow for the formation of a lagoon, they often feature a largely uniform depth, and are considered to be essential for the evolution and sustenance of a wide array of marine life species.
All these types of lagoons are extremely rich in marine flora and fauna, and sheltered by coral formations or islands, they can in most cases maintain a well-balanced ecosystem, away from the strong waves of the ocean.
Deep reef corals are among the most fascinating coral formations and can be found – although more sporadically than in the fore reef zone – in a variety of species and formations ranging from depths of 50-70 feet to more than 100-200 feet where abrupt dives usually lead down to the ocean floor.
The structure of the deep fore reef, as well as the corals and biosphere found in this area are quite fascinating. Marked by a slope that gets steeper and steeper with every advancing seaward step, this zone of the coral reef features decreasing sunlight and temperature, as well as fewer waves and more complex currents, which are the main ingredients that make it ripe for a diverse ecosystem.
The deep reef is located beyond the reef crest and fore reef, and features a continually increasing slope that leads towards the depths of the ocean/sea in which the coral is situated.
Healthier coral populations are found here, when compared to the ones found in shallower fore reefs and lagoons. This is, in most cases, due to the calmer waters present here and the fact that the corals are less disturbed by external influences.
Water pressure and other changed conditions, such as the increased presence of sediments and decreased temperature has led deep fore reef corals to be somewhat different, usually flatter and more spread out in order to maintain their stability and maximize sunlight exposure. Also, although most species are different from those found in shallow areas, many coral colonies can thrive both in the deeper and shallower fore reef zones.
Uneven sediments that form a single general slope can be found here that make the presence of corals increasingly sparse as the slope and depth of the reef continues to descend. Compared to the shallow water sediment normally found in most coral reefs, these sediments have the specific quality of smaller grain sizes.
It’s important to note the significant difference between deep water and reef corals. Although deep water corals are very similar in appearance to regular coral reefs you’d find on the deeper slopes of the fore reef, they are actually very different, both when it comes to their structure and the ways in which they interact with their environment.
Unlike deep reef corals, deep water species thrive in areas with little or no sunlight simply because they have evolved not to use sunlight at all for the purpose of getting their energy. Instead, they have developed a curious type of metabolism that allows them to survive by trapping tiny organisms and feeding on them almost as carnivore fish would.
Deep water coral formations have been found in numerous areas throughout the world, sometimes as deep as 6000 feet under sea level, and scientists continue to uncover new species through submarine explorations in all the oceans on the planet, found both as individual polyps and larger coral colonies.
Corals, animals and plant life still thrive both in the deep reef zone and further down, even though conditions are significantly harsher and access to sunlight and higher temperatures may be restricted.
The fore reef is the area of a coral reef formation separating the reef crest from the deep fore reef – and the open sea as well. This reef zone is basically the leading edge of the reef, and a thriving “marketplace” for predators, larger fish and marine creatures that thrive in a more dynamic environment.
This is the place to go when you’re looking for sharks and barracudas, and you will find there is also a unique and special elegance to the way this area of the reef has developed over time, allowing for the easier management of waves that has kept corals alive for hundreds and even thousands of years.
The fore reef, or reef face, is farther away from the shore, beyond the reef crest. Since it is one of the most hospitable environment on the reef, it often features large, tall corals that are quite diverse in size, shape and species.
The reef structure here is quite fascinating, featuring spurs and buttresses of corals, alternated with deep channels and grooves that have formed over time for a special purpose: they basically act like a buffer zone that protects the rest of the coral from the destructive force of ocean waves. They also makes it possible for a variety of smaller animals and fish to thrive here, finding refuge from the waves or forming their own habitats. This, in turn, attracts larger predators and consumers, maintaining the food chain and keeping the coral reef extremely rich and diverse in terms of its biosphere.
At the edge of the fore reef you can often find steeper slopes leading down from 20 to 40 meters or more, to even larger and more diverse coral species, before sediment growth and decreased sunlight makes the presence of corals more sparse.
Many people believe that the fore reef is the last part of the coral reef before one reaches the vast and deep expanse of the ocean. This isn’t exactly true, since the reef face is actually much closer than the outer wall – or deep fore-reef. The latter features far more extreme slopes, and leads to a sharp dive of hundreds of feet or more in some cases.
The fore reef, on the other hand, starts at the end of the reef crest – a point that always remains submerged, even during low tide – and extends outward to the oceanic side of the coral reef formation, without too many sharp angles or slopes being involved.
A Widely Diverse Ecosystem
Sharks, crabs, mantas and a host of other diverse species of creatures dominate the fore reef and make it a truly ripe environment for anything from the smallest creatures and herbivore fish to the largest carnivores to create a true haven here – somewhat like a metropolis of the sea.
Currents sometimes bring in sharks and sea turtles from the open ocean, while nearly vertical walls of coral can often be seen teeming with small and medium sized fish, while dozens of species of sponges, mollusks and crustaceans can often be found in this reef zone as well.
While not as rich in smaller animals and plant life as the back reef or the protected habitats of coral lagoons, the fore reef is definitely one of the richest and most fascinating areas of any coral reef formation.