THE CORAL REEF SYSTEM
Single coral reefs may cover over 100 sq km: massive structures that have been built almost entirely by marine plants and animals. The material of the reef is calcium carbonate: limestone derived from the surrounding waters by the reef organisms. The living reef forms the top layer of the reef adding new limestone to these massive structures at rates that can be measured annually at KG's for every square meter of the reefs surface.
The corals are probably the most obvious life forms on the reefs. All the different colors and shapes made up of thousands of individual polyps, each secreting its own small cup of coral limestone, which provide the building blocks for reef construction. But plants are also important in the development of this system as many secrete limestone! Coralline algae, in particular, form cementing crusts that act as 'mortar' for the coral 'blocks'. Innumerable other plants and animals also contribute, forming fine sand and courser skeletal material which ends up as either sediments on the reef surface or as infill in the many cavities that develop within the reef.
Coral reefs have existed in the earth's shallow seas for a long time, probably in excess of 450 million years; a clear indication of how successful a life form they are. Although the original corals, called 'rugose' corals, became extinct about 200 million years ago, the reef that they formed were probably very similar to modern coral reefs.
HISTORY
The scleractinian corals that succeeded the rugose forms probably evolved in the warm waters of the Tethys sea, a massive ancient ocean that existed between the northern European and Asian land masses and the Southern African and Indian continents. It was eventually closed by the gradual northward migration of the southern continents, a process known as continental drift. The closing was earliest in the west and latest in the east so that the evolving corals were slowly pushed eastwards into the shallow peninsulas and island studded seas of the western Pacific. It is in this area which has by far the greatest diversity with more than 500 coral species known in this region.
The worlds modern reef's systems began to develop during the tertiary period (about 25 million years ago). Reefs have existed and grown successfully many times in the past. This explains why there are reefs and fossils in cold water areas of the world.
The Australian continent was also on the move and slowly drifting northwards from the cold polar latitudes and into the warmer waters of the tropics and by chance ventured into this area rich in coral growth...its northeastern shores in particular were bathed in the ocean waters passing through the coral rich seas. (great Barrier Reef)
With the great environmental fluctuations in the earth's history, sea levels have oscillated from positions slightly higher than the present to at least 150m below the present level. Corals and associated life forms were able to survive these changes showing how resistant to natural disturbances this ecosystem is.
Florida
is the only state in the Continental US to have extensive
shallow coral reef formations near its coasts. The reefs
extend from near Stuart on the Atlantic east coast to the Dry
Tortugas, west of Key West in the Gulf of Mexico. The best
reef development occurs in the Florida Keys. These reefs may
rival some of the Caribbean areas and number about 6000
between Key Biscayne and the Dry Tortugas.
These reefs came into existence 5-7000 years ago when the Wisconsin Ice Age ended and the sea level rose. The growth is slow and estimates range from one to 16' every 1000 years.
Live-bottom biota's
WORM REEFS Aggregations of the tropical reef worm (Phragmatopoma lapidosa ) construct low reefs of tubes consisting of sand grains cemented together by protein. The reefs expand as worm larva settle on existing tube masses. The reef growth is controlled by waves bringing planktonic food and sand to the worms. Found from Cape Canaveral to Key Biscayne and best developed off St. Lucie and Martin Counties...Bath Tub Reef.
Oculina Banks
Coral banks that occur offshore from JAX to St Lucie inlet at depths of 50-100m are another of FLA little known reef types. The banks are constructed by the ivory tree coral (Oculina varicosa).
GEOLOGY The Fla.-Bahamas region is part of an extensive carbonate platform that once extended into the Gulf coast region. The Florida platform is a massive southward-thickening wedge of limestone's that is at least 6000 m thick beneath Cay Sal Bank. These limestone's have been accumulating for about 150 million years. About 15 million years ago, cooling occurred in the northern latitudes, climates cooled and sea levels began to drop...coral reefs retreating to lower latitudes...and during the past million years with each continental glacial advance, sea level dropped further and coral reef communities withdrew from Florida. During interglacial episodes, sea level rose, waters warmed, and corals returned. The Key Largo Limestone , a fossil coralline rock found in the upper keys, provides evidence of coral reef development during the last glacial period before the present. This formation extends from Miami to the dry tortugas and into the straits of Florida. It varies in thickness from 23m to 61m. the last glacial period ended about 18000 yrs ago and sea level began to rise. Modern reef development began about 5000-7000 yrs ago.
