When setting up a reef tank, one important thing to consider is the species we want to keep in it. Some require stronger water flow and strong light while other require dimmer light and low to moderate water flow. Successful reefkeeping begins with a good planning to only mix compatible species and provide each of them with the required conditions.
Coral reefs are divided in zones that are inherently connected and dependent on each other. To help figure it out, lets have a quick look at the different reef zones, water conditions and species living on each of them.
The information below are from various sources (listed below). I have found some contradiction around the web and hope the information here is accurate. However, if you see anything wrong or have something you would like to add to this, please do so by using the comment form at the end of the article.
The outer reef, also known as fore-reef slope is the zone that faces the open ocean where the reef rises up from the depths, often quite vertically. Currents from the ocean bring constant supply of rich plankton, detritus and nutrient to the animals living there.
The deep part of the slope, represents the limit of the reef. The great depth protects the base of the outer reef from the majority of surge caused by the waves. The lack of intense light prohibits the proliferation of many species. At these levels the blue part of the light spectrum dominates.
Corals and sponges living there are non-photosynthetic and get most of their food from nutrient brought from the open ocean and organism living above them. This part of the reef is not really an area which most of our corals come from.
At 100-135 feet, photosynthetic corals slowly take over non-photosynthetic corals. Most 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) along with feather stars (Crinoidea) are very prolific in this zone.
Some of the dominant species present on the outer reef are:
Echinopora, Porites, Turbinaria, Acropora, Dedronephthya, Subergorgia, etc.
Upper reef slope
Just above the reef slope and before the reef crest, at about 30 feet is the upper reef slope. This area is generally the most densely populated zone of the reef. It is typically dominated by large fingered structures of Acropora and massive coral species.
Some of the dominant species present on the Upper reef slope are:
Lemnalia, Lobophytum, Nephthea, Sarcophyton, Sinularia, Xenia, Acropora, Goniastrea, Favia, Favites, Leptoseria, Lobophyllia, Plerogyra, Pocillopora, Porites, Millepora, Stylophora and Palythoa
The reef crest:
|As the slope rise, waves are seen to break over the reef. This area is known as the reef crest. It is the highest point of the reef and is characterized by a line of waves that break along its edge. Corals on the reef crest are subjected to strong waves, strong light, low tide, strong current and storms. Here, current can have a speed of over 365 cm/sec. For comparisons sake, Dana Riddle measured the output velocity of the Maxijet 1000 powerhead at 75 cm/sec 0.5 inches from the nozzle. Where wave action is severe, living corals are practically nonexistent,|
but in situations of more moderate wave action, corals often cover 60-100% of the crest. These areas tend to be dominated by a few species of closely growing corals such as SPS, fire corals and encrusting corals.
The dominant species present are:
Lobophytum, Sarcophyton, Sinularia, Acropora, Favites, Montipora, Pocillopora, Palythoa.
|On the sheltered side of the reef crest is the reef flat which may be relatively barren (zones with sandy substrat) or covered with coral. The flat may range from only a few centimeters to a few meters deep, and large parts may be exposed at low tide. The organisms here must be able to withstand intense ultra violet radiation, desiccation, high salinities and elevated water temperature. The substrate is formed of coral rock and loose sand. The waves have been completely broken by the outer-reef zones so the water motion is usually lower.|
Here, currents can be highly variable, from as few as 3-4 cm/sec, on up to about 60 cm/sec. Typically, they may range about 20 cm/sec.
Nutrients can be high here, both from being washed over the reef and from any land-based run-offs. The shallow depth and calm water allow tremendous amounts of light to penetrate, and many corals thrive. This zone is usually prolific with Acropora, Actiniarians (anemones), Asteroids (starfish), Holothurioids (sea cucumbers), Alcyonaceans, and reef fishes.
Inshore from the reef flat is an area known as the back-reef slope. Here, the reef again slopes downward, but is generally no deeper than 60 feet. The diversity of species begins to increase because of the protection afforded by the outer reef crest and the reef flat. More massive and hemispherical growth forms begin to occur, in addition to the many branching and foliaceous growth forms. Spectacular soft coral communities can be found here. The mixing action of currents and waves in these areas is also ideal for turbinate corals. Water motion on the back-reef slope is difficult to quantify, as it will change drastically from reef to reef.
|Moving shoreward, the back-reef slope typically flattens into a sandy or sediment-covered bottom. Reduced water circulation (15 to 25 cm/sec.), warm temperature, the accumulation of sediments, and periods of tidal emmersions during low tide combine to limit coral growth. However, several genera of corals thirve on the soft bottom of the lagoon, including catalaphyllia, goniopora, trachyphyllia, and fungia along with corals that are tolerant of the high levels of nutrients.|
While coral coverage thins, water conditions in the lagoon foster the development of macroalgae, plants and planktonic life.
Still closer to shore, corals cease to occur as waves cross the sand flats that rise up to the shoreline and break on the beach.
Deeper lagoons with heavy sediments or high turbidity have:
Cataphyllia, Euphyllia, Gonipora, Leptoseris, Pachyseris, and Montipora.
The island or patch reefs that rise out of the lagoon floor consist of:
Acropora, Favia, Favites, Galaxea, Goniastraea, Pavona, Pocillopora, Porites, Seriatopora, Stylophora, Tubipora.
Spread over the sandy bottom can be found:
Heliofungia, Fungia, and Herpolitha.
Other corals present:
Heliopora, Sarcophyton, Lobophytum, Xenia, Cespitularia, Sinularia, Rhodactis Palythoa, Zoanthus.
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