Global threats to coral reefs
Warming seas and coral bleaching
The warming effect of climate change is the most significant threat to the health and continued existence of coral reefs worldwide. A special report by the Intergovernmental Panel on Climate Change (IPCC) states that even if temperature increases are held at 1.5 ℃ above pre-industrial levels, "the majority (70–90%) of warm water (tropical) coral reefs that exist today will disappear."
Coral polyps—the individual animals that collectively make up coral colonies—can only survive within a narrow temperature range. Their low tolerance for temperature increases is aggravated by the effect of warming seas on their symbiotic relationship with a type of algae (zooxanthellae), which live within coral tissues and provide a built-in food supply. When increased water temperatures stress the corals, they eject the algae (the source of color in healthy corals), becoming white or "bleaching." If the warmer temperatures are temporary, corals can survive a bleaching event, but will eventually die if they remain stressed for a prolonged period. Stressed corals are also more susceptible to coral diseases.
About 25% of the carbon dioxide emitted into the atmosphere is absorbed into the oceans, affecting the chemistry of sea water and making it more acidic than they would be in the absence of CO2. Although this is not a new phenomenon, when abnormally high concentrations of CO2 are in the air, increased ocean acidity hinders the ability of corals, mollusks, and other marine organisms to form strong calciferous skeletons or shells. This, in turn, makes corals more susceptible to diseases and destruction by powerful storms.
In addition to the direct effects of increasing water temperatures and acidification, climate change also is resulting in stronger storms and a rising sea level. Hurricanes and other tropical storms will increase in frequency and intensity as sea temperatures become warmer, shattering (flattening) coral structures and killing coral. Likewise, shallow reef corals will become more deeply submerged with rising sea levels, allowing less sunlight to reach them and the symbiotic algae that depend on photosynthesis to survive.
Local threats to coral reefs
Plastic and other marine debris
Plastic bottles, grocery bags, broken fishing lines, mesh, and a vast array of other types of trash discarded by humans represent a particularly potent threat to coral colonies and the diverse animal species that inhabit coral reefs. Trash kills many types of animals, from marine mammals, sea turtles, and seabirds that become entangled in debris to invertebrates that are deprived of food and light after being caught under plastic bags of synthetic materials. Some animals are known to mistake plastic bags for jellyfish, causing obstruction of their digestive systems, and frequently death.
One alien species can upset the balance of an entire coral reef ecosystem and seriously reduce or eliminate populations of native species. The presence of several alien species can have devastating effects. The most common ways that invasive species are introduced to an area outside their range is by being released into the wild by an aquarium owner (intentionally) or by being transported to a new region on the hulls or in the ballast tanks of ships (unintentionally). The latter case occurs when free-swimming larvae of various types of organisms (such as the Green Asian Mussel) are in the seawater that is taken onboard as ballast.
In the Caribbean, Gulf of Mexico, and parts of the Atlantic, the lionfish (a native of Indo-Pacific reefs) has become permanently established and has decimated populations of native fish species by feeding on their young. Their prey normally consist of herbivorous fish that perform a critical function by feeding on marine algae that are harmful to reefs if they are permitted to grow unchecked. By feeding on small fish, lionfish also out-compete commercially important native fish for their source of food. Lionfish have no natural enemies in the Caribbean and ongoing interventions by humans are necessary to avert even greater loss of native fish. For more on lionfish, NOAA provides updated information.
In Hawaii, the predominant threat from invasive species is from five varieties of algae that spread aggressively, hindering the growth of coral colonies that form the foundation of the reef habitat. Although 25% of Hawaii's coral species are endemic, the island chain's central location has established it as a crossroads for trans-Pacific shipping, exposing it to invasion by non-native species that have quickly grown over native corals, blocking sunlight and hindering the absorption of ultraviolet rays by the symbiotic algae living in the coral tissue. More information is available from Hawaii's Division of Aquatic Resources, a department within the state's Department of Land and Natural Resources (DLNR).
Coastal development is a significant cause of coral reef degradation in areas where reefs are present near tropical beach resorts or commercial seaports. The excavation of coastal land for hotels and other tourist facilities, as well as dredging and the removal of mangroves or seagrass beds to facilitate boat access result in particulate matter (sand, silt, mud, and other materials) being released and carried to nearby reefs. The dispersal of sediments occurs through runoff from land-based sources and transported by waves, tides, and currents. Excessive concentrations of sediment disrupts both physical and biological processes that are part of a healthy reef ecosystem.
Sediments do not affect all types of coral in the same way. Some species of coral have greater resiliency, a higher tolerance for reduced sunlight, and an ability to shed sediments than others. The growth characteristics, morphology, and feeding behaviors of each coral species determine its capacity to survive temporary surges in sedimentation, due to the construction of a new hotel or marina. Long-term periods of excessive sedimentation almost always result in greatly reduced areas of live coral and lower growth rates. If major reef-building corals are suppressed or killed by excessive sediments, the entire reef ecosystem may collapse, eliminating the diverse web of organisms—from other types of marine invertebrates to fish—which depend on the shelter that coral formations provide.
