There are three basic types of coral reefs; fringing, barrier and atoll.
Fringing reefs are located very close to shore, and because of water run off they are typically high in nutrients and the water has a high turbidity.
Barrier reefs are further from shore, with a lagoon between the reef and the shore.
And finally atolls are a circular reef with a central lagoon and possibly small islands formed on the reef.
Classes Of Coral Reefs
There are three basic types of coral reefs; fringing, barrier and atoll.
Fringing reefs are located very close to shore, and because of water run off they are typically high in nutrients and the water has a high turbidity.
Barrier reefs are further from shore, with a lagoon between the reef and the shore.
And finally atolls are a circular reef with a central lagoon and possibly small islands formed on the reef.
It is theorised that each of these types of reefs corresponds to a differing age of the entire reef structure.
The youngest is the fringing reef, with the corals colonising a shallow water area close to the land.
If the sea levels then rise or the land subsides, then the reef structure keeps up with this changing depth by growing upward.
Eventually a shallow area with no coral growth will form behind the main reef, called a lagoon, giving a barrier reef.
If the sea level or land subsides so much as to cause the land to disappear below the water surface, then an atoll is formed.
The overall type of the reef whether it is a turbid, high nutrient reef where the stony corals are less common and algae abounds or crystal clear, low nutrient reef where the stony corals can dominate, is dependent of several factors.
These include the proximity to land (therefore water run off which will be high in nutrients), proximity to river mouths (for the same reason as land proximity), and location of deep sea currents (which typically bring nutrient rich water.
Each type of reef is also divided into various zones within each reef.
What Is Coral Reef Zonation?
Zonation is a very important concept when considering any ecosystem, with coral reefs being no exception.
A particular ecosystem will typically be divided into zones, each one having a particular set of physical parameters, such as light intensity, that set it aside from any other part of the ecosystem.
A zone is defined by its physical parameters and location within the ecosystem. Within a particular zone, various organisms will have evolved such that they are adapted to thrive under those specific conditions that are present within that zone.
It can undoubtedly live somewhere else, in a similar zone or sometimes even one vastly different, but there is always one in which it will be perfectly suited and as a result out-compete any other organisms that occupies that niche in the food web.
What Determines Coral Reef Zonation?
Zonation within a reef is typically determined by: the light intensity received, which is dependent of the depth and turbidity of the water; position relative to the open ocean or river mouths; deep ocean currents; and localised water currents.
Each of these parameters interact to give the final conditions that are characteristic of that zone.
The most dominant parameter is the light intensity, which is the major source of energy for the reef community.
And because this is directly related to the depth, then the depth can be used as a very good indicator of the predominant conditions.
As a general rule, as the water depth increases then:
Light intensity drops.
Blue light increasingly becomes the more dominant wavelengths in the light spectrum, as the other wavelengths are absorbed more rapidly by the water.
Wave surges become less intense, but currents can still remain strong
Water temperature falls and becomes more constant
Therefore the particular organisms that are present on the reef alter with the depth, as each one has evolved to fit best into one area.
The organism can also alter its behavior depending on the zone, with the physical conditions have a large effect on its behavior and even its appearance.
This is graphically shown by many of the stony corals.
In the shallow, high light intensity and extreme water motion zones they will form fingered or massive (domed) structures.
As the depth increases, light intensity rapidly drops off and wave surges are reduced, but the currents can still remain strong.
To adapt to this lower availability of light, stony corals then take on thin, flattened plates therefore increasing the surface area that is exposed to the light.
Storms are another factory that can also affect and alter the zonation of a reef.
The force of the waves and associated surges batter the reef structure, break some of the reef building corals weakened by boring organisms, distribute and eroding sediment, and eroding the shores of coral cays.
Why Is This Important For A Reef Aquarist?
This is all well and good, but what does this have to do with someone wanting to have a small part of a reef in their home?
In actual fact it is very important.