Stony corals are the major reef architects. These small marine animals, called polyps, remain in one place throughout their adult lives and produce a hard skeleton made of calcium carbonate, which they extract from the seawater and combine with CO2 for limestone, constructing elaborate limestone skeletons which are left behind when the animal dies. Some corals grow in colonies that continue to enlarge year after year, and some are solitary and live alone. Together they can form enormous colonies that are called coral reefs, coral islands and coral atolls. The largest, being the GBR, is 1,250 miles long. They can exhibit many shapes, sizes, and colors and reefs look like underwater gardens (although they usually lose their colors when removed from the water except red coral). These shapes and patterns are characteristic for the species and are a result of growth patterns of millions of tiny individual polyps that make up the colony.
And though reef corals are classified as animals, there is a complex of microscopic plants called zooxanthellae, which live within the animal tissues (symbiosis) and the animals benefit from the energy that the plants provide through photosynthesis. These dinoflagellates gain nutrients from the corals nitrogen and phosphorus wastes. They are also responsible for most of the colors of the reef.
These specialized habitats provide shelter, food, and breeding sites for numerous plants and animals and form a breakwater for the adjacent coast, providing natural storm protection. They are also important to SE Fla's economy.
Reef development occurs only in areas with specific environmental characteristics:
1. a solid structure for the base,
2.warm and predictable water temperatures and oceanic salinities,
3. clear, transparent waters low in phosphate and nitrogen nutrients,
4.and moderate wave action to disperse wastes and bring oxygen and plankton to the reef.
Corals live in all oceans of the world from the Arctic and Antarctic to the tropics. The largest reefs occur in the warmer portions of the Pacific and Indian oceans however they are also found in the Caribbean and gulf of Mexico and southern Florida.
Numerous species occur in the different areas ranging from 40 or more in the West Indies to 200 or more on the Great Barrier Reef (because of their great skeletons, their fossils have yielded more than 6000 extinct species.) Corals can live in water below 68-70 degrees (reef min. temps) but don't form reefs, just like some corals which can live 19,000 feet below the surface, but reef building corals are in water usually less than 300'.
There are three types of reefs, 1. fringing, which follow the coastline and form along the coast, 2. barrier reefs, which lie parallel to the coast and separated by a narrow lagoon, and 3. atolls, which are associated with rims of extinct volcanoes which sunk back into the ocean leaving a circular rim of coral around a deep lagoon.
Great care must be taken to minimumize human damage and though the reefs are well marked on the charts, every year careless boaters run aground destroying coral colonies hundreds of years old. From the surface reefs have a unique golden brown color. If you see brown, you may be about to run aground. Anchors, hooks traps and touching all injure or damage or destroy corals by leaving it vulnerable to infection by microscopic organisms that can kill the animals.
The Animal
The adult coral, stationary at this stage in life is called a polyp which can reproduce in two different ways. One is by means of eggs that, when fertilized by sperm, develop into tiny swimming larval organisms called planulae. They eventually settle down on the bottom of the ocean, on a rock etc. and develop into polyps. Each polyp builds a limestone skeleton attached to the surface on which the polyp has landed. After establishment, the upper part of the polyp becomes domed shaped, develops a body and a mouth with tentacles around the mouth used to draw in food from the surrounding waters. The tentacles are armed with specialized stinging cells called nematocysts that paralyze tiny prey. (mostly at night)
Another process is by budding which offshoots called buds grow out from the body and remain attached sending out more buds. Some corals, the buds break off to become separate individuals. The largest solitary polyps grow to a diameter of 10 inches (25cm) while polyps of colonial species form from .04-1.2inches in diameter.
