During the day, it may reach 250 per cent of saturation because of the production of oxygen by algae in the reef structure. Likewise, the tremendous amount of CO2 produced by the metabolism can be carried off before it becomes too high.
Reefs can be found wherever a suitable substrate exists in the lighted waters of the tropics, away from areas of continental sediments which would tend to bury the reef in mud or cold upwelling’s which would tend to lower the temperature too much.
They may form barrier reefs (e.g., barrier reef in Australia which contains canal or lagoon between them and coast) adjacent to continents ; atolls or ring-shaped or horse-shoe-shaped islands on the top of submarine heights, or fringing reefs, as in Mandapam, Appa Islands and Hare Islands all modern reefs, the main reef structure is formed by madreporarian corals of following familes-Poritidae (Porites minicoinsis, Porites mannarensis), Acroporitidae (Acropora intermedia), Astraeidae and Milliporidae. Secretions of calcareous algae go into the building up of reefs.
Equally important are calcareous Bryozoa, lamellibranches and gastropod mollusks and Foraminifera, which become significant elements in the total reef structure. Although the name ‘coral-reef’ implies that corals should be the most abundant organisms, algae of many sorts comprise over three times the biomass of the corals. In addition, the calcareous algae are not only producers; their skeletons act to cement the entire reef to make it more resistant to the pounding surf.
Coral reefs normally exist in water with mean temperature 20-5°C, but they can withstand very high temperatures. The growth of coral reef is influenced by salinity and reduced tides. Moderate water movement around the coral reefs help in the circulation of food.