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Coral Zones in the Reef Tank

Mastering this neglected dimension of reefkeeping is vital in properly placing corals and ensuring their survivability in any reef tank.

By Richard Harker |

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We know that the keys to successful reefkeeping include strong light, adequate water motion and good water quality. Yet sometimes corals that do well in one tank will do poorly in another tank. A coral may develop a beautiful shape in your tank, and it may look entirely different in a friend’s aquarium. Coral zonation may explain what is happening. The placement of your corals plays an important role in creating a thriving reef tank full of healthy corals.

Natural coral reefs around the world are like snowflakes — no two are alike. Even reefs in close proximity develop differently. Broadly speaking, however, coral reefs can be categorized into several types, which are called zones.
The way a coral reef matures, how it develops and which animals thrive on it differ by zones. Zones, in turn, are differentiated by their relationship with the open water. Exposure to storms, strong winds and strong water currents determine the type of corals that can grow, the size of the reef and the likelihood that it will grow into a major reef. Coral reefs exposed to the worst storms evolve differently from coral reefs protected from storms.

Euphylia ancora
Delicate large polyp stony corals such as this Euphylia ancora, grow in calm lagoons. Other corals that dp well in lagoonal zones include elegance and plate corals. Photo by Richard Harker

On the natural reef, corals begin life as planulae drifting in the water currents flowing through the reef. Coral planulae have limited control over where they settle as they drift over the substrate. If they drift near a patch of live rock devoid of algae, they will attach to the rock and start growing.

All types of corals settle out throughout the various coral reef zones. The zones do not determine where corals begin to grow. The zones determine whether they will survive. This is why zonation is so important for hobbyists. A small coral fragment can probably survive in virtually any reef tank. However, as the small fragment grows, it will thrive in some reef tanks and die in others.

The Four Zones of the Coral Reef

The four zones of coral growth are the fore reef, the back reef, patch reef and lagoon. If we look down at a tropical island from the air, we can clearly see the zones.
Fore reef. The fore reef is the area farthest from shore where waves break. It is the zone that protects tropical islands from storm damage. This area is subjected to repeated storms and strong currents. To survive, the fore reef has to be sturdy — a fortress, well-strengthened by dense and well-cemented coral skeletons.

The fore reef is almost entirely made up of coral skeletons cemented by coraline algae. Sand does not accumulate around the fore reef because currents carry it away as soon as it develops.

patch reef
If they understand the reef zones, today's aquarists can create a realistic patch reef like this display. Photo by Richard Harker
The corals that survive on the fore reef are predominately corals that can take a pounding and survive. Boulder and brain corals, along with very stout Acropora, are the few corals that can survive in this hostile environment. In the Pacific, this might mean Acropora humilis or A. gemmifera, and Pocillopora eydouxi; in the Caribbean, these strong corals include staghorn (A. palmata) and boulder corals (Montastrea annularis).

Damage to fore reefs occurs regularly. Even stout corals well designed to handle the force of a storm are frequently damaged, so most corals in this zone remain small. They rarely reach the size of corals farther toward the shore.
Back reef. As we move closer to land, the next coral zone is the back reef. Because the potentially damaging surge and strong currents are considerably lower in this area, a larger variety of corals can survive in this zone. There is greater diversity, and the corals can grow larger. The back reef is the typical coral garden we envision when we build reef tanks.

Great Barrier Reef
In an aerial view of a reef, it's sometiomes possible to see the four different coral zones: the fore reef, back reef, patch reef, and lagoon. This is the Great Barrier Reef. Photo by Richard Harker/span>
We can find pretty much any type of coral in this zone. Slow-growing sturdy fore-reef Acropora tend to get crowded out by faster-growing back-reef corals, such as Acropora microphthalma and A. formosa, or Seriatopora spp. and Pocillopora damicornis; but a little of everything grows in this zone.

Patch reef. As we move toward the shore, we enter the patch reef zone. The name says it all. The size of coral gardens declines, replaced by small outcroppings of corals surrounded by sand. The sand is made up of ground coral skeletons, and shells carried by storms and strong currents from the fore reef to calmer patch reef areas.
Patch reefs are small because there is less solid rock for planulae to settle. Tidal currents and storms suspend sand in the water, ultimately smothering larger corals when it settles.