(comptons CD Rom1990)
The coral reef has a very high order of internal organization, greater even than the tropical rain forests they are often compared to. Both receive high levels of solar radiation, the ultimate source of all ecosystem energy. (p63 Great Barrier Reef).
CORAL BIOLOGY 63 coral taxa in keys
2
CLASSES of reef corals exist, HYDROZOA AND ANTHOZOA. In the
keys, Millepora (fire coral) is the only hydrozoan coral on
shallow reefs. Stylaster and Distichophora are found in deep
habitats. The anthozoans include octocorals, zoanthids, stony
corals, false corals, and anemones. Reef corals are colonial,
often containing thousands of individual members or polyps.
---------Fire Coral-(M. complanata)--------------------Fan Coral....Keys
FIRECORAL...MILLEPORINA The stinging/burning comes from nematocysts. 2 species of firecoral are found in Florida reef. The bladed is a keel shaped (M. complanata) restricted to shallow windward reef tops. Crenulated (M. alcicornis) is a branching species found in a much wider range of reef habitats. Fire corals are successful in colonizing living octocoral branches.
In 1842, Charles Darwin devised a scheme to describe coral reefs that still dominates their classification. He described them as "fringing reefs" attached to the mainland or island, "barrier reefs", and open ocean atolls. Fringing reefs, the simplest reef landforms, are built upwards and outwards in shallow seas adjacent to islands or continents. Barrier reefs occur further offshore, separated from the mainland shore by a shallow lagoon. In contrast, atolls are found in the deeper oceans, having usually developed over volcanic foundations that have subsided beneath the sea. Coral growth in this case produce saucer like reefs with central lagoons.
Coral reefs can only grow up to about the level of a low spring tide, and reefs (as opposed to individual corals) will probably not develop in water more than 40m deep. Reefs are, therefore, strongly influenced by sea level. Over the time that most of the world's reefs have grown, there have been major sea level changes. There were two main reasons for sea level changes. One, the worlds ocean basins have changed in size and shape because of sea floor spreading and plate tectonics and the other is the ice ages.
ZONES OF THE MODERN REEF.
In the last 8000 years, reefs have grown to what they are today. They have grown from the older reef platforms about 15-20m deep. The modern reefs are relatively thin and their shape reflects accurately the shape of the surface over which they are growing.
To understand the growth of modern reefs, its best to begin at the surface, where five easily defined zones can be identified:
1. the reef front,
2. the reef crest,
3. the coral flat,
4. the sand flat, and
5. the lagoon.
The upper slope of the modern reef front extends from the mean low-water level of spring tides down to a depth of 10 to 20m. Fairly steep, sometimes vertical, sometimes terraced, and often serrated by coral covered spurs and channels, the area has a lot of coral cover. On some reefs, the front is protected by a line of patch reefs, many of which are joined to form an outer front.
The exposed windward reef crest, or outer reef flat, lies above the mean low-water level of spring tides. It is composed of either a seaweed, stepped surface of encrusted coralline algae and devoid of corals, or a flat coralline-encrusted surface that may reach half a meter above the mean low-water level of spring tides, as on a ribbon reef. The upper surface of the reef crest is sometimes covered by a green turf of fleshy algae, which provide a home for countless small organisms, particularly foraminifera. These algae pavements can reach 200-300m wide and form in response to the high energy conditions typical of windward edges. These algal pavements are replaced by extensive coral development in lower energy or more leeward parts of the edge of the reef.
The coral flat or reef flat, occurs on the sheltered or lee side of the algal flat. It usually consists of an aligned coral zone where most corals are encrusted by coralline algae. These aligned corals occur in patches 1 to 2 m wide and 20m long formed by coral growing parallel to the direction of waves refracted across the top of the reef. The water movement is channeled across the grooves between the aligned assemblages of coral and in this way, coral growth on the reef top controls the backward flow of water across the reef. Nutrients and sediments are carried back through these channels to the sand flat and lagoon areas. This zone is usually separated from the lagoon by a sand flat, sometimes up to 500m wide. The sand flat is made up of broken skeletons of corals, coralline algae, and other reef organisms derived from the reef front and coral flat and transported toward the lagoon as sands and gravels. Foraminifer from the algal turf of the reef crest also form part of this sediment.
