Coral reefs under threat

Clockwise from top left: Coral Triangle region known for its biodiversity, evidenced by the multitude of organisms in this small section of House Reef in the Philippines. In 2007, Acropora corals in the Tumon Bay Marine Preserve in Guam bleached due to higher water temperatures caused by global climate change. Photo: Dave Burdick. Coral reef habitat. Photo: Brian J. Skerry. Coral polyps form a living mat over a calcium carbonate skeleton .photo: Schmahl/fgbnms

Coral reefs are the largest structures created by any group of animals in the world. They have existed on earth for over 240 million years. Reefs are ecologically important ecosystems and have a high biodiversity. They are a source of food and medicine, and they protect the coast from wave erosion. Coral reefs are home to over 25 percent of all marine life and are among the world's most fragile and endangered ecosystems. Coral reef is a mound-like structure built by tiny animal corals, mainly composed of calcium carbonate. There are two types of coral, hard and soft. The actual architects of coral reef are hard corals, which are referred to as reef-building corals. Corals are sessile animals, meaning they are not mobile but stay fixed to one place. Soft corals do not build reef.
Hard corals are colonial organisms composed of hundreds of thousands of individuals. An individual coral animal is known as polyp or coral polyp. A polyp generally ranges in size from tiny (no bigger than a pinhead) up to a foot in diameter. Growth of these reef structures varies greatly, depending on the species of coral and environmental conditions -- growing on average a half inch (1.3 centimeters) a year. It is estimated that 800 different hard coral species are involved in building reefs. Corals can form a reef in a wide range of shapes that can stretch for tens or even hundreds of miles.
Reef-building coral lives a symbiotic life. Inside the sac of each coral polyp lives microscopic one-celled algae called zooxanthellae. Through photosynthesis the algae gives off oxygen and other nutrients that the coral polyp needs to live and in return the polyp gives the algae carbon dioxide and other substances that algae needs. Another byproduct of the symbiotic relationship with zooxanthellae is colour. Generally, their brilliant colour comes from the zooxanthellae living inside their tissue. Although hard corals have a wide distribution in the world's oceans, the varieties that form reefs are typically restricted to relatively shallow, warm tropical waters between latitudes 30o north and 30o south. Reefs grow best in warm(optimum water temperature 25°C-29°C), shallow (maximum depth of about 70 meters.), clear, sunny, and agitated waters. The best salinity is between 34 and 37 parts per 1000. The coral cannot grow in polluted water.
Based on current estimates, shallow water coral reefs occupy approximately 284,430 square kilometers (110,000 square miles), and are found in the waters of 109 countries. Based upon geographic distribution, 60% of the world’s reefs are found in the Indian Ocean and Red Sea, 25% are located in the Pacific Ocean, and 15% in the Caribbean.
The geological record indicates that ancestors of modern coral reef ecosystems were formed at least 240 million years ago. The coral reefs existing today began growing as early as 50 million years before. A coral reef is like a metropolis under the sea. The Great Barrier Reef off the north-east coast of Australia is the largest coral reef and is considered to be one of the seven Natural Wonders of the world. It is over 2000 km long and 150 km wide. The Great Barrier Reef is not in fact one single reef, but a system of about 3000 individual coral reefs.

