Coral and Coral Reef

Reefs tend to develop a definite depth profile and associated coral zonation under the influence of constant wave activity. This results from the decrease in wave energy with water depth. The reef crest is the shallowest reef area and subject to the highest wave energy; here coral and algae encrust the substrate to avoid being broken and swept away. The reef crest is located at the top of the seaward reef slope and may be exposed at low tide. Waves and tides cut channels across the reef crest. As tides rise and fall, water moves back and forth through these channels between the open sea and the lagoon.

Wave and storm energy are important controls on the character of the reef crest. Coralline algae tend to dominate reef crests if hurricanes are frequent and average daily wave conditions are rough. Grazing fish, which would normally consume the algae, are deterred by the consistent high wave energy. In areas with infrequent storms and calmer daily wave conditions, encrusting corals or robust branching corals tend to inhabit reef crests.

Moving down the seaward reef slope, the reef front lies just below the reef crest. Corals here are diverse and show the greatest range of forms. At the top of the slope, wave energy is high and coral forms are usually encrusting to massive, such as brain corals. Further down the slope, in deeper water, massive corals dominate, then give way to delicate branching corals as wave energy decreases with depth. Finally, at the base of the reef front, plate-like corals take advantage of the low wave energy. By orienting their flat, upper surface toward the sun, they attain maximum exposure to the diminished light of the deep reef. Further downslope, the fore reef consists of limestone boulders, coral branches and smaller sediments, all transported from above, as well as sponges, soft corals and algae thriving in place.

Shoreward of the reef crest lies the shallow reef flat. Reef rubble occurs here in high-energy reefs. In lower energy settings, carbonate sand may be present. These sediments are supplied by storm waves breaking on the reef crest. Even closer to shore is the back reef area, where fine-grained sediment inhibits reef growth; however, scattered stubby, branching or low, knobby corals usually develop in water depths of 3–4 ft (1–1.3 m).

Beyond the back reef, the water begins to deepen again—to as much as 100 ft (30 m) or more—within the lagoon (i.e., water between the reef and the shore, or completely surrounded by the reef). Here the sea floor is generally protected by the reef from significant wave agitation, so fine-grained sediments compose the lagoon floor. Hardy corals occur in scattered clusters, known as patch reefs.