Essentials of Oceanography
H.V. Thurman, 1993, Essentials of Oceanography, 4th Edition, Macmillan Publishing, N.Y., p., 336-341.
List of Figures

CORAL REEFS
Although corals are found throughout the ocean, coral accumulations that might he classified as reefs are restricted to the warmer-water regions where the average monthly temperature exceeds 18°C (64°F) throughout the year (figure 15-19). Such temperature conditions are found primarily between the tropics, although reefs grow to latitudes approaching 35°N and S on the western margins of ocean basins where warm-water masses move into the high-latitude areas and raise average temperatures. Also shown in figure 15-19 is the greater diversity of reef-building corals on the western side of ocean basins. More than 50 genera of corals are found in a broad area of the western Pacific Ocean and a narrow belt of the western Indian Ocean. Fewer than 30 genera are found in the Atlantic Ocean with the greatest diversity occurring in the Caribbean Sea. The explanation for this pattern of diversity may be related to past ocean current patterns, but such an explanation is controversial.

Because of changes in wave energy, salinity, water depth, temperature, and other less obvious factors, there is a well-developed vertical and horizontal zonation of the reef slope. These zones are readily identified by the assemblages of plant and animal life found in and near them (figure 15-20).

Not only coral, but algae, mollusks, and foraminifers make important contributions to the reef structure. Reef-building corals are hermatypic. They have a mutualistic relationship with the algae zooxanthellae that live within the tissue of the coral polyp. Not only reef-building corals but other reef animals have a similar symbiotic relationship with algae. These heterotrophs that derive part of their nutrition from algal symbionts are called mixotrophs. They include corals, foraminifers, sponges, and mollusks (figure 15-21). Because light is essential for algal photosynthesis, reef-building corals are restricted to clear, shallow bottom waters. The algae contribute not only to nourishing the coral, but could contribute to the calcification capability of corals by extracting carbon dioxide from the animals' body fluids. This would increase the concentration of the carbonate ion needed for the precipitation of calcium carbonate. The corals contribute to the relationship by providing a supply of nutrients to the zooxanthellae. This exchange between algal symbionts and corals, as well as other mixotrophs, supports high levels of biological productivity within the reef community even though the level of nutrient concentration in the surrounding water is low.

Reef growth also requires that the water have a relatively normal salinity and that it be free from particulate matter. Therefore, we see very little coral reef growth near the mouths of rivers that lower the salinity and carry large quantities of suspended material that would choke the reef colony.

Coral reefs actually contain up to three times as much plant as animal biomass. The zooxanthellae account for less than 5 percent of the reef's plant mass; most of the rest is filamentous green algae. However, zooxanthellae account for up to 75 percent of the biomass of reef-building corals and provide the coral with up to 90 percent of its nutrition.

The greatest depth to which active coral growth extends is 150 m (492 ft), below which there is not enough sunlight to support hermatypic mutualism. Water motions are not great at these depths, so relatively delicate plate corals can live in the outer slope of the reef from 150 m (492 ft) up to about 50 m (164 ft), where light intensity exceeds 4 percent of the surface intensity (figure 15-20). From 50 m to about 20 m (66 ft), the strength of water motion from breaking waves increases on the side of the reef facing into the prevailing current flow. Correspondingly, the mass of coral growth and the strength of the coral structure supporting it increase toward the top of this zone, where light intensity exceeds 20 percent of surface value. Head corals are dominant. The buttress zone extends from 20 m to the low-tide line; within it more-massive branching corals, Acropora, and encrusting algae such as the red alga, Lithothamnion, withstand the crashing waves. Light intensity exceeds 60 percent of surface intensity in the buttress zone. The waves cut surge channels across the algal ridge, which is inhabited by only a few animals-such as snails, limpets, and the slate pencil urchin, Heterocentrotus - that can withstand the constant beating of the surf. The surge channels extend down the reef slope as debris channels that carry the products of wave erosion. Small reef fish find protection from larger predators such as sharks, barracuda, and jacks in the debris grooves extending between the buttress ridges. The reef flat extends across the reef beyond the lagoons of atolls and barrier reefs. Here the reef may be under a few centimeters to a few meters of water at low tide. A variety of beautiful reef fish inhabit this shallow water. The sand of reef debris and foraminifer tests fills in the deeper holes and provides a home for sea cucumbers, worms, and a variety of mollusks. In the protected water behind the Lithothamnion ridge lagoonal reefs may form, where species of Porites and Acropora grow into beautiful large colonies. Gorgonian coral, anemones, crustaceans, mollusks, and echinoderms of great variety also will be found in the lagoon reef (figure 15-22).

An obvious example of commensalism is the behavior of the shrimpfish, which swims head down among the long slender spines of the reef sea urchins. The urchin's spines are a significant deterrent to any predator, and the sea urchin is neither hindered nor aided by the presence of the little fish. The clown fish receives a similar protection by swimming among the tentacles of two species of sea anemones. This relationship is believed to be mutual since the anemones benefit by the clown fish serving as bait to draw other fish within reach of anemone tentacles and actually carrying food to them (figure 15-23).

