Coral Reefs
Definition
- Tropical communities built from calcium carbonate deposits from several different groups of cnidarians
- Examples
- Scleractinian coral
- Soft coral
- Blue coral
- Organ-pipe coral
- Gorgonian coral - pictured at right!
- Black coral
Structural Organisms
- Polyp cnidarians and zooxanthellae (dinoflagellate algae) produce CaCO3 skeletons
- Mutualistic relationship
- cnidarians protect algae and provide CO2, nitrogen, phosphorus
- Zooxanthellae help provide food and CaCO3 skeleton
- Coral polyps are huge colonies of individual organisms attached by a thin sheet of tissue
- Coralline algae also contribute to reef growth and deposit large amounts of CaCO3 and cement sediments together
Coral Polyps and Zooxanthellae
Conditions for Reef Growth
- Shallow water light penetration required for zooxanthellae
- Warm water at least 20°C (68°F), but no more than 30-35°C because too much heat results in bleaching (coral expels zooxanthellae, which gives the coral its color)
- Salinity sensitive to reduce salinity, do not do well near rivers
- Sediment vulnerable to high levels of sediment, damaged by human activities like mining, dredging, logging that increase flow of sediment into reef
- Pollution very sensitive to low concentrations of chemicals (pesticides, fertilizers), change in nutrient distribution increases seaweed population and corals cannot compete well for space and light
Kinds of Reefs
- Fringing Reefs
- Develop near rocky shorelines in a narrow band inner reef flat, outer reef slope
- Especially vulnerable to sediment, freshwater runoff, and human disturbance
- Most diversity and growth on reef crest upper edge of reef slope good wave circulation, farther from land

- Barrier reefs
- Separated from shore by a deep lagoon back reef slope, reef flat, fore reef slope
- Small sand islands (cays, or keys) may form on top of reef crest
- Richest growth on outer reef crest less sediment backwash

- Atolls
- Ring of reefs and cays surrounding a shallow central lagoon inner back slope, reef flat, outer fore slope
- Can occur very far from land and grow from depths of thousands of meters
- Reef crests strongly influenced by wind and wave action
- Darwins second most famous theory atolls start when a deep sea volcano erupts and forms a volcanic island, corals from a fringing reef, then the island sinks
Reef Ecology
- Nutrient recycling helps organisms survive in tropical waters that are usually nutrient-poor
- High rates of nitrogen fixation by cyanobacteria, digestion of zooplankton increases nutrient supply
- Zooxanthellae and turf algae provide the most photosynthetic activity
- Competition for anchoring space
- fast growers block out sunlight
- some attack and digest neighbors
- spiky surfaces protect from predators
- Competition for food somewhat alleviated by occupation in different ecological niches
- Predation and grazing control population and type of coral and algae present in reefs
Random Pictures from our Gallery!