Coral Reef Structure

Coral Reef Structure

The Bay Islands are a series of underwater mountain peaks surrounded by deep water. Consequently, the dive sites are extremely close to the shore, sometimes even within swimming distance of it.

The basic structure of the reef formation is fairly uniform and varies little from area to area. Its general features are schematically described below.

Understanding the reef formation will enable you to find your way around more easily, thus enhancing your diving enjoyment. In general it is possible to divide the reef into six major areas, which I will label as zones.

Zone 1: Reef Flat The first region slopes almost imperceptibly down from the shore-line. This sandy expanse is frequently covered with turtle grass and usually ends in a fringing reef made up of elkhorn or staghorn corals with small coral heads. The fringing reef protects the lagoon from the wave action of the ocean.  
Zone 2: Ridge The second area starts off as a moderately inclined sand plain covered in clusters of coral heads, called a patch reef or algal ridge. Encrusting coralline algae flourish where waves break on the reef, forming an elevated bank. These corals are short & stout to survive the constant onslaught of wave action. As water from the waves returns seaward, its load of suspended sand and gravel scours a series of narrow ridges. The alternating ridges and valleys are known as spurs and grooves. Elsewhere the coral heads are more pronounced and form a labyrinth of caves and canyons like Valley of the Kings. In some locations, the second and third reef zones merge into one another, and the coral heads form large pinnacles which are actually part of the ceiling of the wall.  
Zone 3: Terrace On many reefs there is a terrace below the algal ridge. In this zone we often find stout brain corals, and seafans which are able to withstand the wave surge and strong currents which characterize this area. Parrot fish & surgeon fish populate this area, feeding on alge. Down the slope from the submarine terrace, there is increasing coral cover and fishes that prefer areas of rich coral growth, like angelfish, and a wide variety of smaller fish that depend on the nooks and crannies of the coral to hide.
Zone 4: Sub-Terrace Some outer reefs have flattened terraces that interrupt the gradient of the reef slope. It is here, protected from the wave surge overhead yet still exposed to abundant sunlight, that we find lush gardens of corals with expansive body forms designed to maximize their exposure to sunlight.
Zone 5: Reef Wall The Reef Slope is also known as the wall. Descending the slope, we find corals continue to expand horizontally in shape in order to capture as much sunlight as possible. Branching forms found in shallow water are largely replaced by plate like forms below a depth of 180 ft. The reef wall is where most deep dives at the beginning of the day are made. Spur & groove systems and their resulting tunnels and channels will extend to about this point.
Zone 6: Drop-off On many reefs the reef slope may suddenly drop off, forming a nearly vertical wall. At depths below 180 feet, the overlying water reduces the amount of light so much that few sunlight-dependent hard corals can survive. In these dim, quiet waters the delicate corals not dependent upon photosynthesis become common. Free from the competition of zooxanthellae-based corals which dominate the waters above, fragile lace corals send out thin branches to trap plankton.

A day of diving on Roatan typically includes a deep first dive, followed by a short rest and then a shallow 'drop-off dive' at a wall close to shore, where the diver can control their depth and maintain a shallow profile, and swim back to the beach at their own pace.

In order to more fully enjoy your time on the reef, it is helpful to learn some of the terms that you will encounter:

A pinnacle is frequently encountered feature consisting of a large block of coral which rears up in the shape of a tower. The pinnacle is more prominent than the surrounding spur to which it is usually connected, although you will sometimes come across large, isolated pinnacles.

Spurs are long strips or fingers of rock and coral separated by deep sandy channels, known as canyons or grooves. The spurs are sometimes so close to one another that they meet above the grooves, and take on the appearance of a tunnel. Tunnels are also termed crevices, ravines or swim-through's. The term chimney, on the other hand, refers to a completely sealed horizontal cave, or a partly closed hallow canyon which begins at the wall lip, or edge, and continues halfway down the length of the wall, extending to the sea floor at times. The wall lip lies at an average of 45-60ft, and dives can be planned to 130 ft. The wall or drop-off is actually at the end of the shelf, the point at which the really deep water begins. Sometimes it takes the form of a very steep sand slope, while elsewhere it is a steep wall fissured by caves and canyons where protruding pinnacles are very much in evidence. In certain locations, the wall is obliterated behind a veritable waterfall of sand, originating in the sand plain above. This spectacular river of sand is called a sand chute.

Most dives are similar in general outline. Set off from the mooring, which is generally located some 30-45ft above the sand plain. Continue through the grooves and canyons to the wall (even if your boat is located above the wall itself, depending on such factors as wind and current). You should try to take a different groove on the way back to the mooring rope, which you ascend to perform a safety stop. Some dive boats are equipped with a safety bar which is lowered into the water to a depth of 10ft to let you rest comfortably during the safety stop. Some larger dive boats have bars at 15 or 20 ft as well. A regulator and extra tank or a hooka hose are frequently attached to the bar, so the diver can still make a safety stop and not worry about low air in their own tank.

Remember one important principle: most spur and groove systems point in the general direction of the shore. They resemble fingers stretching from dry land out to sea, from shallow to deep water and the drop-off. Should you lose your group in the vicinity of the wall, do not ascend to the surface to decide on the direction home. Simply enter a groove and swim along it or just above it. In most cases, it will lead you to shallow water (with better visibility) where you should be able to locate the mooring rope. Remember not to touch the coral at any time while swimming close to it.

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Drew Thompson 2000

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