Coral Reefs and Nutrient Levels.
When human populations increase in lands adjacent to coral reefs, the reefs deteriorate. There are many aspects of human behavior that would obviously damage the reef, but one of the more subtle is the inevitable increase in the nutrient levels of the reef waters. As nutrient levels increase in reef waters, the dominant benthic community changes. At low nutrient levels the hermatypic corals and other reef animals that contain algal symbionts thrive. As nutrient levels in the water increase, conditions favor development of fleshy benthic plants at moderate nutrient levels and suspension feeders at high nutrient levels. At high nutrient levels, phytoplankton mass exceeds the benthic plant mass, and benthic populations tied to the phytoplankton food web dominate. The clarity of water is reduced by increased phytoplankton biomass, and the fast-growing members of the phytoplankton-based ecosystem destroy the reef structure by overgrowing the slow-growing coral and by bioerosion. Bioerosion by sea urchins and sponges is particularly effective in damaging the reef.

The Crown-of-Thorns Phenomenon.
Since 1962 the crown-of-thorns sea star, Acanthaster planci has caused destruction of living coral on many reefs throughout the western Pacific Ocean (figure 15-24). Some investigators believe this is a modern phenomenon brought about by the activities of humans. However, there is little evidence to point to such a cause. A 1989 study of the Great Barrier Reef has indicated that during the past 8,000 years A planci has been even more abundant on the reefs studied than it is today. If this is true, the sea star may be an integral part of the reef ecology in this region rather than a destructive upstart taking advantage of human actions that have modified the reef in some way favorable to its proliferation.

Bleaching of Coral Reef Communities.
The bleaching or loss of color by coral reef organisms has occurred on a local basis numerous times in the past. However, a mass mortality of at least 70 percent of the corals along the Pacific Central American coast occurred as a result of a bleaching episode associated with the severe El Niño of 1982-83. Although the cause is not known for certain, this eastern Pacific bleaching may have been caused by the increased water temperatures associated with the El Niño event. Whatever the cause, it will take many years for these reefs to recover. Two species of Panamanian coral became extinct during this event.

One thing is clear, the direct cause of the bleaching is the expulsion of symbiont algae, zooxanthellae. Essentially all reef-building corals and some other reef mixotrophs are nourished by these algae that live within their tissue. The loss of this source of nourishment can be life threatening to the reef dwellers.

Figure 15-25 shows the bleaching of a round starlet coral, Siderastrea siderea, on Enrique Reef, Puerto Rico. The photograph was taken in November 1987. Normally a rusty brown color, this coral has been bleached on its lower left half.

The broad scope of human-induced changes throughout Earth; increased concentrations of green-house gases in the lower atmosphere; depletion of ozone in the upper atmosphere; pollution of the oceans with petroleum, plastics, sewage, and so on; and the bleaching of coral reefs may all be symptoms of a worldwide pathology that will require major modifications in human behavior before it can be cured.

List of Figures

FIGURE 15-19 CORAL REEF DISTRIBUTION AND DIVERSITY. Coral reef development is restricted to the low-latitude area between the two 180C (640F) temperature lines shown on the map. Minimum water temperatures of 180C in surface waters of the Northern and Southern hemispheres occur in February and August, respectively. In each ocean basin, the coral reef belt is wider and the diversity of coral genera is greater on the western side of the ocean basins. (After Stehli and Wells, 1971.)

FIGURE 15-20 CORAL REEF ZONATION

FIGURE 15-21 CORAL REEF INHABITANTS THAT DEPEND ON ALGAL SYMBIONTS.
A) polyps of a reef coral extend their tentacles to capture tiny planktonic organisms from the surrounding water. However, for most reef-building corals, most of their nourishment is provided by symbiotic algae, zooxanthellae, that live in the coral tissue.
B) the blue-gray sponge on the left is Niphates digitalis, a totally heterotrophic sponge. On the right is Angelas sp., which contains some cyanobacterial symbionts. It is, like most reef-building corals, a mixotroph. Photo was taken in 20 m (60 ft) of water at Carrie Bow Cay, Belize.
C) a giant clam, Tridacna gigas. These suspension feeders also depend on symbiotic algae living in the mantle tissue.
(Photos A by Christopher Newbert, B by C. R. Wilkerson, Australian Institute of Marine Science, C by Ken Lucas/Biological Photo Service.)

FIGURE 15-22 NONREEFBUILDING INHABITANTS.
A) some of the more unusual of the great variety of colorful and strange fishes associated with coral reef and other shallow-water, tropical environments are the puffers. When a potential predator comes too close, they gulp water to fill a ventral stomach pouch, expanding their loose, scaleless skin to produce a spherical shape. Actually, puffers are seldom bothered by predators as they have viscera and skin that often contain deadly neurotoxin. Though they are feeble swimmers, puffers like this swelled-up member of the genus Arothron are virtually free from attack.
B) many members of the coral family do not secrete the calcium carbonate of the reef builders. An example is a soft gorgonian coral, shown with feeding polyps extending from its branches. (Photos by Christopher Newbert.)

FIGURE 15-23 MUTUALISM. The clown fish, which lives unharmed among the stinging tentacles of the sea anemone, may pay for this protection by bringing food to the anemone. This symbiotic relationship that benefits both participants is called mutualism. (Photo by Christopher Newbert.)

FIGURE 15-24 CROWN-OF-THORNS SEA STAR. This Crown-of-thorns sea star is being attacked by one of its few predators, the Pacific triton. (Photo © Christopher Newbert.)

FIGURE 15-25 BLEACHED CORAL This round starlet coral, Siderastrea siderea, on Enrique Reef, Puerto Rico, has been bleached across the lower left side of the image. (Photo by Dr. Lucy Bunkley Williams.)