Importance of coral
Coral reefs play an important role in terms of biodiversity, coastal protection, fisheries, medicine, tourism and recreation etc.
Biodiversity: Coral reefs act as homes and nurseries for 25% of marine life. Although reefs cover only 0.2 per cent of the world’s ocean floor, these provide habitat for 250,000 known species, including more than 4,000 fish and about 800 species of hard coral. Over 25 percent of the world's fish biodiversity and between 9 and 12 percent of the world's total fisheries are associated with coral reefs. Many species have yet to be discovered. Scientists believe that one to nine million species are associated with coral reefs.
Coastal protection: Coral reefs buffer shorelines from currents, waves, and storms, helping to prevent loss of life, property damage, and erosion. In Florida, the absence of coral reefs would cause parts of the state to be submerged.
Fisheries: Fisheries related to coral ecosystems range from subsistence fishing, commercial fisheries, aquaculture, food industry, recreational fishing, the aquarium/marine ornamental trade, and the curio and fashion industries. The fish that grow and live on coral reefs are a significant food source for over a billion people worldwide.
Medicine: Coral reefs and creatures found in coral ecosystems are important sources of medicines. Coral reefs have been used in the treatment of cancer, HIV, cardiovascular diseases, ulcers. Coral’s porous limestone skeletons have been used for human bone grafts. The medicines and other potentially useful compounds identified to date have led to coral ecosystems being referred to as the medicine cabinets of the 21st century by some, and the list of approved and potential new drugs is ever growing.
Tourism and recreation: Healthy coral ecosystems support local businesses and economies, as well as provide jobs through tourism and recreation. Coral reef attracts millions of tourists, scuba divers and snorkelers every year. Local economies receive billions of dollars from these visitors through diving tours, recreational fishing trips, hotels, restaurants, and other businesses based near reef ecosystems.
According to the WMO/CBD, coral reefs, referred to as the ‘tropical rainforests of the ocean’ are facing unprecedented threats because of climate change, including damage from increasingly severe tropical cyclones, bleaching events and ocean acidification. About 20 percent of the original area of coral reefs has been lost, with a further 25 percent threatened in the next century.

Climate change threats
Climate change impacts have been identified as one of the greatest global threats to coral reef ecosystems. Increasing concentrations of carbon dioxide (CO2) leads to “double trouble” for coral reefs. First, the trapping of heat in the atmosphere leads to ocean warming, which can cause extensive coral bleaching events and mass mortalities. A slight rise in maximum water temperatures -- only 1 to 2 degrees -- can stress the corals. The global devastation of coral reefs from record warming of the sea surface in 1997/98 was the first example of what is likely to occur in the future under a warming climate.
Second, high CO2 levels lead to ocean acidification. Ocean acidification refers to the reduction in ocean pH as CO2 is taken up by the ocean surface. An even more critical factor, however, is that a drop in pH leads to a loss of carbonate ions, which are required for corals, to build their skeletons.
The warmer water associated with more nutrients (human or animal wastes and fertilizers that runoff into the ocean can increase nitrogen levels) also encourage the growth of harmful algae on top of the coral, which kills it, because it blocks out the sun. Without the sun, the zooxanthellae cannot perform photosynthesis and so they die. Without the zooxanthellae, the coral polyps cannot survive.
With temperature rise, mass bleaching, and infectious disease outbreaks are likely to become more frequent. The frequency of coral diseases appears to have increased significantly over the last 10 years, causing widespread mortality among reef-building corals. Many scientists believe the increase is related to deteriorating water quality associated with anthropogenic pollutants and increased sea surface temperatures.
A report's lead author Professor Ove Hoegh-Guldberg, an expert on coral bleaching at Sydney University, states that coral reefs could be eliminated from most areas of the world by 2100; even the world's largest, the Great Barrier Reef, could be dead within 30 years unless measures are taken now to slow climate change.
Moreover, anthropogenic activities are also major threats to coral reefs. Pollution, over-fishing, destructive fishing practices using dynamite or cyanide, collecting live corals for the aquarium market, coastal development, tourism and mining coral for building materials or jewelry, oil spill are some of the many ways that people damage reefs all around the world every day. Researches say that, taking into consideration the present condition, if the rate of destruction continues, almost 70% of the world’s coral reef will be destroyed by the year 2050.
Worldwide, about 500 million people have some level of dependence upon coral reefs. Besides, coral reefs support millions of species and are the sources of food, rich genetic resources, medicine and many more.
Healthy coral reefs mean a healthy world. The Joint Technical Commission for Oceanography and Marine Meteorology (JCOMM) says that “An immediate global response to reduce anthropogenic drivers of climate change is imperative to ensure the survival of these invaluable and diverse ecosystems,”

Md. Abu Sayed is an agriculturalist and environmentalist. [email protected]