Sand flats have been built by prevailing swell and waves that continually destroy the reef front and transport the broken products backwards. The surface of the sand flat, covered by one half to two meters of water, depending upon the tides, is often sparsely covered by small colonies of branching corals which can grow from fragments brought from the coral flat which grow when they come to rest. What can happen is that as these pieces start to grow a small solid patch eventually forms after larva of massive corals settle and grow. In this way, the coral flat extends backward across the sand flat, and the sand flat will migrate into the lagoon.
The lagoon is best seen in platform reefs, where they reach 5-10m deep with the deepest part on the leeward side of the reef. Lagoons are sometimes open with very few patch reefs rising from the floor or often very crowded with patch reefs making navigation of the lagoon difficult.. These patch reefs whether they form discrete circular or network-like structures, usually rise vertically from the lagoon floor. They are made up of many species of branching and massive corals, the dead parts of which are encrusted by coralline algae. The lagoon floor consists of coral sand, which becomes finer away from the windward margin and patches of mud (very fine) sometimes occur in the quiet waters of the leeward margins. Because the lagoons are depositories for sediments and organic material created on the windward margin, they are slowly filling up.
About 6000 yrs ago, sea level established and maintained a position close to what it is today. In ten meters of water, corals grow upwards toward the light, so that the reef surface grows upward from the deeper, quiet water to the high energy surface environment characterized by heavy pounding of the surf. The types of corals that make up the reef change as the environment changes.
The composition of the reef also varies with the position of the reef on the continental shelf. The structure of the outer shelf-reefs generally reflects the influence of powerful waves, and here most of the reef consists of coral skeletons forming a porous, but relatively stable framework. Reefs in the middle and inner parts are dominated by accumulations of sand, and the coral framework forms a much smaller proportion of the reef, reflecting the calmer sea, or lower energy conditions compared with the outer reef.
3 stages of reef development can be seen.
1. vertical growth to sea level...a reef growing slowly from a deep substrate may not yet have reached sea level or only recently have reached sea level and is characterized by vigorous growth of free-standing branching or massive corals upwards to sea level. Such a reef is termed juvenile.
A reef growing from relatively shallow substrate, or one growing rapidly from a deeper substrate, will reach sea level quickly and will be subject to the influences of shallow water for a long time...many have been at sea level for 4 to 6000years. Swell, waves and currents help develop these and these reefs are called mature.
Reefs growing from very shallow substrates, or small reefs that have grown rapidly from deeper substrates, may have been at sea level even longer than mature reefs and because a long time at the surface leads to destruction of the reef, lagoons will have been filled in and the typical zonation destroyed. Erosion is thought to exceed production, and such reefs are called senile.
Its believed that reefs progress through the growth stages from juvenile to mature to senile. Different reefs require different lengths of time to follow this progression, the time being dependant on depth and size of substrate, rates of growth, length of time at sea level, and intensity of the processes operating at sea level. As reef growth has seldom lasted for more than 5000-15000 years, some large lagoon reefs may not have sufficient time to progress through the complete cycle in one high-sea-level growth phase and could need more than one growth phase to reach a senile condition.
Maintenance and Destruction
Reefs come in a vast array of shapes and sizes and many of these differences result from erosion while periodically exposed during low-sea-levels rather than from growth differences. While the reefs are biologically complex with the many plants and animals, they are also ecologically organized with a series of zones across the reef, each serving a specific role in providing food, consuming wastes, recycling nutrients, and creating and maintaining the limestone structure that is the reef.
The clear boundaries and isolation make coral reefs excellent examples of the natural order of things.
Many of the worlds tropical oceans have few plant nutrients, which result in little plankton growth (the absence which results in the beautiful blue color usually associated with tropical seas), but coral reefs show no particular preference for the most barren areas of the ocean and can live in water very low in plant nutrients and plankton to water that is quite enriched. The enriched environments often find the reef not being able to compete with other marine systems dense beds of large algae populated by filter feeding animals such as barnacles. Though reefs don't prefer to struggle for nutrition, they have developed the ability to thrive in barren areas of the ocean more effectively than other marine systems by creating and recycling their own internal food supply.