If it is understood where a particular genus/species thrives and exists on a natural reef, then the conditions that it requires within an aquarium can be inferred.
For example if a coral is found on the reef rock rim, then it is exposed to high light intensity and strong wave surges.
To keep such a coral health and happy in an aquarium under similar conditions then intense lighting and devices that cause surges throughout the tank are required.
On the other hand if it is found on the reef slope, then it is exposed to low light levels, very few surges and strong currents.
Such a coral requires lower lighting levels and more constant water currents to be kept in an aquarium.
This article deals mainly with corals, both Alcyonaceans (Soft Corals) and Scleractinians (Stony Corals) orders, the main species that are found in each of the zones, and the outstanding physical properties that are characteristic of that zone.
Each zone is presented in order of its appearance from the ocean to the land.
Note that this is a generalised progression, and a real reef may vary from this and miss some of the zones.
As usual, nature will not follow strict rules.
Reef Front / Forereef Front
The reef front is the first part of the reef as land is approached, i.e. the front of the reef.
As the sea floor starts to approach the surface, then enough light starts to penetrate and supply the energy required for a reef community to exist.
This is divided into two sub-zones, the reef slope and upper reef slope.
Reef Slope / Deep Forereef
Below 20m in depth on the reef front is the reef slope.
At these levels the blue part of the light spectrum dominates and the light available is vastly lower than that at the surface.
Corals expand horizontally in shape in order to capture as much sunlight as possible.
Therefore any branching species that are found in shallower waters are largely replaced by plate-like forms of the same species.
Gorgonian fans (Gorgonacea) are very prolific in this zone, along with the feather stars (Crinoidea) that are associated with gorgonians.
The dominant species present are:
Scleractinians: Echinopora, Porites, Turbinaria, and Acropora.
Alcyonaceans: Dedronephthya.
Gorgonacea: Subergorgia etc.
Upper Reef Slope / Seaward Platform
Above the reef slope to around 5m is the upper reef slope, with this being the most densely populated zone of the reef.
This region typically has a spur and groove feature, with fingers of coral formations penetrating into the ocean with coral sand channels in-between.
The spurs are typically dominated by large fingered structures of Acropora and massive coral species.
The spur formations provide calmer regions where fleshy green algae, sponges and encrusting corals can grow.
The sandy groove regions support little coral or algae growths because of the strong scouring surges and tidal run-off running through these grooves.
But the occasional tough algae growths such as Halimeda can survive.
The grooves often open out to a region of rubble and coarse sand.
This entire zone provides a region where nutrients are concentrated then transported into the reef flat area by algae growing then becoming detached.
Corals have to take on expansive body forms designed to maximise the exposure to the sunlight, but are not limited to the vast horizontal plates characteristic of the reef front.
The shallower regions also have more Scleractinians because of their more robust structure, higher growth rates under intense light and territorial defense mechanisms.
Feather stars, sponges and other suspension feeders expose themselves to intertidal currents on structures that jut out into the currents, such as gorgonians.
Inside overhangs and caverns, azooxanthellae corals, sponges and soft corals are dominant.
The dominant species present are:
Alcyonaceans: Lemnalia, Lobophytum, Nephthea, Sarcophyton, Sinularia, and Xenia.
Scleractinians: Acropora, Goniastrea, Favia, Favites, Leptoseria, Lobophyllia, Plerogyra, Pocillopora, Porites, Millepora, and Stylophora. Zoanthidea: Palythoa.
Reef Rock Rim
The rock rim is the highest energy zone of a coral reef ecosystem, with very intense light, and intense wave action and surges.
Parts of it may be exposed at low tide.
Corals that live within this zone are typically very short and fingered or massive in structure to withstand the strong wave action and as don't have to spread out to capture light.
Almost all surfaces in this zone are exposed to some light (not just the upper area as in the deeper reef front zone) so the massive and fingered structures utilised this fact and are strong enough to resist the strong currents.
Coralline algae can cements this region together, forming a solid terraced like pavement.
