Geographic Distribution of Coral Reefs
Coral reefs are found only in tropical waters between 30°N and 30°S latitude
Reefs extend further toward the poles along the western sides of ocean basins due to the warming influence of western boundary currents
Reefs form narrower belts close to the equator along the eastern margins of ocean basins due to the cooling influence of eastern boundary currents

Coral Reef Environments
Corals require
bright sunlight (<50-70 m water depths)
limited turbidity
warm water (>18°C annual average temperature)
normal to hypersaline water
strong wave action and surface currents
Reefs will not develop where there is strong influx of clastic sediment or fresh water
Reefs will thrive along the eastern margins of land masses taking advantage of westerly equatorial water flow

Structure of Coral Reefs - I
Coral reefs are built on top of bedrock platforms
Reefs are essentially rigid masses of calcium carbonate derived primarily from colonial type coral skeletons
Reefs build upward with new corals growing on top of older dead coral masses
Reefs may also be built in part from coralline algae, worms, and clams (molluscs) that secrete calcium carbonate shells

Colonial Corals
Corals grow throughout the world's oceans
Corals which grow together as colonial organisms, however, only live in tropical waters and are the primary type of reef corals
Solitary corals may grow anywhere, even in polar waters
Colonial corals need warm water because they grow in symbiosis with small plants called zooanthellae (dinoflagellates) which needs lots of sun light

Structure of Coral Reefs - II
Active reef surfaces are normally within 2-10 m of the ocean surface.
Reefs may be mound-, ridge-, or ring-shaped depending on the type of platform on which the reef develops, wave action, and rate of subsidence of the platform
The outer edges of the reefs where wave action is most intense, is also where coral growth is most intense
In the reef interior, more fragile coral forms will grow

Charles Darwin and Reefs
Darwin studied Pacific and Indian Ocean reefs for three years while he was a naturalist aboard HMS Beagle in the 1830's
He noted that reefs were of three basic types:
fringing reefs (reefs formed along the coastline of islands)
barrier reefs (reefs surrounding an island with a protected lagoon in between)
atolls (reef with lagoons but no central island)
He deduced a single sequence of events which could cause each type to evolve

Coral Reefs and Plate Tectonics
Coral reefs develop along continental margins or the eroded tops of oceanic volcanoes
Oceanic volcanoes are commonly either mid-ocean ridge or island-arc volcanoes
Wave action first develops a platform around each volcano for initial coral reef development
As volcanoes subside during oceanic crustal cooling, coral reefs grow up and over volcanoes until ultimately only the reef is visible (coral atoll)

Ecology of Coral Reefs
Coral Reefs are made up of many interacting organisms. Important group include:
Colonial corals and symbiotic zooanthellae
coralline algae - carbonate component
Snails and clams - carbonate component
Echinoderms - sea urchins, sea cucumbers, starfish, sea lilies
Fish - hundreds of different types
Bacteria and other early decomposers

Nutrients in the Reef Environment
Tropical surface waters are normally depleted in nutrients and so biomass is limited
Biomass in reefs is extremely large - where do their nutrients come from?
Reefs appear to be especially efficient at trapping nutrients and keeping them in the reef environment as they are recycled

Major Reef Systems of the World
Australia - Great Barrier Reef
Indonesia, Micronesia
Caribbean Sea - Great Carbonate Banks
Red Sea
Persian Gulf


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