Coral Reefs

Tropical communities built from calcium carbonate deposits from several different groups of cnidarians
Examples

Polyp cnidarians and zooxanthellae (dinoflagellate algae) produce CaCO₃ skeletons

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

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

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

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

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
Darwin’s 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

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

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