Why are coral reefs found only in clear tropical waters?
Coral reef is independent ecosystem. Both algae and coral polyps build coral reef.
Coral reef consists of polyps and algae. Both unicellular algae and filamentous algae are present in polyps. In dead skeleton of coral there is filamentous algae. While unicellular algae is in the body of polyps. The polyp releases carbon dioxide . The carbon dioxide is used by algae . The complete recycling of matter takes place in corals. For photosynthesis of algae it needs light and clear water.
This is the reason why in turbid water corals are unable to survive.