Chemical runoffs can either be destructive such as herbicides and insecticides or enriching, such as fertilizers. In either case the viability of a coral reef community is threatened. Inshore reefs whether they be stable or in decline, do not develop the structured zonation and balanced communities of the outer reefs because they exist in enriched and biologically active waters and internal recycling of the food supply in not essential.
Needs
A coral reef requires only the basic plant nutrients plus a copious supply of calcium for the construction of the calcium carbonate of the coral skeletons and other limestone forming materials of the reef. Carbon dioxide and calcium are abundant in sea water. N, P and some trace elements may only be present in limited quantities in the clear oceanic waters and the reef must recycle these materials to maintain its intense biological activity. All plant and animal matter grown must be totally consumed or fully degraded within the reef community to prevent the loss of any nutrients. Extensive mats of blue-green algae on the reef may provide the reef with N by converting the N in the atmosphere to the soluble inorganic N nutrients.
There are very small but important losses and gains from any reef system and that is the exchange of larval forms with other reefs. This ensures inbreeding and that any species depleted by disease or other stress will be replaced by larval input from another reef or reefs.
About 3/4 of the CO2 that is removed from sea water by a coral reef in each 24hr period is used directly in photosynthesis within the reef algae. This creates about 20g of new organic matter for every square meter of shallow, reef-flat environment per day and can be up to 50g a day in areas of intense biological activity. This is the reefs food supply.
Most of the limestone making up the solid underlying mass of the coral reef consists of skeletons of once living corals. This material may remain where it grows, though it is perhaps more commonly broken off and redeposited in other parts of the reef. One thing certain, corals cannot cement their skeletons together in a solid reef structure, they mearly form a vast quantity of limestone. They provide the aggregate of the reef concrete. At least some of the corals in this rich surface growth are likely top add to the structural framework of their own reef when they die.
The encrusting coralline algae hold together the sand and framework materials of the reef to create a solid surface. They have a role in building a real reef rather than a pile of uncemented coral debris and sand. Once the reef has been totally consolidated by these algae, chemical precipitation of more carbonates in the reef structure provides additional cementing of the remaining loose calcium carbonate materials. Its a slow process but it makes the entire structure more rigid. The final reef material is a porous but strong limestone, though it frequently contains uncemented regions.
NUTRIENTS AND REEF PRODUCTIVITY
Direct feeding by the polyps usually only supplies only 10 per cent of the corals energy needs. In such cases, most energy comes from photosynthesis by zooxanthellae, which obtain nutrients from metabolic waste products of the coral host as well as from the seawater or detritus. Coral reef primary productivity is usually very much higher than areas where these nutrients are more abundant but recycling rates of the corals and zooxanthellae are high but the harvestable primary productivity is low because most of the organic matter produced is metabolized and nutrients are freed to be reused in photosynthesis.
Almost every gram of organic matter created by a coral reef is consumed and eventually finds its way back to the sea water as carbon dioxide. Before most of the organic matter is finally degraded back to CO2, it may move through all or part of the complex and finely balanced coral reef food web.
What is the result of a reef creating and totally consuming so much food material? First, it allows the continuing maintenance by reproduction of the coral reef community by feeding the sun's energy into the system. Second, and perhaps most significantly, it provides the energy needed to remove the other quarter of the total of CO2 used by the reef in a 24-hour period. This additional CO2 is used in the biological formation of calcium carbonate or limestone. The limestone created us mostly retained by the system, giving rise to the physical growth of the reef structure.
Reefs may be thought of as assemblages of beautiful animals but in fact they are dominated by the activities of plants. The whole system is driven by the photosynthetic activities of plants, just like most ecosystems existing in light. Even the corals function principally as plants, deriving as much as 90% of their total and energy requirements from the tiny algae, known as ZOOXANTHELLAE, contained within their coral tissues. The remaining requirements are met through feeding on reef plankton.
