Physical Characteristics of Coral Reefs

Coral animals can be found in several parts of the ocean, but the reef-building types only live in places that meet a narrow range of environmental conditions. Reef-building corals have very specific habitat requirements. They are finicky about the amount of salt in the water, water temperature and depth, movement of currents, and available nutrients.

Salinity refers to the amount of dissolved minerals, or salts, in ocean water. The average salinity of ocean water is 35 parts per thousand, which can be written as 35‰. The symbol ‰ is similar to percent but refers to parts per thousand instead of parts per hundred. Salinity is low in areas where freshwater flows into the ocean, such as near the mouths of rivers. Salinity is high in places where water evaporates from slowmoving or stagnant pools of salt water.

Reef-building corals favor waters where the salinity is about 34 parts per thousand by weight, a little lower than average sea salinity. Coral reefs do not exist in places where freshwater runs into the ocean and drastically reduces the salinity. That is why there are no coral reefs in the part of the Atlantic Ocean where the Amazon River meets the sea, even though other physical factors of the region are ideal.

Although some species of coral can be found in deep, cold ocean waters, stony coral, the type that forms hard skeletons, primarily exist in warm ocean waters. Some reef-building coral species are hardier than others, but water temperatures between 68°F (20°C) and 96.8°F (36°C) are suitable for most, with 75.2°F (24°C) being the ideal. For this reason coral reefs are predominately scattered throughout the tropical and subtropical western Atlantic and Indo-Pacific Oceans between the tropics of Cancer and of Capricorn. These are the areas of the world that experience only small changes in weather between seasons. In the tropical Pacific Ocean, the reefs are widely distributed, but in the western Atlantic Ocean they are confined to the Florida Keys, Bermuda, the Bahamas, the Gulf of Mexico, and areas in the Caribbean Sea.

The number of different coral species that compose a reef is dependent on the ocean in which the reef is located. The Indo-Pacific coral reefs are rich in species diversity, boasting more than 500 different coral species, while the Atlantic Ocean reefs are made up of between 60 and 70 coral species. Scientists are not sure why there is such a difference in species diversity in the two locations, but they suspect that the most recent ice age was more damaging to the Atlantic Ocean reefs than to others.

Temperature affects corals in several ways. The coral animals constantly convert dissolved carbon dioxide and calcium into calcium carbonate, a compound that forms their skeletons. In warm water, calcium carbonate reaches saturation levels very quickly. At saturation, a dissolved compound precipitates, changing from a dissolved form to a solid one. The ability to convert dissolved calcium carbonate to the solid form helps corals create plenty of skeletal material. On the other extreme, if water temperatures get too high, the consequences are disastrous. Coral are unable to create any calcium carbonate to build or repair skeletons. In addition, corals cannot reproduce in water that is too warm.

Because reef-building corals form important relationships with microscopic green organisms, they grow best if they receive plenty of sunlight. Sunlight does not penetrate water deeper than 150 feet (about 46 m), so corals cannot grow below that depth.

In addition, coral reefs are very sensitive to the amount of dissolved nutrients in the water. Coral animals thrive in nutrientpoor conditions, because high levels of nutrients can stimulate the growth of tiny marine plants. Overgrowth of these water plants, a phenomenon called algal bloom, can make the water dark and murky, preventing corals’ resident algae from receiving enough light. Nutrient-poor, or oligotropic, waters, which are characteristically blue in color, are typical of coral reefs.

"Red Tide" alga bloom. Source from:
"Red Tide" alga bloom. Source from:

The distribution and growth of reefs in the ocean is also influenced by the flow of ocean currents. Clear, moving water is extremely important to the survival of reef-building coral. Moving water carries food, nutrients, and oxygen to the living coral animals. Reefs are rarely found where there are large amounts of suspended matter because debris, silt, or other particulates can smother the fragile coral animals.