Coral reefs, the “rainforests of the sea,” are some of the most biodiverse and productive ecosystems on earth. They occupy only .2% of the ocean, yet are home to a quarter of all marine species: crustaceans, reptiles, seaweeds, bacteria, fungi, and over 4000 species of fish make their home in coral reefs. With an annual global economic value of $375 billion, coral reefs provide food and resources for over 500 million people in 94 countries and territories. But tragically, coral reefs are in crisis.
Coral reefs are endangered by natural phenomena such as hurricanes, El Nino, and diseases; local threats including overfishing, destructive fishing techniques, coastal development, pollution, and careless tourism; and the global effects of climate change—warming seas and increasing levels of CO2 in the atmosphere. According to Reefs at Risk Revisited, a recent report by the World Resources Institute (WRI), 75% of the world’s coral reefs are at risk from local and global stresses. Ten percent of coral reefs have already been damaged beyond repair, and if we continue with business as usual, WRI projects that 90% of coral reefs will be in danger by 2030, and all of them by 2050.
Coral reefs are colonies of individual animals called polyps, which are related to sea anemones. The polyps, which have tentacles to feed on plankton at night, play host to zooxanthellae, symbiotic algae that live within their tissues and give the coral its color. The coral provides CO2 and waste products that the algae need for photosynthesis. In turn, the algae nourish the coral with oxygen and the organic products of photosynthesis. The coral uses these compounds to synthesize calcium carbonate (limestone) with which it constructs its skeleton—the coral reef.
The symbiotic relationship between corals and zooxanthellae can only exist within the narrow band of environmental conditions found in tropical and subtropical waters. The water must be clear and shallow so that the light algae need for photosynthesis can penetrate, and water temperatures must ideally remain between 23˚ and 29˚ C (77˚ to 84˚ F). The number of coral species in each reef varies: the Great Barrier Reef off Australia has over 500 species of coral while a Caribbean reef has about 50. Today many reefs have 40%-50% less coral than they did just 30 years ago.
Natural phenomena such as hurricanes or prolonged cold and rainy weather can harm coral reefs. The El Nino weather pattern, which can result in lower sea level, altered salinity due to too much rainfall, and elevated sea-surface temperatures, can also damage coral. When corals become overheated, they react to the stress by expelling their symbiotic zooxanthellae, which results in coral bleaching.
If the bleaching is mild or short-term, corals can recover as algae recolonize, but if the bleaching lasts too long, corals starve to death. Other natural phenomena that stress coral reefs include predators such as parrotfish, barnacles, crabs and crown-of-thorns starfish, and diseases.
Of local threats to coral reefs, overfishing and damaging fishing techniques such as deep water trawling and the use of explosives and cyanide, are the most destructive. When herbivorous fish that eat seaweed are overfished, uncontrolled seaweed growth can smother corals. Coastal development results in erosion, and runoff containing the excess sediment can block the light zooxanthellae need. Nutrient-rich fertilizer runoff and sewage effluent can boost algae growth, which starves the water of oxygen, causing eutrophication. Pollution from land, including hot water releases from power plants, pathogens, and trash, and from marine activities, such as fuel leaks and oil spills, also endangers coral reefs. Tourism, while relying on the appeal of coral reefs, can be damaging when careless divers trample on corals or break off pieces as souvenirs. In addition more and more coral and tropical fish are being harvested for the aquarium trade.
The global effects of climate change are also having critical impacts on coral reefs, and “the evidence is overwhelming that the ability of corals and the reefs they build to keep pace with the current rate of climate change has been exceeded” according to a recent study. The average temperature of tropical oceans has increased by .7˚ C which, combined with natural fluctuations of warmer ocean temperatures, has resulted in extensive coral bleaching around the globe, involving thousands of square miles of reefs. When El Nino occurred in 1997-1998, widespread and severe coral reef bleaching occurred in the Indo-Pacific region and the Caribbean, killing 16% of the world’s coral reefs in 12 months.