Boat and anchor damage
No threat to coral reefs is as immediate and direct as vessel groundings and boat anchors that are deployed without regard to the coral under the surface. Boat operators who are careless or indiscriminate in operating their vessels can cause instant damage to reefs by grinding and dislodging shallow coral structures, which can convert the affected area to rubble, kill live coral, and take fifty years or more to recover. Similarly, boat anchors from all sizes of vessels, from small recreational craft to large cruise ships break apart coral structures, causing long-lasting ecological harm and leaving ugly scars that impair the aesthetic qualities of reefs.
Many coral species are slow-growing and will add only one to seven inches a year if they manage to survive a grounding or anchoring incident at all. Almost all damage from boat groundings and anchors is preventable. Boat operators should not be operating without proper charts, sounding equipment, knowledge of local conditions, and recognition of what the color of the water indicates about depth and the features on the bottom. Likewise, boaters should use existing moorings that are often provided in frequently visited areas of reef or avoid anchoring directly over a reef entirely if no mooring is available.
Sewage, nutrients, & runoff from land-based sources
This category of threat includes land-based sources of nutrient-rich pollutants, such as inadequately-treated sewage, fertilizer, and eroded topsoil, as well as runoff from livestock operations, landfills, and storm drains. According to NOAA, land-based runoff can overwhelm the ability of coral polyps to filter these types of pollutants from their systems. The presence of these substances causes an increase in the nitrogen-phosphorus ratio normally present in seawater, starving corals of phosphorus, as well as decreasing their temperature threshold for bleaching—exacerbating the effects of climate change.1
Oil and chemical spills
Millions of tons of petroleum products are spilled or diffuse into oceans every year, with a significant amount affecting coral reefs.2 According to NOAA's Office of Response and Operation, there are three principal ways that oil spills and other chemicals (such as oil dispersants) reach coral reefs:
- Surface oil slicks can be washed onto coral in the intertidal zone, where retreating tides leave exposed corals coated in oil.
- When seas are rough, oil (which is lighter than water and does not actually mix with it) is shaken up into small particles within the water column.
- Spilled oil becomes denser over time and may mix with sand, sinking below the water surface and smothering corals living below.
Oil and chemicals that are hazardous to humans are highly toxic to coral polyps. When they come in contact with coral, they will kill or greatly impede coral growth, development, and reproduction. The species and maturity of corals greatly affects their sensitivity to a variety of substances not found in seawater under healthy conditions. A lengthy exposure to small concentrations of hazardous chemicals may be as lethal as a brief event that saturates coral with these substances.3
Damage from harmful diving and tourism practices
The beauty of corals is also a source of vulnerability. The commercial harvesting of live corals and other unsustainable practices seriously degrade both the health and the aesthetic qualities of reefs. In some regions, black and red corals are heavily collected for jewelry making. In other cases, fragments of branching coral are broken off for sale as souvenirs or home decor. The shelves of many large department stores in the US are laden with coral stripped from reefs in the Philippines, Indonesia, and elsewhere.
Unintentional actions by tourists can also be harmful to coral reefs. Inexperienced snorkelers and divers often touch live coral surfaces with their hands or swim fins, injuring or killing the delicate polyps. In areas where coral reefs are located near popular beach resorts, the heavy use of sunscreen and other lotions by bathers result in high concentrations of harmful chemicals that can weaken or kill corals and other sensitive marine organisms.
Algal blooms refers to the excessive growth and proliferation of algae in any type of aquatic environment. They are usually caused when nutrient-rich pollutants (nitrogen and phosphorus) are present in high concentrations due to runoff from agricultural operations (fertilizer, livestock) and inadequately treated sewage (see "Sewage, nutrients, & runoff from land-based sources", above). The effect of excessive nutrients in the water is compounded by increasing temperatures caused by climate change. The algae may be native species that are normally found in small quantities or invasive algae that has been transported to a new area in the ballast tanks of ships.
Some types of algal blooms, such as the algae responsible for "red tide," can produce toxins that are fatal to coral, fish, sea turtles, and marine mammals. When the algae dies and decays, it often depletes oxygen levels in the water, suffocating coral and other animals in the reef community. Even algae that do not emit toxins may cloud seawater, blocking the sunlight necessary for the symbiotic zooxanthellae living within corals to photosynthesize and produce food for the coral. (Note: Coral polyps also get a portion of their nourishment from plankton, which they kill and gather with their tentacles). If coral reefs are affected for a lengthy period by algal blooms or reoccur in frequent intervals, the collapse of entire reef ecosystems may occur. In 2010, a team of scientists studying coral reefs in the Gulf of Oman made an alarming discovery. After a single large-scale bloom, a large coral reef was completely destroyed in three weeks.4
Overfishing and bottom-trawling
No ecosystem can survive the sustained harvesting of species faster than they are replenished by natural growth and reproduction. In the case of coral reefs, a variety of highly destructive and wasteful techniques are used to catch fish at unsustainable levels and which ravage or obliterate reef systems. Most of these techniques indiscriminately gather all types of fish, accompanied by the disposal of large quantities of dead bycatch—fish that have no market value (such as small, herbivorous reef fish) but perform important functions in a coral reef, such as keeping algae in check.
