Marine Science Chapters


Coral Reef Zonation

The windward side of a tropical island with a barrier reef Ironshore
The shoreline of a barrier reef island gets the roughest weather on the windward (east) side (left). The beaches here may be made of old coral reefs (now exposed) called ironshore (right).

Rubble beach Rubble beach close up
Rubble beaches are also common on the windward side of a barrier reef island (left). The rubble (right) may be composed of broken coral and shell.

The windward side of the coral reef is on the eastern side due to the direction the Trade Winds blow. The Trade Winds, dominating on Earth around the equator from about 30 degrees north to 30 degrees south, blow pretty consistently from east to west. Thus the eastern sides of tropical islands have the biggest waves and receive the brunt of most of the storms moved from east to west by the Trades.

Looking out to the windward reef from a tropical island Looking back to the tropical island from the windward reef
Looking out to the windward reef (that may be 100 yards, or more, away) from a tropical island (left). Looking back to the tropical island from the windward reef at low tide (right).

Standing on the windward reef, called an 'algal ridge,' at low tide The algal ridge
Standing on the windward reef, called an 'algal ridge', at low tide (left). The algal ridge (right).

Algal ridge made of coralline algae Coralline algae up close
The algal ridge is made almost entirely of pink encrusting coralline algae (left). Encrusting coralline algae up close (right).

Butresses and grooves in the algal ridge A small coral head on the algal ridge
The algal ridge forms buttresses (relative high parts of the reef) and grooves (channels in the reef) from the waves, storms, and tidal draining of the reef flat. It is mostly covered with the encrusting pink coralline algae (left) but a few hardy corals may be found here (right).

An algal ridge is common on the windward reef. It extends up to the surface of the ocean and forms a protective cap on the coral reef. The algal ridge is composed mostly of coralline algae. Coralline algae secrete large amounts of calcium carbonate in their tissues. Encrusting corallines ( common on algal ridges) adhere this calcium to the reef and cover the coral like a layer of pink cement. The coralline algae can withstand extensive wave pounding and needs a high oxygen content in the water. So, the areas of breaking waves are the best for it. As it grows up to the surface, this windward algal ridge actually serves as a protection for the lagoon behind it. At low tide the top of the ridge may be exposed but only a couple of times a month at extreme low tides. The pink (mauve) areas shown in these pictures show an extensive algal ridge exposed at an extreme low tide.

Octopus caught in the lagoon Spanish dancer slug

Spanish dancer head
Octopus caught in the lagoon of a coral reef (left) may be on the menu for dinner. The Spanish dancer slug gets to be over a foot long (right top and right bottom) and can actually swim by rippling its sides and twisting its body.

The lagoon of a barrier reef
The lagoon of a barrier reef is a protected and interesting place to snorkel.

Turtle grass in the lagoon
Garden eels in the lagoon

Conch snails in the lagoon
The coral reef lagoons may have beds of turtle grass (left) and sandy areas with garden eels (top right) and conch snails (bottom right).

Patch reef Patch reef
Patch reefs often occur in the lagoons (above). These are areas where the solid reef is still at the surface and has not been covered with sediment. Each patch reef is very different.

The lagoon harbors many delicate species unable to survive on the open reef. It may have patches of turtle grass, areas of a sandy bottom (broken fine pieces of coral), and patch reefs (reef areas surrounded by sediment). These lagoons are fun areas for snorklers to explore and may provide hours of safe reef viewing. Lagoons can be the center of an atoll, or as a ring around a crustal island of a barrier reef.

The leeward side of a tropical island with a barrier reef Looking from the beach, across the lagoon, to the leeward reef
The leeward side (left) of tropical islands (west) is protected and is quite different than the windward side. The shorelines of the leeward side are often composed of fine sandy beaches. (right) Looking from a leeward sandy beach, across the lagoon, to the leeward reef (where the waves are breaking).

Sea urchin on coral Sting ray
Sea urchins (left) and sting rays (right) may be found in the leeward lagoon.

Angel fish Hog fish
Angel fish (left) and hog fish (right) may be found in the lagoon. The hog fish is stirring up the sand to expose small crustaceans so it can eat them.

The leeward side of the reef is on the western side and is protected by the algal ridge. In fringing and barrier reef stages it is also protected by the crustal oceanic island. This is the side of the island where the buttress and groove formations are most prevalent. As one descends the side of a typical reef four different areas are usually encountered - the buttress zone, transition zone, the living base, and the dead base.

