Threats to Coral Reefs

Nestled in tropical ocean waters, coral reefs provide invaluable resources to both human and marine life. Coral reefs are estimated to contain one-quarter of the undersea worlds diverse species while covering less than 0.2% of the ocean floor. However, coral reefs are in serious danger due to both natural and man-made causes. Population growth and development has altered the coral reef environment. Destructive fishing practices, land-based sources of pollution such as agricultural runoff, and excessive coastal development all have detrimental effects on delicate reefs. Global warming due to increased concentrations of greenhouse gases may result in sea-level rise and higher ocean temperatures, both of which have the potential to be destructive to coral reefs. In Florida, living animals whose skeletons make up the foundation of the multi-colored underwater reefs, corals are a diverse and ecologically important member of the aquatic ecosystem. Florida boasts the only living coral reefs in the continental United States. These ancient, slow-growing structures are now imperiled by their own enormous popularity.

Boat anchors and groundings, heavy use by divers and snorkelers, and pollution all cause damage that is damaging to the reefs. James W. Hyde BIOL 1040 Corals are made up of tiny cup-shaped animals called polyps, which are similar to jellyfish. They begin life in tropical waters as free-floating larvae but eventually settle on the ocean bottom and form large, sedentary colonies. A single polyp may eventually become as large as a saucer or stay as small as the head of a pin. All polyps start off small and some single polyps become large. Billions of polyps working together, generation after generation, create one of natures most spectacular and enduring wonders: a coral reef. A coral polyp is made up primarily of clear tentacles and a mouth. It feeds by waving its tentacles through the surrounding water to attract tiny wandering animals called zooplankton, which the polyp stuns with stinging cells located on its tentacles. When a polyp is feeding or defending its territory, it stretches itself out of its cup and spreads its tentacles like the branches of a tree.

At rest, it pulls its tentacles into the cup and closes its mouth tightly. (Corals, Architects of the Reef, 1997) Corals, up until the mid-19th century were incorrectly classified as plants. Corals are animals that collect their own food instead of producing it themselves as most plants do. Corals do have within their systems an assortment of microscopic plants called zooxanthellae, which do use sunlight to photosynthesize. (Cousteau, 1985) According to Cousteau, the coral polyps give these brown in color algae a safe haven from predators. The algal in return, is believed to provide James W. Hyde BIOL 1040 the polyps with a natural sunscreen and additional oxygen. The algae also provides carbon compounds to the coral, which are used to supplement its zooplankton diet. Coral reefs, for the most part require clear water, bright light, oceanic salinties, and water temperatures that average approximately 70 degrees. Most polyps, rest during the day, while the algae produce energy.

The polyps become active at night to gather food. Corals usually reproduce during the night and at certain times of the year. Corals generally spawn in response to lunar cycles. The most well known example of spawning in response to lunar cycles are the mass spawning events in the Pacific and Caribbean stony corals. During this stage, the corals produce a milky cloud of eggs and sperm. If this took place during the day, the eggs and sperm would be quickly eaten by fish and other predators. After the eggs are fertilized and they hatch into larvae, they drift with the current for several days to several weeks before settling on a hard surface. As soon as they land they begin to construct skeletons to attach themselves to the rock. At this time the larvae change from shapeless masses to polyps with tentacles and mouths. The polyp will remain in one place its entire life, growing new polyps by cloning itself. This process is called budding. (Corals, Architects of the Reef, 1997) During their lives, coral polyps extract calcium from the seawater and mix it with carbon dioxide to build protective limestone skeletons.

