Coral reefs are accumulated dead skeletal matter built up by carbonate-secreting organisms.
Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals. Similar to a sea anemone but with a hard skeleton. Built only underwater and at shallow depths, coral reefs need a stable foundation upon which to grow, provided by shallow continental shelves and submerged volcanoes.
Southeast Asia accounts for 32.3% of that figure, while the Pacific including Australia accounts for 40.8%. Atlantic and Caribbean coral reefs account for 7.6%.
Although corals exist both in temperate and tropical waters, shallow-water reefs form only in a zone extending from 30° N to 30° S of the equator.
Fringing reef – Grows seaward from the rocky shores of islands and continents.
Barrier Reefs – Parallel shorelines of continents and islands and are separated from land by shallow lagoons.
Atoll Reef - a coral island (or islands) that encircles a lagoon partially or completely.
Volcanic island becomes extinct. As the island and ocean floor subside, coral growth builds a fringing reef.
As the subsidence continues, the fringing reef becomes a larger barrier reef further from the shore with a bigger and deeper lagoon inside. Ultimately, the island sinks below the sea, and the barrier reef becomes an atoll enclosing an open lagoon.
Reef Flat - shallow zone of the back reef located closest to the shore. The flats shallow waters experience wide variations in temperature and salinity and contribute to the accumulation of sediments. These factors, along with occasional exposure during low tides, tend to limit coral growth on this section of the reef.
buttress zone (outer part of fore reef) - coral reef here is characterized by spurs, or buttresses, of coral that jut out from the wall. In between the spurs are deep grooves, or channels, that help to mitigate the force of oncoming waves by channelling spent waves back out to sea where they collide with oncoming waves, thus cancelling out their strength. The grooves also drain sediment off the reef.
Reef wall – zone which receives less sunlight but is abundant with corals because of the reduced wave activity.
Temperature - Coral reefs thrive in water temperatures of 25° and 29° Celsius. Certain corals have evolved to survive outside this temperature range Wave Motions- Water movements help take away waste and bring food.
Nutrients - Corals survive only in clear water, without much sediment, where light can breach the surface of the water. Since there isnt much debris floating around, the water is usually low in nutrients. These waters would not naturally have many animals living in them because of the lack of nutrients, but since corals live here, many thousands of animals survive by eating either the coral, or the animals that eat the coral.
Light- Light is incredibly important for coral reefs. The reason corals are able to survive in such non-nutritious water is because of the zooxanthellae algae produces food for the coral as well as itself. In order to photosynthesize to produce food however, the algae needs light. Without light, this ecosystem cannot exist.
Salinity- Corals tend to live in salty water, surviving in a range of 27-40 parts per thousand (ppt) but preferring 36 ppt. Depth- Corals live in shallow water and are usually found to a depth of 60 meters but deep water corals go far deeper to about 2000 meters.