Coral Reef Formation
- Corals are the result of a symbiotic relationship between an animal known as a coral polyp, and a zooxanthellae algae
- The polyp provides shelter and protection
- The algae carry out photosynthesis and produce carbon compounds such as carbohydrates
- Some species of coral polyps secrete calcium carbonate which builds up in layers to form a hard outer structure; over time these deposits from thousands of individuals can form a coral reef
CC BY 2.0, via Wikimedia Commons
Some coral species secrete layers of calcium carbonate that forms the hard structure of coral reefs
Distribution of coral reefs
- The distribution of reef-building corals is limited to the topics and subtropics
- Non reef-building species can be found in other regions
- The formation of coral reefs is limited by abiotic factors, including:
- Water depth
- pH
- Salinity (salt concentration)
- Water clarity
- Temperature
- Coral has a narrow range of tolerance for all of these abiotic factors, resulting in its limited distribution
Coral reef distribution map
The distribution of reef building corals is limited by abiotic factors
Abiotic factors affecting coral reef formation table
Abiotic factor | Effect on reef formation |
Water depth | Corals can only grow at shallow depths where light can penetrate the water at high enough levels for the zooxanthellae to photosynthesise. |
pH |
The hard outer layer secreted by coral polyps is made from calcium carbonate, which dissolves when the pH is too low. Corals need carbonate ions to build calcium carbonate. The H+ ions that are present at lower pH levels combine with carbonate ions to form hydrogen carbonate ions, reducing the availability of carbonate ions for reef-building. Increased carbon dioxide released from the burning of fossil fuels dissolves in the oceans and lowers the pH pf the water, reducing coral growth. |
Salinity |
Since corals are marine animals they need salty water, within a 32-42 % range, to survive. Freshwater run-off from land can reduce salt concentrations and limit coral growth. |
Water clarity |
Water clarity must be good for light to penetrate through the water. Sediment from land run-off, and water pollution, can reduce water clarity and limit coral growth. |
Temperature |
Corals have a range of tolerance of roughly 20-28 °C, though they grow best in water temperatures above 23 °C. While they can withstand short periods of higher temperatures, rising sea temperatures causes the polyps to expel their algae symbionts, leading to coral bleaching. |