Also can be present low, sand binding algae mats that entrap sediment.
The dominant species present are:
Alcyonaceans: Lobophytum, Sarcophyton, and Sinularia.
Scleractinians: Acropora, Favites, Montipora and Pocillopora.
Zoanthidea: Palythoa.
Reef Flat
The reef flat behind the reef rock rim is also a rigorous environment.
The organisms here must be able to withstand intense ultra violet radiation, desiccation, high salinities and elevated water temperature.
This zone is divided into two sub-zones, living coral sub zone and sand sub-zone.
Coral cover decreases inward, with sand covering the inner part of the reef flat.
This entire zone is usually most prolific with Acropora, Actiniarians (anemones), Asteroids (starfish), Holothurioids (sea cucumbers), Alcyonaceans, and reef fishes.
Living Coral Sub-Zone
Within the living coral sub-zone tongues of the reef structure penetrate into the lagoon with narrow sand channels in-between.
This is very similar to the upper reef slope.
The coral sand that is present is produced from coralline algae, foraminifers, calcareous algae, and the breakdown of the reef structure.
Echinoids (sea urchins), Asteroids (starfish), Holothurioids (sea cucumbers) and molluscs are abundant over this entire zone.
The dominant species present are:
Scleractinians: Acropora, Pocillopora, Gonipora, Platygyra, Seriatopora, Lobophyllia, Tubipora, Montipora, Fungia, Goniastrea, Favia, Favites, and Porites.
Sand Sub-Zone / Lagoon
Gradually the reef structure gives way to vast areas of coral sand eroded from the main reef structure.
Dotted throughout this zone are small 'islands' of Scleractinians that rise up out of the sand.
In the calmer regions delicate branched corals form intricate growths.
The lagoon is constantly supplied with nutrients and sediment removed from the reef front and reef rock rim zones, with some lagoons becoming muddy from the accumulated sediment.
Holothurioids are prolific inhabitants and constantly rework the surface sediment.
Deeper lagoons with heavy sediments or high turbidity have:
Scleractinians: Cataphyllia, Euphyllia, Gonipora, Leptoseris, Pachyseris, and Montipora.
The island or patch reefs that rise out of the lagoon floor consist of:
Scleractinians: Acropora, Favia, Favites, Galaxea, Goniastraea, Pavona, Pocillopora, Porites, Seriatopora, Stylophora, and Tubipora.
Spread over the sandy bottom can be found:
Scleractinians: Heliofungia, Fungia, and Herpolitha.
Other families present:
Alcyonaceans: Heliopora, Sarcophyton, Lobophytum, Xenia, Cespitularia, and Sinularia.
Coralliomorpharia: Rhodactis.
Zoanthidea: Palythoa, and Zoanthus.
Cay
Intertidal Zone
This is another high energy zone, with the organisms that live here adapted to withstand intense ultra violet radiation, desiccation and high salinities.
There is usually water retained in tidal pools even at low tide.
Some corals can survive this harsh environment completely exposed to the air.
Gastropods are very common in this zone, and usually the most conspicuous inhabitants.
Specific family information has yet to be found.
Beach
Finally then comes the beach.
This region is of no interest to current aquaria, but hey imagine it, your own Pacific island beach!
Any thing inhabiting this region is very resilient.
Some obvious organisms are Molluscs (mainly mussels that bury themselves in the sand) and Decapods (crabs).
But there is also a multitude of interstitial sand fauna that extensively inhabits this zone.
References
Aw, Michael, Tropical reef life: a marine awareness guide, OceaNEnvironment:Carlingford, 1997.
Delbeek, J. Charles, and Sprung, Julian, The reef aquarium: a comprehensive guide to the identification and care of tropical marine invertebrates, vol. 1, Ricordea:Coconut Grove, 1995.
Moe, Martin A. Jr., The marine aquarium reference: systems and invertebrates, rev. ed., Green Turtle:Plantation, 1993.
