CORAL REEFS ARE DYING OUT AT AN ALARMING RATE, MAINLY BECAUSE OF THREATS TO THEIR ECOSYSTEM BY HUMANS.
Coral reefs are aragonite structures produced by living organisms, found in shallow, tropical marine waters with little to no nutrients in the water. High nutrient levels such as that found in runoff from agricultural areas can harm the reef by encouraging the growth of algae. In most reefs, the predominant organisms are stony corals, colonial cnidarians that secrete an exoskeleton of calcium carbonate (limestone). The accumulation of skeletal material, broken and piled up by wave action and bioeroders, produces a massive calcareous formation that supports the living corals and a great variety of other animal and plant life. Although corals are found both in temperate and tropical waters, reefs are formed only in a zone extending at most from 30°N to 30°S of the equator; although the reef-forming corals do not grow at depths of over 30 m (100 ft) temperature has less of an effect on distribution but it is generally accepted that no corals exist in waters below 18 °C.
The building blocks of coral reefs are the generations of reef-building , and other organisms that are composed of calcium carbonate. For example, as a coral head grows, it lays down a skeletal structure encasing each new polyp. Waves, grazing fish (such as parrotfish), sea urchins, sponges, and other forces and organisms break down the coral skeletons into fragments that settle into spaces in the reef structure. Many other organisms living in the reef community contribute their skeletal calcium carbonate in the same manner. Coralline algae [i.e zooxanthelate,filamentous algae] are important contributors to the structure of the reef in those parts of the reef subjected to the greatest forces by waves (such as the reef front facing the open ocean). These algae contribute to reef-building by depositing limestone in sheets over the surface of the reef and thereby contributing also to the structural integrity of the reef.
Reef-building or hermatypir corals are only found in the photic zone (above 50 m depth), the depth to which sufficient sunlight penetrates the water for photosynthesis to occur. The coral polyps do not photosynthesize, but have a symbiotic relationship with single-celled algae called zooxanthellae; these algal cells within the tissues of the coral polyps carry out photosynthesis and produce excess organic nutrients that are then used by the coral polyps. Because of this relationship, coral reefs grow much faster in clear water, which admits more sunlight. Indeed, the relationship is responsible for coral reefs in the sense that without their symbionts, coral growth would be too slow for the corals to form impressive reef structures. Corals can get up to 90% of their nutrients from their zooxanthellae symbionts.
Although corals are found growing in most areas of a healthy coral reef, the elevation of the reef flat relative to sea level (and considering tidal range) imposes significant constraints on coral growth. In general, only a small number of hardy coral species can thrive on the reef flat, and these cannot grow above a certain height because the polyps can withstand only limited exposure to the air at low tide. Of course some reef flats carry a meter or so of water over the surface, and then coral growth can be prolific. It is the upward growth of coralline algae on the outer part of the reef flat that ultimately results in an overall rise in the surface elevation of a reef, which slopes gently downward in towards the shore or lagoon and very steeply downward in the seaward direction. Prolific growth of these algae is a response to water motion bringing in inorganic nutrients and removing waste products. The damaging effects of exposure at low tide on the algae is ameliorated somewhat by constantly breaking waves on the reef edge. Nonetheless, it is the case that mature reefs are in equilibrium with both sea level and wave regime with respect to their elevation, and excess production of limestone moves away from the margin to expand the reef laterally and fill in low areas.
The more prolific growths of corals are to be found in water deeper than where the bottom is exposed at low tides: on the frontal reef slope (forereef), in lagoons, and along reef channels that bisect the flat. Under conditions of clear, moving seawater, corals provide the bulk of the skeletal material comprising the reef and the structural complexity that results in a high diversity of reef associated fishes and invertebrates.
Coral reefs can take a variety of forms such as:
•Apron reef – short reef resembling a fringing reef, but more sloped; extending out and downward from a point or peninsular shore.
•Fringing reef – reef that is directly attached to a shore or borders it with an intervening shallow channel or lagoon.
•Barrier reef – reef separated from a mainland or island shore by a deep lagoon.
•Patch reef – an isolated, often circular reef, usually within a lagoon or embayment.
•Ribbon reef – long, narrow, somewhat winding reef, usually associated with an atoll lagoon.
•Table reef – isolated reef, approaching an atoll type, but without a lagoon.
•Atoll reef – a more or less circular or continuous barrier reef extending all the way around a lagoon without a central island.
•Bank Reef – Bank reefs are larger than patch reefs and are linear or semi-circular in outline.
Locations of coral reefs.