Lagoon. The final zone is the lagoon. Lagoons are where we find the calmest waters. The only water motion in these areas are generally tidal currents that periodically flow across the outer reef zones and reach the lagoon.
Lagoons are often characterized by nutrient-rich turbid water. As a result, many stony corals do poorly. Poor water flow found in lagoons impedes coral respiration, slowing growth. In contrast, algae thrive in lagoons, often over-growing corals.

For these reasons, this zone is generally a mix of algae and fleshy large polyp corals. Euphyllialike hammer corals and elegance corals (Catalaphyllia spp.) prefer the sandy substrate and nutrient-rich waters of lagoons.

Zonation in the Reef Tank

While natural reef zones are differentiated by their distance from the open ocean, our reef tank zones are generally differentiated by their distance from the top of the tank. In a sense, a reef tank is a natural coral reef tilted on its side, with the fore reef at the top of the tank, and the lagoonal and patch reef at the bottom of the tank.
The primary reason reef tanks turn out like this is because of the way most hobbyists construct their tanks. We typically use powerheads and recirculating loops that return water near the surface of the water. As a result, the majority of water motion is at the top of the tank. That means the highest energy zone is at the top of the tank. This is where sturdy fore-reef corals should be placed.

The structure of the tank breaks up water flow and dissipates a great deal of the water motion energy, so the middle of the tank is like the back reef. More delicate corals that might not survive in the strong currents at the top of the tank can grow here. That would include branching Acropora and more delicate bottlebrush corals, such as Acropora bushyensis.
One can estimate water motion by what happens to sand placed in the tank. If sand is constantly suspended and only collects in corners and protected areas, water energy approaches fore-reef levels. If the tank can maintain a thick sandbed, water energy is low like that found in a lagoonal patch reef.

Hobbyists with sandbeds in their tanks should focus on lagoonal soft corals and large polyp corals on the bottom of the tank. Some lagoonal Acropora can also grow in this zone.
The one area where natural reef zonation differs from reef tank zonation is light. Light levels do not vary much from one zone to the next on the natural reef. The most dense coral growth is always near the surface of the water where corals receive intense light.

Because we light our reef tanks from above, light levels decline rapidly toward the lower zones of the tank. The decline can be minimized by minimizing the height of the tank, using little rock and constructing the structure with a gradual slope. This is why the best tank shape for a reef tank has a large footprint and short height.

Designing a Reef Tank Around Zonation

Zonation determines which corals survive, and it also determines how corals grow. Corals subjected to strong water motion tend to develop strong structures that can withstand the energy. The same coral moved into a tank with much weaker water motion will grow faster with a weaker structure. This morphological adaptability of corals explains why the same coral can grow differently in different tanks.

We often construct a reef tank and then stock it with whatever corals we can find or afford. A better approach is to begin by deciding which corals you want to keep. Do you prefer soft corals and fleshy large polyp corals? Do you favor small polyp stony corals typically found in stronger water energy zones?

Designing a reef tank with specific animals in mind will create a healthier environment for the tank’s inhabitants and result in less frustration. Let’s say you prefer large polyp corals, and your goal is to create a patch reef and lagoonal reef tank. Large polyp corals grow in sand unattached to rock. Create a tank with a little rock, a lot of sand and moderate water motion. Lay the large polyp corals in the sand. Attach small cuttings of lagoonal small polyp stony corals to a few rocks, and keep water motion low enough to prevent sand from accumulating on the large polyp corals.

If your goal is to create a reef tank that mimics the back reef, you have to design the tank differently. The back reef has small patches of sand in protected areas, but is mostly rock and gravely bits of skeleton. You’ll want to create strong water motion — strong enough to move around and suspend sand. If you want to grow corals throughout the tank, create water motion at the bottom of the tank, not just at the top. This can be done by returning water from the sump at the bottom of the tank or creating closed loops for circulation, moving water from the top of the tank to the bottom.
We hobbyists tend to be too anxious to fill a new tank with mature coral colonies. While adding large corals to a new tank makes the tank more complete, it is fraught with potential problems. The plastic nature of small coral colonies means that a small cutting of a coral is more likely to adapt and thrive in a new tank, while a larger colony of the same coral will more likely fail to adapt and perish. Regardless of the zone you wish to replicate, start with coral cuttings and very small colonies.


Richard Harker has spoken at numerous meetings, including MACNA and the Western Marine Conference. He has studied and photographed coral reefs at Australia’s Great Barrier Reef, Fiji and the Solomon Islands, as well as other locations.

 

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