Coral reefs are found around the world. Their biodiversity is great, but they are in danger of being destroyed.
Corals have been building reefs for millions of years. The reefs we see today have all developed over the last 8,000 to 10,000 years (see attached map). In Lake Champlain in Vermont, there is a 450 million years old rock structure that is said to be the oldest reef on earth. Scientists have estimated that it once stretched from Newfoundland to Tennessee. Today, coral reefs cover about .2 percent of the ocean’s bottom, but they account for more than 25 percent of all marine species, second only to tropical rainforests on earth. They provide food and habitation for hundreds of species of fish and invertebrates, protect the shoreline from erosion, act as the primary producer through the symbiotic interaction with zooxanthellae, provide incalculable pleasures for ocean enthusiasts, and even might someday be a source of antimicrobial and antiviral drugs.
Reef Locations
Coral reefs are found in virtually all of the oceans in tropical waters (see attached map). In a remarkable venture called the Millennium Project, the National Aeronautics and Space Administration, using the Landsat satellite, has created a series of global images that pinpoint all the coral reefs around the world. Most of them are in tropical waters, but some are found in temperate seas and a few even exist in cold waters.
Types of Reefs
Coral species can be divided into two types, soft and hard. Soft corals are solitary or small colonies and can be found in deeper water as most of them have no zooxanthellae in the polyps. Soft corals generally do not contribute to the actual reef building, although they form a calcium carbonate skeleton and can be a major part of the foundation rubble.
Hard corals form the major groundwork for coral reefs. Reefs can be divided into three main types—fringing, barrier, and atoll (see attached graphic).
Fringing reefs form in shallow waters along the coastlines of continents and islands. Growth is upward until the reef reaches near the surface and then out into deeper water until the zooxanthellae, algae living in the polyps, cannot get enough sunlight to live, although fewer and fewer reef polyps are found toward the open ocean. Toward shore there is a wave breaking area called the lagoon/reef flat that is often strewn with broken coral pieces and free sand.
Barrier reefs also form along coastlines but further away from land and in somewhat deeper water. There is usually a fringing reef along shore and a lagoon between the fringing reef and the barrier reef. Barrier reefs can form in open ocean water if the underlying seabed is shallow enough to allow corals to develop.
Atoll reefs form rings around islands with a lagoon in the middle. The theory (actually formulated by Charles Darwin) behind atoll formation is this: (1) a volcano breaks the surface and builds an island land area, (2) a fringing reef is built around the island, (3) the island slowly sinks into the mantle under the ocean and/or the sea level rises, and (4) the coral animals in the fringing reef continue to grow keeping their growth up with the sinking of the land mass.
Ecology
Living polyps are found only on the surface of the reef. Each polyp produces a small amount of calcium carbonate as a protective tube for its habitat. When the individual polyp dies, the limestone rock remains. Over centuries countless layers of limestone is produced from these small secretions, and enormous reef structures are built.
In general the reef ecology involves the following components moving from land to open ocean: the intertidal zone; a lagoon; the land side of the reef mass, the back reef; the top of the reef, the reef crest; and the ocean side of the reef, the buttress zone (see attached graphic).
Intertidal Zone. The intertidal zone with a reef offshore is protected both from heavy breaking waves and from large, open-ocean predators. This sheltered part of the reef ecology can produce a tremendous biomass especially on rocky coasts.
Lagoon. This area, although it is only 100 feet to a mile or so wide and a few inches to 30 feet deep, produces a great number of ecological micro-niches. These niches are produced by variations in temperature, beds of algae and sea grasses, concentrated intertidal interactions, diminished water circulation, a reduction of surf and tidal force, and vast quantities of plankton. It generally does not include many living coral polyps. Although living pieces of coral might be flung around during storms, these do not continue to thrive in the lagoon. Benthic animals—crustaceans, anemones, molluscs, and worms—are found in great numbers, and schools of fish are abundant.
Back Reef. The back reef supports the greatest biodiversity of all the reef ecology. Here is found living hard corals and a great biomass of all the intertidal invertebrates. This area also is the home to hundreds of species of small to moderate-sized fish and invertebrates living commensally with the coral.
Reef Crest. In the reef crest is found the mass of the reef. Much of it might be out of the water at low tide, and the few coral polyps present are under constant stress. Thus, the crest has almost no living coral, but there is often great masses of calcareous algae that contribute to the buildup of the reef. Because of the harshness of the niche, biodiversity is less than the lagoon and the back crest, but it is the part of the reef where the greatest amount of calcium carbonate has been deposited. Over the years the crest will move toward the open ocean and deeper water.
Buttress Zone. The ocean side of the reef is a rough area of ridges that supports the powerful, unrelenting force of the incoming waves and provides a pathway to carry away the coral fragments and sediments into deep water. The buttress zone is often associated with the edge of the continental ridge with its related drop off into abyssal depths. This ecological niche is high in oxygen, sometimes rich in nutrients from upwelling. and is home to a great number of species—non-reef-building corals, sponges, anemones, crustaceans, sea whips, sea fans, large predatory fish and whales, and many more.
Conservation
"We are at a very critical time in the history of coral reefs," said Carrie Manfrino, president of the Central Caribbean Marine Institute on Little Cayman Island. It is like working with a sick patient. How well we treat that patient will determine if the patient survives. We could potentially see the end of hard coral reefs in our lifetime." This might be one man’s opinion, but the facts speak for themselves; many of the coral reefs around the world are sick. The threats are unrelenting:
Global Warming. Global warming acts on ocean waters just as it does on land. Corals have a narrow temperature range. Warming of the ocean water cannot be tolerated, and they die. Some of this tolerance incompatibility is with the zooxanthellae, and when they die, it is not long until the coral polyp also dies. This can be seen in many places as bleaching when the green color is lost.
Acidic Water. With the increase in carbon dioxide through global warming the coral animals, which need alkaline waters, are placed under stress.
Pollution. The pollution threat is real—fertilizer, sewage, mining, farming and logging of rainforests, petroleum products, industrial chemical waste products, and heavy metals.
Misuse of Fertilizer. Much of the fertilizer, especially the nitrogen and phosphorus, placed on lawns and crop fields ends up in the ocean. This can produce algae blooms with some of the algae growing on the reef. This shuts out the light causing the zooxanthellae to die thus eventually killing the polyp. In addition, for the last several years a species of brown seaweed has been growing on reefs along the Atlantic coast in such great numbers that it threatens all the reefs. Most scientists think that fertilizer is a factor in the blooms of these algae and seaweeds.
Aquarium Trade. Poisoning to stun fish (too much cyanide kills the majority of them) and exploding dynamite in capturing animals immeasurably harms the reefs. Furthermore, collecting pieces of coral for the aquarium trade threatens to eliminate some species of coral.
Direct Physical Damage. Sometimes the reefs are damaged and/or broken apart during snorkeling and scuba diving, boats anchoring on the reef, drawing hooks and nets over the reef by fishermen, and other people-initiated activities.
What is being done to help solve the problems? Fortunately, most governments recognize that there is a problem. However, up to this time very little except talk has been done. It could be that we are past the time when anything can be done. There are some conservation initiatives that are in place, and the world waits to see if they will have any results. For this to be the last generation to witness coral reefs, one of the grandest sights on earth, would be a great tragedy, not to mention what it would do to the ecosystems of our ocean coasts around the world.
More information can be obtained by going to the National Oceanic and Atmospheric Administration’s site.