Coral reefs are estimated to cover 284,300 square kilometres, with the Indo-Pacific region (including the Red Sea, Indian Ocean, Southeast Asia and the Pacific) accounting for 91.9% of the total. Southeast Asia accounts for 32.3% of that figure, while the Pacific including Australia accounts for 40.8%. Atlantic and Caribbean coral reefs only account for 7.6% of the world total.
Coral reefs are either restricted or absent from along the west coast of the Americas, as well as the west coast of Africa. This is due primarily to upwelling and strong cold coastal currents that reduce water temperatures in these areas. Corals are also restricted from off the coastline of South Asia from Pakistan to Bangladesh. They are also restricted along the coast around north-eastern South America and Bangladesh due to the release of vast quantities of freshwater from the Amazon and Ganges Rivers respectively.
Ecology and Biodiversity
Coral reefs support an extraordinary biodiversity; although they are located in nutrient-poor tropical waters. The process of nutrient cycling between corals, zooxanthellae, and other reef organisms provides an explanation for why coral reefs flourish in these waters; recycling ensures that fewer nutrients are needed overall to support the community.
Cyanobacteria also provide soluble nitrates for the coral reef through the process of nitrogen fixation. Corals absorb nutrients, including inorganic nitrogen and phosphorus, directly from the water, and they feed upon zooplankton that are carried past the polyps by water motion. Thus, primary productivity on a coral reef is very high, which results in the highest values per square meter, at 5-10g C m-2 day-1. Producers in coral reef communities include the symbiotic zooxanthellae, coralline algae, and various seaweeds, especially small types called turf algae.
Coral reefs are home to a variety of tropical or reef fish, such as the colorful parrotfish, angelfish, damselfish and butterflyfish. Other fish groups found on coral reefs include groupers, snappers, grunts and wrasses. Over 4,000 species of fish inhabit coral reefs.
Reefs are also home to a large variety of other organisms, including sponges, Cnidarians (which includes some types of corals and jellyfish), worms, crustaceans (including shrimp, spiny lobsters and crabs), molluscs (including cephalopods), echinoderms (including starfish, sea urchins and sea cucumbers), sea squirts, sea turtles and sea snakes. Aside from humans, mammals are rare on coral reefs, with visiting cetaceans such as dolphins being the main group. A few of these varied species feed directly on corals, while others graze on algae on the reef and participate in complex food webs.
A number of invertebrates, collectively called cryptofauna, inhabit the coral rock substrate itself, either boring into the limestone surface or living in pre-existing voids and crevices. Those animals boring into the rock include sponges, bivalve molluscs, and Sipunculans. Those settling on the reef include many other species, particularly crustaceans and Polychaete worms.
Due to their vast biodiversity, many governments world-wide take measures to protect their coral reefs. In Australia, the Great Barrier Reef is protected by the Great Barrier Reef Marine Park Authority, and is the subject of many plans and pieces of legislation, including a Biodiversity Action Plan.
Algae and Coral Reef
Researchers found evidence of algae dominance in locations of healthy coral reefs. In surveys done around largely uninhabited US Pacific islands, algae consists of a large percentage of the surveyed coral locations. The algae population consists of turf algae, coralline algae, and macroalgae.
Land Development and Pollution
Extensive and poorly managed land development can threaten the survival of coral reefs. Within the last 20 years, once prolific mangrove forests, which absorb massive amounts of nutrients and sediment from runoff caused by farming and construction of roads, buildings, ports, channels, and harbors, are being destroyed. Nutrient-rich water causes fleshy algae and phytoplankton to thrive in coastal areas in suffocating amounts known as algal blooms. Coral reefs are biological assemblages adapted to waters with low nutrient content, and the addition of nutrients favors species that disrupt the balance of the reef communities. Both the loss of wetlands and mangrove habitats are considered to be significant factors affecting water quality on inshore reefs.
Poor water quality has also been shown to encourage the spread of infectious diseases among corals. Suntan oils, lotions and products containing sunscreens negatively affect the coral ecosystem because sunscreens cause the rapid and complete bleaching of hard corals, even at extremely low concentrations. The effect of sunscreens is due to organic UV filters, which are able to induce the lytic viral cycle in symbiotic zooxanthellae with latent infections, according to the NIEHS, “National Institute of Environmental Health Sciences”. They conclude that sunscreens, by promoting viral infection, will play an important role in coral bleaching in areas prone to high levels of recreational use by humans.
The Florida-Caribbean Cruise Association has already signed a groundbreaking conservation agreement as of January 15, 2008 with Cozumel, Mexico “the world's most visited cruise destination” to enforce strict compliance of laws and regulations of Coral Reef conservation and protection. Cruise ship passengers, tour operators, service providers and local communities will be educated on the new laws including proper waste management procedures and the elimination of all sunscreen products used in or near coral reef regions.