These skeletons form the gradually evolving framework of the huge and colorful coral reefs. The living polyps grow on a graveyard filled with James W. Hyde BIOL 1040 countless skeletons of other polyps. Coral reefs are built polyp by polyp, this process is usually very slow: estimates of growth range from one to sixteen feet every 1,000 years. (Corals, Architects of the Reef, 1997) The shallow reef builders such as Staghorn and Elkhorn can grow much faster. Corals are one of the most productive ecosystems on the Earth (Griggs, 1984). Much of the productivity derives from the substantial concentration of marine biodiversity in coral reefs. The coral reef is the most complex, species-rich, and productive marine ecosystem (Bryant, 1998; Stafford-Deitsh, 1993). One estimate (Reaka-Kudla, 1996) proposes that coral reefs have about 1 million species, with only 10% described. Large species of fish and other organisms typically school along the reef, while small ones pack into the extensive network of crevices.

The benefit from coral reef services is both immediate and long-term, making them a priority for conservation and a major resource for sustainable development. Tourism – Where coral reefs exist, substantial income can be derives from tourism. People from all over the world travel to reefs to snorkel, dive, fish and bask on the coral sands. Fisheries – Most of the worlds poor are located within the coastal zones of developing regions, and depend directly on reef species to meet their protein needs(Bryant, 1998). Although reefs cover less than 0.2% of the ocean area, they contain 25% of marine species (Roberts, 1998) James W. Hyde BIOL 1040 Mainland and Island Protection – The protective services provided by reefs reduce storm damage, coastal erosion and flooding by intense wave action. Over geologic time, coral reefs have enabled the formation of lagoons and calm shorelines where seagrass beds and mangroves can flourish, providing habitat for numerous species at the interface of land and sea (Bryant, 1998). Medicine – About half of the potential pharmaceuticals being explored are from the oceans, many from coral reef ecosystems. Several promising drugs have already been identified, developed, and tested (Carte, 1996).

Ecological Indicators – Due to the potential vulnerability of modern corals to high temperature, coral reefs may be among the first systems to show signs of ecological stress from global warming (IPCC, 1998). Detecting this change will require an extensive network of monitoring sites throughout the coral reef regions of the world. Coral bleaching is the whitening of coral colonies due to the loss of symbiotic zooxanthellae from the tissues of polyps. This loss exposes the white calcium carbonate skeletons of the coral colony. Without zooxanthellae, the coral polyps have less energy available for growth or reproduction. Scientists arent sure why bleaching occurs. Hypotheses include James W. Hyde BIOL 1040 elevated water temperatures, ultraviolet radiation, and diseases or viruses affecting the zooxanthellae (Brown and Ogden, 1993). Bleaching is a natural phenomenon and is not lethal unless it occurs for long periods of time.

Changes in sea level are detrimental to established corals and reefs. If there is a drop in sea level the corals are exposed, corals need water. If a rise occurs it decreases the amount of available sunlight and may inhibit growth. Added emissions of carbon dioxide and other trace gases (greenhouse gases) into our atmosphere may be causing a gradual warming of our planet. This warming could cause the polar ice caps to melt, thereby raising sea level. Rises in sea level can also release nutrients trapped in soil (Coral and Coral Reefs, 1996). Four coral conditions have been identified as diseases: white band disease (WBD), and black band disease (BBD), bacterial infection and shut down reaction (Richmond, 1993). They are also susceptible to tumors and parasitic worms.

These maladies are all stress related, and anthropogenic stresses can increase a corals susceptibility to these diseases. Diseases such as BBD and WBD actually kill coral tissue while advancing in a band around the coral and leaving the white coral skeleton behind. Edmonds (1991) stated that BBD, caused by cyanophyte, may have a role in maintaining coral diversity because it is most prevalent in coral species that form large colonies and provide a structural framework for the reef. When BBD kills part of these colonies, the skeleton is available to be colonized by other coral species recruits (Edmonds, 1991). James W. Hyde BIOL 1040 WBD, which is believed to be caused by a bacteria pathogen yet unknown, has much of the same effect on corals, leaving behind a white, lifeless coral skeleton. Gladfelter (1982) does not see WBD as being beneficial to reefs. He feels that WBD destroys the reef structure because the dead coral skeleton brought about by the algae is colonized by algae, invertebrates, gastropods, and boring clionid sponges that work to weaken the coral skeletons making them more susceptible to breakage during storms. The exact method by which the diseases are transmitted are unknown.