As currently practiced at the large-scale, industrial level, bottom-trawling methods have evolved to become brutally efficient for catching fish, while destroying large areas of deep-water coral reefs. Commercial trawlers used to avoid areas where coral reefs covered the seafloor because the sharp, heavy formations would snag and cut through fishing nets. The modern generation of "rockhopper" industrial trawling equipment has transformed the fishing industry, allowing fishing vessels to drag almost any type of bottom and even move boulder-size objects, razing everything in their paths like a lawnmower.
In some parts of the world, cyanide is used by commercial fishers to make them easier to catch. While larger fish are only temporarily stunned by the cyanide (which is squirted into the water where fish are present), small fish and invertebrates are fatally poisoned in the process. Cyanide is used to collect fish for the aquarium trade, as well as for the restaurant sector. According to the World Wildlife Fund, approximately 20,000 tons of fish are consumed in the restaurants of Hong Kong, Singapore, and mainland China, with a square meter of reef destroyed for every live fish harvested.5
The use of dynamite and other explosives is another wasteful method used for commercial fishing. Explosive charges are detonated underwater, resulting in dead fish floating to the surface, which can be easily skimmed off the surface of the water. These violent blasts result in the complete destruction of the environment where the fish are collected, with coral formations in the affected area turned instantly to rubble.
Coral diseases usually affect corals that are under stress due to abnormal conditions that affect a reef ecosystem. Many of the threats described above, such as climate change-induced stressors (increased surface temperatures and acidification), can create conditions that make corals vulnerable to diseases. Pollutants from oil spills, other chemicals, and land-based runoff also weaken the ability of coral to resist disease. Coral diseases are especially prevalent when multiple factors, such as high temperatures, pollutants, and biological stressors (e.g., bacteria, viruses, and fungi) are present at the same time, with each factor aggravating the adverse effects of the others.
The prevalence of coral diseases has steadily increased during the past decade, resulted in significant die-offs of reef-building coral. The exact causes have not been proven, but there is broad consensus within the scientific community that warming seas and man-made pollutants are partly to blame. Injuries to coral colonies also create entry points that allow bacteria and pathogens to get the upper hand. In Australia, where coral diseases have killed at least 6 percent of the corals on the Great Barrier Reef, scientists determined that diseases were more dominant in areas where fishing and other human activities have damaged corals.6
Coral diseases include black-band disease (affecting brain corals, shelf corals, and star corals), yellow-band (yellow blotch) disease, and red-band disease manifest themselves as discolored bands, lesions, or blotches on the coral surface, which can expand rapidly over time, consuming healthy coral cells and killing entire coral colonies within months.7 Other types of diseases can cause large areas of coral tissue to peel away from the coral skeleton, exposing the white limestone base. Algae and various types of encrusting invertebrates then rapidly exploit exposed coral skeletons as a base for aggressive growth. This group of diseases include white-band (affecting elkhorn and staghorn corals), bacterial bleaching, white plague (also caused by a bacterial pathogen), white pox (characterized by circular lesions), rapid wasting, stony coral tissue loss disease, and Aspergillosis (a lesion producing fungal infections in Caribbean soft corals) among others.
 Athena Chan, Sewage, Fertilizers Contribute To Coral Death, Study Finds,TechTimes.com, 17 July 2019.
 Fragoso ados Santos, H., Duarte, G., Rachid, C. et al. Impact of oil spills on coral reefs can be reduced by bioremediation using probiotic microbiota. Sci Rep 5, 18268 (2015). https://doi.org/10.1038/srep18268.
 NOAA Office of response and operation (website), How do oil spills affect coral reefs?, 6 December 2013, https://response.restoration.noaa.gov/about/media/how-do-oil-spills-affect-coral-reefs.html.
 Bauman, A.G., J.A. Burt, D.A. Feary, E. Marquis, and P. Usseglio. 2010. Tropical harmful algal blooms: An emerging threat to coral reef communities? Marine Pollution Bulletin 60: 2117 – 2122. https://doi.org/10.1016/j.marpolbul.2010.08.015.
 World Wildlife Fund (website), Fishing problems: Destructive fishing practices, https://wwf.panda.org/our_work/oceans/problems/destructive_fishing/.
 Yale Environment 360 (10 January 2019), As Disease Ravages Coral Reefs, Scientists Scramble for Solutions, Ret Talbot, Yale School of Forestry and Environmental Studies,
 National Ocean Service, NOAA, Coral Diseases, https://oceanservice.noaa.gov/education/tutorial_corals/coral10_disease.html.
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