Marine Communities: The Coral Reef. Is a coral reef more like a tropical rain forest or a desert?. Lecture Outline. I. Types of Coral Reefs. II. Reef Zones. III. Coral Anatomy. IV. The Ecology of a Coral Reef. V. The Distribution of Coral Reefs. VI. Environmental Stresses to Coral Reefs.
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Is a coral reef more like a tropical rain forest or a desert?
I. Types of Coral Reefs
II. Reef Zones
III. Coral Anatomy
IV. The Ecology of a Coral Reef
V. The Distribution of Coral Reefs
VI. Environmental Stresses to Coral Reefs
1. Fringing Reefs
2. Barrier Reefs
Fringing reefs grow in shallow waters and closely border the coast or are separated from it by a narrow stretch of water.
The reefs of the Florida Keys and most areas of the Caribbean are fringing reefs.
NASA PhotoBarrier Reefs
A barrier reef is separated from land by a broad, deep lagoon.
These reefs grow parallel to the coast and are large and continuous.
An atoll is a ring-shaped coral island and its surrounding reef, nearly or totally enclosing a lagoon.
In his book The Voyage of the Beagle (December 1831 to October 1836), Charles Darwin proposed a theory for the evolution of atolls that includes stages as fringing reefs and barrier reefs.
Darwin based his theory on the assumption that the volcanic islands in the Pacific are sinking into the ocean, something he had no way of testing in the mid 19th century.
Scientific evidence of the sinking of Pacific islands was obtained in the 1950’s by the Ocean Drilling Project.
In the earliest stage, a fringing reef forms a limestone shoreline around an island or along the continent.
As the island subsides and the coral continues to grow upward, the expanding lagoon separates the reef from the shoreline turning it into a barrier reef.
In the final stage the land vanishes entirely below the sea and an atollformsaround a shallow lagoon.
Coral reefs are divided into 4 major zones. These zones are determined by the amount of light and wave energy .
The Reef Face
The Buttress Zone
The Flat-Top Zone or Algal Ridge
The Rubble Zone or Reef Terrace
The Lagoon Zone
The reef face is the very deepest portion of the reef, extending from the base of the reef to about 20 m below the sea surface.
This portion of the reef receives low levels of light.
At these depths, water motion is not energetic, and where light is sufficient for coral growth relatively delicate corals, like the staghorn coral, can survive.
The buttress zone extends from a depth of 20 m to the low-tide line.
This reef zone is exposed to the greatest wave turbulence so that only the most durable forms of coral, like brain coral and elkhorn coral, can exist.
The flat-top zone extends across the reef crest.
The reef here may be anywhere from a few centimeters to a few meters below the surface at low tide.
Light is abundant, and wave energy has been dissipated by the coral heads in the buttress zone.
A variety of smaller reef fish inhabit this zone but few large predators.
The rubble zone is a shallow (3-5 m) debris field immediately shoreward of the flat-top zone.
Large blocks of coral are transported to the rubble zone during storms where they will be colonized by dense growths of algae.
Few other organisms call this part of the reef home.
The lagoon zone runs from the backside of the rubble zone all the way to shore.
The lagoon zone averages 3 to 15 meters in depth and may be as wide as 6 to 8 kilometers.
This zone is scattered with benthic grasses, sand bottom, and patch reefs.
Scattered within this zone are small patch reefs that occur with no pattern.
These patch reefs are usually only a few acres in area, but because they occur in shallow protected waters, they have many of the thin, delicate corals that cannot survive the heavy surf occurring in the buttress and flat-top zones.
Patch reefs are inhabited by populations of small tropical fish, but few large predators. In many of these patch reefs the soft coral are as abundant as the hard corals.
With some variations, most coral reefs have all these zones.
For an atoll, the lagoon zone is in the center, with the other three zones around the lagoon.
For a barrier reef, the lagoon zone gets deeper, and then there is 30 to 40 kilometers of coastal water before the shoreline.
For some of the fringing reefs in the Caribbean, the rubble zone is the beach, and the lagoon zone will be on the backside of the island away from the direction of storm waves.
The bulk of a coral reef is composed of loose to well-cemented organic debris, fragments of shells and skeletons, made of calcium carbonate (CaCO3).
The living portion of the reef is just a thin layer at the surface.
The coral animal is a sac-like polyp, a member of the Cnidaria family.
The polyp removes calcium from the sea water and secretes a calcium carbonate shell, called a corallite, which forms the foundation of the reef.
The primary soft-tissue structures of the polyp are the tentacles, a mouth (which also serves as the anus), and a gut cavity.
Zooxanthallae are dinoflagellates (algae) that live within the tissues of all reef-building (hermatypic) corals and can comprise up to 75% of the polyp’s body weight.
A symbiotic relationship that benefits both participants is called mutualism.
The waste products of the polyp provides carbon dioxide and nutrients for the zooxanthallae. The zooxanthallae in turn produce oxygen and sugars for the coral polyp.
By removing the CO2 from the coral, the zooxanthallae reduce the acidity in the coral, enabling the rapid formation of calcium carbonate (rapid calcification) necessary for reef building.
Coral reefs are distributed around the globe, but are limited to areas of the ocean where the average monthly temperatures are above 18°C (64 °F).
In general, the coral reef belt is wider and the diversity of coral organisms is greater on the western side of the ocean basins.
The symbiotic relationship with the zooxanthallae enable coral to thrive in low-nutrient (oligotrophic) waters.
In high-nutrient (eutrophic) environments, algae and other species will compete with coral for the available nutrients and phytoplankton. Eventually, the coral dies off.
Coral cannot survive in fresh, brackish water or highly turbid water.
Coral reefs are important to the environment and to people in a number of ways:
Major stresses to Florida reefs:
Man-made reefs provide an alternative to natural reef systems.