Even though healthy corals may get BBD through contact with an infected coral, diseased corals are aggregated naturally on the reef or can be separated by great distances (Edmonds, 1991). The use of cyanide to harvest live coral reef fishes is a serious threat to some of the worlds richest coral reefs. Cyanide is used by fishermen to stun and capture reef fish alive; however, the cyanide also kills untargeted corals and many other coral reef organisms. Cyanide fishing to collect aquarium fishes was first reported in the Philippines in the 1960s. Since then, more than a million kilograms of cyanide has been used on coral reefs in that country (Coral Reefs: Cyanide Fishing and Live Reef Fish Trade, 1998). Recently this destructive fishing technique has spread to other countries to meet the growing worldwide demand for ornamental aquarium fishes and for live reef food fish (especially in the restaurant trade in Hong Kong, James W. Hyde BIOL 1040 Coral reefs are critical to food security.

Coral reefs and associated sea grass and mangrove habitats provide fishery resources that represent a critical source of food. Reefs contain over 4,000 species of fishes as well as crustaceans, molluses, and other edible invertebrates. Overfishing threatens these resources throughout the world. Reef fisheries have already been greatly diminished by overfishing and habitat destruction. Overfishing is widespread, occurring on most of the worlds reefs (Richmond, 1993). The commercial extraction of corals for aquarium and ornamental specimens, jewelry, and construction materials is joining the threats as cause for concern to the biodiversity, structure, and function of coral ecosystems. Extraction of corals for curios, jewelry, and aquarium trade is driven by international demand. Coral for aquarium and curio trade involves hard corals in the Orders Coenothecalia (blue coral), Milleporina (fire coral), Scleractinia (stony coral), Stolonifera (organ pipe corals), and Stylasterina (lace corals).

Ornamental corals are species with ornate skeletal structures and include the fast-growing, branching corals like Acropora and pocillipora. Corals which are harvested for jewelry are black corals (Order Antipatharia) and precious corals (pink, gold, and bamboo corals). These types of corals are deepwater spices that occur at very low densities, are recruitment limited, and grow very slowly. Precious corals are generally extracted using destructive trawls and tangle-net dredges. Commercial extraction of corals has damaged essential reef habitats and is believed to be causing local depletions of rare species in some areas (The James W. Hyde BIOL 1040 International Trade in Corals, 1998) Land-based sources of marine pollution pose the greatest threats to coral reefs because of their widespread impact on water quality.

Accounting for 80% of all marine pollution, land based sources of marine pollution include coastal development, agricultural practices, industrial activities, and inland deforestation (Coral Reefs and Land-Based Sources of Marine Pollution, 1998). Ocean pollution poisons coral polyps. Pollution takes on many forms including oil slicks, pesticides and other chemicals, heavy metals and garbage. Fertilizer runoff and untreated sewage introduce added nutrients to coastal ecosystems. These elevated nutrient levels promote algae growth. Unfortunately, high concentrations of algae or solid sewage can overwhelm and smother the polyps. Under normal conditions, herbivores fish and some invertebrates keep the algae population in check, but overfishing has diminished populations of herbivors Deforestation degrades more than just land habitats.

When tropical forests are cut down to clear land for agriculture, pasture, or homes, topsoil washes down rivers into coastal ecosystems. Soil that settles on reefs smothers coral polyps and blocks out the sunlight needed for corals to live. Tourism presents specific threats, both direct and indirect, to coral reef ecosystems, often leading to the destruction of the economic and natural resource based upon which it depends. Direct tourism threats include: damage to corals by divers, snorkelers, and boat anchors; and damage by cruise and recreational boat groundings. Indirect impacts on reefs by tourism include: James W. Hyde BIOL 1040 sedimentation and pollution associated with coastal tourism development and port dredging; wastewater, gray