The 2010 El Nino has also resulted in massive bleaching around the world. Scientists don’t yet understand the long-term impacts of coral bleaching, but they do know that bleaching leaves corals vulnerable to disease, stunts their growth, and affects their reproduction, while severe bleaching kills them.
The 30 million tons of carbon dioxide our oceans absorb every day is changing the chemistry of seawater and increasing acidification. Today, coral reefs are experiencing warmer ocean temperatures and more acidity than they have at any time in the last 400,000 years. Acidification reduces the water’s carrying capacity for calcium carbonate that corals need to build their skeletons. Even a small decrease in the coral’s ability to construct its skeleton can leave it vulnerable to erosion, and research suggests that coral reefs will begin to dissolve if atmospheric carbon dioxide levels double this century. It’s estimated that by 2050, only 15% of coral reefs will have enough calcium carbonate for adequate growth.
A new study also shows that ocean acidification profoundly alters coral reef ecosystems. As C02 levels rise and acidification increases, the biodiversity of coral reefs drops, resulting in the elimination of key species needed for healthy reef formation. “The decline of the structurally complex corals means the reef will be much simpler and there will be less habitat for the hundreds of thousands of species we associate with today’s coral reefs,” said Katherina Fabricius, a scientist at the Australian Institute of Marine Science.
All of these factors act in concert on coral reefs, and complex interactions between the threats leave coral reefs even more vulnerable. Climate change will also bring sea level rise that may result in drowned coral reefs, and more intense storms that produce excessive nutrient or sediment runoff. The overfishing of herbivorous fish and excess nutrients decrease coral’s resilience in the face of increased CO2. Rising ocean acidity lowers the threshold at which corals bleach.
If we reach 450 parts per million of C02 in the atmosphere (as of 2010, we were at 388 ppm) ocean temperatures will rise 2˚ C, calcium carbonate levels in the oceans will decrease, and we will largely destroy all our coral reefs. Coral reefs provide us with food, construction materials (limestone) and new medicines—more than half of new cancer drug research is focused on marine organisms. They offer shoreline protection and maintain water quality. And they are a draw for tourists, sometimes providing up to 80% of a country’s total income. Losing the coral reefs would have profound social and economic impacts on many countries, especially small island nations like Haiti, Fiji, Indonesia, and the Philippines that depend on coral reefs for their livelihoods.
What can be done to save these precious and beautiful ecosystems? Reefs at Risk Revisited calls for the expansion of Marine Protected Areas (MPAs) where fishing and fishing methods are regulated, curbing unsustainable fishing, better management of coastal development, and the reduction of both land and marine-based pollution. It also recommends comprehensive ecosystem management that includes all stakeholders, and stresses the need to invest in scientific research and educate the public about the importance of coral reefs. It’s crucial, of course, for national and international bodies, and for all of us to address the threats of climate change by curbing carbon emissions.
The Coral Restoration Foundation protects and restores coral reefs through creating coral nurseries and transplanting corals into degraded reef areas. Concerned individuals can adopt a coral through the Coral Restoration Foundation or a coral reef through the Nature Conservancy, which uses the funds to conduct research, promote marine conservation and support the creation of MPAs. MPAs, which are being created worldwide, protect biodiversity and help communities manage resources sustainably.
The Great Barrier Reef is the largest and richest coral reef in the world because it has been protected since the early 1970s. The creation of an MPA off St. Lucia in the Caribbean has resulted in a tripling of the fish population.
Everyone can help coral reefs by practicing sustainable fishing, and eating only sustainably caught fish. When vacationing near coral reefs, be careful not to touch them and don’t buy souvenirs of coral or other marine species.
CERC students who want to learn more about coral reefs can take CERC’s spring break Coral Reef Ecology course in Bermuda in March 2012. It deals with the biology and microbiology of corals, the ecology of coral communities, anthropogenic factors that impact coral reefs, and coral reef restoration and sustainability.