Coral and Coral Reef

Although corals live in nearly all marine environments, hermatypic corals thrive in a rather narrow set of environmental conditions. These limitations also restrict the geographic distribution of well-developed coral reef tracts.

Coral reefs typically occur in water depths less than 190–230 ft (60–70 m) and maximum growth rates occur at depths less than 60 ft (18 m). This is because the corals, or rather their symbiotic zooxanthellae, depend on light for growth. The algae need access to light to accomplish their photosynthesis. Too much sediment in the water also causes problems, by limiting light penetration or suffocating organisms, and thereby slowing reef Kayangel atoll, Belau. Photograph by Douglas Faulkner. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. growth. Consequently, the amount of light and the clarity and depth of the water are important influences on the development of coral reefs.

Corals thrive in oligotrophic water; that is, water with low concentrations of nutrients such as phosphate, ammonium, and nitrate. Currents and wave activity help supply the continuous but low concentrations of nutrients that corals and algae require for survival, while also removing waste materials.

Water temperature is also an important environmental influence on the growth of stony corals. Typically, a water temperature of 74–78°F (23–26°C) is most conducive to coral growth, and temperatures must generally remain above 67°F (19°C) throughout the year. As reviewed below, global warming is having a disastrous impact on coral reefs worldwide by causing water temperatures to transgress these narrow bounds.

Stony corals also prefer marine waters with stable salinity. The salt concentration of the water must range between 35 and 38 parts per thousand, and the concentration of oxygen must remain high. Another important factor is the need for continuous submersion under water, although some corals can survive temporary exposure during low tide.