Zones of a Coral Reef

1. Zones of a Coral Reef Types of Coral Reefs

2. The Beach The beach zone is derived from sand and gravel produced by the biological and physical erosion of reef limestone. Rocky intertidal areas may retain water in tide pools even at low tide. The organisms that live in tide pools are adapted to withstand intense ultraviolet radiation, desiccation and high salinities.                                             

3. The Reef Flat The reef flat is a rigorous environment. The organisms that live here must be adapted to withstand intense ultraviolet radiation, high salinities, and elevated water temperatures. Although some areas of the reef flat are barren pavement, pockets of branching corals do occur where there is sufficient water flow to prevent over-heating or desiccation.

4. Algal Ridge Encrusting coralline algae flourish where waves break on the reef, forming an elevated bank known as the algal ridge. 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. Corals growing on top of the spurs are stoutly formed and compressed in shape to withstand the extreme force of breaking waves.

5. Submarine Terrace On many reefs there is a terrace below the algal ridge. In this zone we often find stout fire corals which are able to withstand the wave surge and strong currents which characterize this area. Here surgeon fishes and parrot fishes rove over the submarine terrace grazing on benthic algae. As we move down the slope from the submarine terrace, we find increasing coral cover and fishes that prefer areas of rich coral growth, like this regal angelfish.

6. 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.

7. Reef Slope 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 sixty meters. Below sixty meters, we also encounter the first forests of gorgonian corals. These delicate sea fans orient their fronds to maximize contact with the passing currents which carry plankton.

8. Drop Off On many reefs the reef slope may suddenly drop off, forming a nearly vertical wall. At depths below seventy to eighty meters, the overlying water reduces the amount of light so much that few sunlight-dependent hard corals can survive.

9. Drop Off 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

10. Drop Off Although the orange tube coral does possess a limestone skeleton, it lacks zooxanthellae and is a carnivorous plankton feeder. Being independent of sunshine allows this coral to descend to depths of over one thousand meters

11. Fringing Reefs Develop near shore throughout the tropics Primarily on rocky shores – but will grow where ever there is a solid substrate to attach to Grow in a narrow band along the shoreline Longest is near the red sea measuring 2,500 miles

12. Structure of a Fringing Reef

13. Structure of a Fringing Reef Consists of a inner reef flat and a reef slope The reef flat is greatly impacted by sediment, human impact and freshwater runoff because it is so close to shore Bottom is primarily sand and coral rubble Can be vegetated by sea grasses and seaweeds

14. Structure of a Fringing Reef Continued The reef slope is typically very steep and covered with a dense layer of living coral Because of the slope, sediment and coral rubble form a pile at the base of the reef slope Depending on depth this can provide a substrate for the expansion of the kelp This also provides a substrate for turtle grass in some parts of the world

15. Barrier Reefs Form a band parallel to shore like the fringing reefs – but occur at a further distance out to sea Between the shore and the reef is a lagoon Usually containing a sandy bottom Protected from waves by the reef May contain patches of coral The largest barrier reef is the great barrier reef in australia – 1,200 miles long covering an area 80,000 square miles Waves and currents may pile sand onto the reef flats to form islands called keys or cays

18. Structure of a Barrier Reef Consists of a back and fore reef slope and a reef flat The back reef slope is protected from waves by the reef itself but high waves can carry in sediment – this prevents a lot of coral growth The reef flat is a nearly flat platform The greatest coral growth is on the reef crest The crest is covered with sand channels probably caused by wind, waves and currents The steepness of the fore reef slope is dependant on the strength of the waves its exposed to

19. Atolls Defined as a ring of reefs surrounding a lagoon that has an island in the middle Most atolls occur in the West Pacific Ocean Can be found far from land Range in size from less than a mile to 20 miles in diameter

22. Formation of an Atoll Begins when a deep sea volcano builds a volcanic island – like Hawaii Corals create a fringing reef around the island The island then begins to erode and the sediment covers the inner slope of the fringing reef leaving a barrier reef Eventually we end up with an atoll