Buttress zone top Buttress zone near the edge
The top of the buttress zone (left) and the edge of the buttress zone (right) on the leeward side of a coral reef are below the surface of the water.

Buttress zone edge Down the side of the buttress zone
As you swim over the edge of the buttress zone (left) the side of the reef may seem like a wall extending to the depths. Diving down the side of the buttress zone (right) shows many coral reef species.

Crab Sea whips Sponges
Crabs (left), sea whips (center), and sponges (right) are common as you descend down the side of the buttress zone.

The buttress zone is found from low tide to 20 meters deep. It may extend upwards from the outer rim of the lagoon but not to as shallow a depth as the algal ridge. This is an area of some surf, high oxygen, and lots of light. Often some of the hardier coral live here and there may be many buttresses and grooves. Tunnels may be present leading from the lagoon through the buttress zone either from the coral reef growing together across a groove or from the erosive forces of sand from the lagoon. Some of these tunnels provide divers with amazing short journeys from the lagoon, through the inside of the reef, to the open ocean.

Anemone tentacles Sea anemone

The deeper you go on the outside of the coral reef, the darker it gets, and the reef building corals get fewer and fewer. Anemones (left and top right) and snails (bottom right) are some of the creatures seen here.

Nudibranch Bivalves
Nudibranchs (left) and bivalves (right) from a coral reef.

Shrimp Arrow crab
Shrimp (left) and arrow crabs (right) become more common, the deeper you go. These are crustaceans and are scavengers, collecting anything that falls from above as food.

Basket seastar Sea cucumber
Basket seatars (left) and sea cucumbers (right) are also part of the reef community.

The transition zone is found from 20 to 50 meters deep and has a variety of species. Light is decreasing in this area and zooxanthellae do not do as well as farther up in shallower water. The number of filter feeders (like sponges) is increasing in this area and the number of reef building corals is decreasing. There is a good deal of organic material, from the upper reef, that falls here, providing food for filter feeders (like sponges).

Plate coral Plate coral close up
Plate coral (left) becomes more common the deeper you go on coral reefs. Its growth is like a plate, growing with its top surface facing up (toward the sun). All of the polyps are on the top surface (thus getting the maximum amount of light for their zooxanthellae) (right).

Plate coral with orange sponge Incurrent pore of orange sponge
Plate coral, living at depths, may have an orange sponge (Mycale) living on its undersurface (left). Sponges are filter feeders, taking in water through microscopic pores, filtering out the organic material inside their body, and exiting the water through visible excurrent pores (right).

Big vase sponge Vase sponge with crinoid
Many sponges get very large the deeper you descend. The sponge on the left is over four feet high. A smaller vase sponge (right) has a crinoid (feather star) nestled inside.

Burgundy sponge Excurrent pore of burgundy sponge
Burgundy sponge (left) has several visible excurrent pores (right) which are called oscula. The inside of the body of a sponge is a complex of pores, carrying water that exits just under the oscula.

Crusty sponge Excurrent pore of crusty sponge
Crusty sponge (left) also has several visible excurrent pores. You can see many of the exits of the internal pores that are found inside of the body of this sponge in the close-up on the right.

The living base is found from 50 to 150 meters deep and is dominated by sponges. Living species are common here but the light is so reduced that it is hard for the reef building corals to survive. Some corals do live here especially those that grow very flat and plate-like with their polyps on the top facing the surface (to collect the most light for their zooxanthellae).

View to the surface at 150 feet
A view to the surface at 150 feet shows the clear, blue tropical water.

The dead base is found below 150 meters and has few living species as compared to the reef areas closer to the surface. This base is almost solid calcium carbonate from the corallites of the reef building corals - as well as ahermatypic corals, snails, sea urchin spines, and coralline algae. This is the area where the original fringing reef started, when the oceanic island first formed.

Sponges and fish
Sponges and fish are common on coral reefs.

These zones are just guidelines as each reef has its own history and form. Although the zones may be used to define areas on 'classical' coral reefs (fringing, barrier, atoll formations) they can also be applied to continental reefs like Australia's Great Barrier Reef. Australia's Great Barrier Reef is not a classic barrier reef, because it is not found around a sinking island (Australia is continental and not sinking).

Coral reef zonation
Typical coral reef zonation (buttress - to 20 meters, transition - between 20 and 50 meters, living base - from 50 to 150 meters, and dead base - below 150 meters) showing the algal ridge on the windward side (east) of a tropical island.

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(Revised 30 June 2003)
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