Apart from the corals, the major plants providing the food supply of the reef are fine filamentous algae that form a rapid growing fuzz or turf over almost all available surfaces within the reef. This turf is grazed by animals, which in turn are preyed on by larger animals and so the food web expands. Also, all living materials die and undergo microbial decay, forming the basis of yet another component of the complex food web. The reef also creates and consumes its own plankton. Each animal and plant serve a vital role in the reef's finely tuned balance.
The Factory
The 10 to 30 grams of calcium carbonate or limestone created everyday for each square meter of the active parts of the reef are retained somewhere within the general reef environment. The reef thus exhibits real, measurable growth over hundreds of years. The energy to enable the creation of all this limestone comes into the system through algal photosynthesis. Much of the limestone originates as growing coral but other organisms, such as many algae, tiny single celled forams, and shells make major contributions. The cements in the limestone rock are partially deposited by algae and partially chemical precipitation.
Photosynthesis by zooxanthellae also promotes production of skeletal limestone's that make up the reef framework. Zooxanthellae provide coral with the energy needed to calcify. (They also prevent phosphate poisoning of calcification by removing phosphatic wastes and any phosphates from the microenvironment (phosphate inhibits the precipitation of aragonite crystals, the skeletal building blocks of corals))
Photosynthesis uses CO2 and water from respiration raising the pH of the system end enhancing aragonite precipitation.
During calcification, calcium ions, which are abundant in sea water, combine with bicarbonate ions, also found in the environment and the following reaction occurs...
1 Ca2 + 2HCO3 -----> Ca(HCO3)2 and then calcium carbonate and carbonic acid are produced...
2 Ca(HCO3)2----->CaCO3 + H2CO3
but carbonic acid can't exist so carbonic acid must be ionized
3 H2CO3---> H+ + HCO3- or converted to water and CO2
4 H2CO3----> H2) + CO2
Reactions 3 and 4 are catalyzed by the enzyme carbonic anhydrase, which is found in the calicoblastic epithelium the corals tissue layer where calcium metabolism is most active. Calcium Carbonate is apparently stored then actively transported in a system of microvessels to the deposition sites where it is deposited as aragonite crystals. It has also been suggested that the zooxanthellae provide glycolate, which is converted to glyoxylate and combined with urea to form allantoic acid. This may be the medium by which calcium and CO2 are transported to sites of calcification.
Destruction
One of the obvious of all reef destroying processes are storms. But not all reef destruction is caused by physical forces. Many animals and even some algae live out their lives inside the reef structure and even in the skeletons of still-living corals and other organisms with hard skeletons. Some of these graze off of the reef surfaces and remove considerable amounts of the limestone at the same time. These boring and grazing organisms weaken the structure of their host or the reef itself by physically disrupting the limestone materials. This in turn, forms more fine sand and calcium carbonate detritus in the reef system. But the boring organisms also dissolve, using acid secretions, much of the calcium carbonate they remove (about 5-25% of total CaCO3 deposited by the reef)..
Readers digest Great Barrier Reef pp64-86.
CORALS EXTRA
BIOLOGICAL INTERACTIONS
Of course, grazing and predation can be readily observed but on coral reefs, disease, competition, chemical warfare, bioerosion and symbioses are also common.
Pathogens may injure and kill corals, and black band, one such pathogen usually attaches to a coral following tissue damage.
Competition include overgrowth denying light and water movement.
Allelopathy (chemical defense and offense) is also used to prevent overgrowth and gain living space.
Damselfish destroy coral tissue and farm/defend algae on the dead coral.
The black sea urchin crops algae from the reef and this provides settlement habitat for coral larvae.
Sponges bore into coral skeletons, weakening them, some bind to them and some protect the undersurface from attacks by boring organisms.
1
State Of
1.How old is the recent
Caribbean Coral Reef System?
2
List 5 socioeconomic importance’s of coral reefs and give an
example of each.
3.
How are reefs a buffer for the CO2 cycle?
4.
What 3 human activities threaten
5
How does upland clearing effect coral reef?
6 How does sewage
discharge effect coral reefs?
7. The discovery of
what about coral has made the destruction of reefs an international
interest?