Coral reefs exist in tropical areas worldwide (Figure 8). In general, reefs are abundant in areas with shallow coastlines and clear, warm water where riverine discharge of sediments is low. Large coral reefs are rarely found in areas above 29° latitude where ocean temperatures fall below 18 °C for extended periods as this slows coral growth and their capacity to build large reefs; however, zooxanthellate corals can be found in areas with water temperatures as low as 11 °C. In addition, herbivory is often less intense in cooler waters meaning that seaweeds are more abundant in temperate areas and that competition between corals and seaweeds is more intense. The combination of cooler water temperatures and more intense competition with abundant seaweeds likely interact to limit the latitudinal range of large coral reefs. However, when the physical and
ecological criteria are met, the results can be phenomenal. For example, the most biologically diverse reefs occur in the tropical Indo-Pacific in the areas around Indonesia and the Philippines and house over 550 species of coral and thousands of species of fish. The Great Barrier Reef off northeastern Australia is the largest reef in the world with more than 2800 individual reefs occupying over 1800 km of the Australian coastline and can be seen from outer space.
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