Reef zonation
By Mike Kirda (exerpts from the article Reef Zones 6/21/98)

Reef zonation is pretty widely accepted within the scientific community. Corals often have a preference for one area of the reef over another and the zones are rather consistent throughout the world. This is not always true- there are many species with show no such affiliation, however, you can often tell what area of the reef they come from just by looking at their shape or morphology.

Coral Reef lower slope
Depth: 20 to over 100 meters Not really an area which most of our corals come from. Some species are taken down to about 90 feet, but this is not that typical of a collection zone. lighting levels- Maximum PAR readings at 120 meter depth was 2.55 micro-Einsteins/m2/s (0.15% of surface levels). Water temperatures ranged from 22.6 to 30 degrees Celsius. {1} Veron states that on flat substrates, bottlebrush forms of Acropora can be dominant. (CAIP, pg. 31) More likely, they get their nutrition from feeding rather than zooxanthellae at this depth. Often there are fairly large mono-specific stands.  

Coral Reef Upper reef slope
Depth: 2 to 20 meters. The angle of the slope can vary from a gentle slope to a nearly vertical wall. The transect profiles from numerous references give all sorts of slopes and curves- there is tremendous diversity here. On the windward side, the upper slope often features Spur and Groove formations. These formations essentially channel the wave forces into a groove, which often will continue all the way up to the reef crest. Weins mentions that the reef crest can be a very treacherous place to explore as the tops of the groove are often "paved" over by coralline algae. The channeled force apparently can create numerous geyser-like water spouts at the reef crest. Coral diversity is often highest at about 10 or 20 meters. Light levels are high, so do not limit coral growth. As you move up, the monospecific stands are replaced by a high diversity of species, typically not dominated by any one species. Veron notes that if the slope is not too steep, that large stands of elkhorn Acropora may predominate at shallow depths.

Reef fronts (Reef Crest)
Depth: 0 to 2 meters This is the part of the reef that takes the brunt of the wave action. The corals that survive and thrive here are pretty amazing. They survive exposure to tropical sunlight levels that are very near surface level irradiance (2000 E/m2). They can survive wave surge/swell, which can have speeds 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. {2} At low tides, the corals are often completely out of the water. In order to protect themselves from such an extreme environment, the corals tend to have "short and stubby" (digitate) or encrusting growth forms. This enables them to withstand the waves. To survive exposure to the sun/rain, the corals tend to be heavy slimers. Otherwise, the reef crest is dominated by crustose coralline algae.  

Coral Reef Outer reef flat
Depth: 0 to 2 meters This area, the portion of the reef flat just past the reef crest, is generally not highly populated with corals. This area also gets hammered by waves and seems to have a rocky substrate that is usually exposed.  

Coral Reef Inner reef flat
Depth 0.5 to 2 meters. Veron indicates that there is a clear demarcation typically between the inner and outer reef flats as the substrate of the inner ones has lots of loose rubble and some partly consolidated sandy areas. Where the rock is exposed, there is a good coverage of corals. At low tide, the tops of the corals can be partly exposed. As this area is behind the reef crest, it is rather well protected from wave action. Consequently, the water motion is not as great. 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. As the water is so shallow, irradiance levels are very near surface levels, as much as 80 to 95% of surface levels.

Coral Reef outer slope
Depth: 3 to 30 meters In clear waters, this area can be very similar to the off-shore Forereef slope. Often there is a reef crest area as well. The upper portion is dominated by massive colonies, often Porities. Unlike the Forereef slopes, Acropora are much less abundant typically, but often richer in soft corals and other genera. Veron also mentions that species of Pavona, Goniopora, Porites, Turbinaria and other species can form huge.

Coral Reef Inner intertidal mudflat
Depth: 0 to 1 meter Veron explains that for the this area, there is often no clear demarcation between the mangrove swamps and a true intertidal reef flat and goes on to say that you can even find some corals growing on mangrove roots. This area will have to contend with mud and freshwater runoff.

Lagoons
Depth: 3 to 30 meters Often the lagoon is littered with patch reefs that grow in pillar-like fashion from the floor, all the way to the water's surface. In some of the larger lagoons, the number of patch reefs can number in the thousands. Depending on the atoll, the lagoon may be very open to the ocean, or may be completely cut off. For an example, for an atoll with many cays and channels, a complete turnover of water in the lagoon may take 30 to 40 days. Circulation within the lagoon would be primarily tidal or wind driven. During the summer, the wind driven currents provide much of the circulation, and have been measured at 15 to 25 cm/sec. During the winter this form of circulation can drop by 80%. In a large lagoon, the tidal currents in the channels have been measured at up to 400 cm/sec. In the moat areas behind the reef flats, the rocky substrates are often completely eroded, leaving a sandy bottom. Veron puts these types of lagoons at a few meters deep, usually completely surrounded by reef. The bottom of the lagoon may be rocky, or, in deeper areas, covered in sediment. Where there is a hard substrate, the coral growth, protected from wave action, is usually elaborate. Sometimes there is little to no growth, but sometimes the corals grow in or on the sands. Veron mentions huge stands of branching Acropora growing in the sand. In larger lagoonal areas, there may be only patch reefs. Water quality in the lagoons is usually mixed- sometimes the waters are quite turbid, which reduces and alters the light that reaches the corals. Often the conditions within the same lagoon can vary from location to location- one section may have very clear water, while another may be very turbid

Back reef margins
Depth: 0 to 3 meters Veron describes this as areas of reef flat divided by sandy-floored fissures. The area typically receives a decent amount of current, but is protected from wave action. As such, it is home to very active reef growth. Veron notes that the coral composition of this area will change markedly from place to place on the reef.

Back reef slopes
Depth: 0 to 20 meters As this area is usually protected from waves, it can support very lush coral growth. However, it varies according to the slope- typically, the less slope, the higher the coral growth. Near vertical walls may have next to zero growth however. Veron indicates that this area can abound with caves and canyons, and that as you reach the deeper sections, you can often find spectacular soft coral communities. Water motion is difficult to quantify, as it will change drastically from reef to reef depending on the location of local currents. It will be less that what you would find on the windward side.

Outer intertidal mudflat Depth: 1 to 3 meters Corals begin to appear as the water gets deeper. One of the more interesting features are "micro-atolls", consisting often of Porities sp. that grow up to the water’s surface, then outwards. Often the middle section dies off and erodes, leaving a miniature "lagoon". The outer edges will continue to grow up and out. The colonies can get quite massive at many meters across.

References: {1} Reed, J.K., Deepest Distribution of Atlantic Hermatypic Corals discovered in the Bahamas, Proceedings of the Fifth International Coral Reef Congress, Tahiti, 1985, Vol. 6 {2} Riddle, Dana, Water Motion in the Aquarium, Part 2, Aquarium Frontiers, Jan/Feb. 1997., pg. 13. {3} Pichon, M., Dynamic Aspects of Coral Reef Benthic Structures and Zonation, Proceedings of the Fifth International Coral Reef Congress, Tahiti, 1985, Vol. 6