Coral bleaching

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Bleached corals
Lodestone Reef Valentines Day 2016, Green Chromis on Coral.jpg
Healthy corals

Coral bleaching occurs when coral polyps expel algae that live inside their tissues. Normally, coral polyps live in an endosymbiotic relationship with this algae crucial for the health of the coral and the reef. [1] The algae provides up to 90% of the coral's energy. Bleached corals continue to live but begin to starve after bleaching. [2] Some corals recover.

Coral Marine invertebrates of the class Anthozoa

Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically live in compact colonies of many identical individual polyps. Corals species include the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.

Polyp one of two forms found in the phylum Cnidaria (zoology)

A polyp in zoology is one of two forms found in the phylum Cnidaria, the other being the medusa. Polyps are roughly cylindrical in shape and elongated at the axis of the vase-shaped body. In solitary polyps, the aboral end is attached to the substrate by means of a disc-like holdfast called the pedal disc, while in colonies of polyps it is connected to other polyps, either directly or indirectly. The oral end contains the mouth, and is surrounded by a circlet of tentacles.

Algae Group of eukaryotic organisms

Algae is an informal term for a large, diverse group of photosynthetic eukaryotic organisms that are not necessarily closely related, and is thus polyphyletic. Including organisms ranging from unicellular microalgae genera, such as Chlorella and the diatoms, to multicellular forms, such as the giant kelp, a large brown alga which may grow up to 50 m in length. Most are aquatic and autotrophic and lack many of the distinct cell and tissue types, such as stomata, xylem, and phloem, which are found in land plants. The largest and most complex marine algae are called seaweeds, while the most complex freshwater forms are the Charophyta, a division of green algae which includes, for example, Spirogyra and the stoneworts.


Above-average sea water temperatures caused by global warming is the leading cause of coral bleaching. [2] According to the United Nations Environment Programme, between 2014 and 2016 the longest recorded global bleaching events killed coral on an unprecedented scale. In 2016, bleaching of coral on the Great Barrier Reef killed between 29 and 50 percent of the reef's coral. [3] [4] [5] In 2017, the bleaching extended into the central region of the reef. [6] [7] The average interval between bleaching events has halved between 1980 and 2016. [8]

Global warming rise in the average temperature of the Earths climate system and its related effects

Global warming is a long-term rise in the average temperature of the Earth's climate system, an aspect of climate change shown by temperature measurements and by multiple effects of the warming. The term commonly refers to the mainly human-caused observed warming since pre-industrial times and its projected continuation, though there were also much earlier periods of global warming. In the modern context the terms global warming and climate change are commonly used interchangeably, but climate change includes both global warming and its effects, such as changes to precipitation and impacts that differ by region. Many of the observed warming changes since the 1950s are unprecedented in the instrumental temperature record, and in historical and paleoclimate proxy records of climate change over thousands to millions of years.

United Nations Environment Programme organization

The United Nations Environment Programme (UNEP), an agency of the United Nations, coordinates the organization's environmental activities and assists developing countries in implementing environmentally sound policies and practices. It was founded by Maurice Strong, its first director, as a result of the United Nations Conference on the Human Environment in June 1972 and has overall responsibility for environmental problems among United Nations agencies; however, international talks on specialized issues, such as addressing climate change or combating desertification, are overseen by other UN organizations, like the Bonn-based Secretariat of the United Nations Framework Convention on Climate Change and the United Nations Convention to Combat Desertification. UNEP's activities cover a wide range of issues regarding the atmosphere, marine and terrestrial ecosystems, environmental governance and green economy. It has played a significant role in developing international environmental conventions, promoting environmental science and information and illustrating the way those can be implemented in conjunction with policy, working on the development and implementation of policy with national governments, regional institutions in conjunction with environmental non-governmental organizations (NGOs). UNEP has also been active in funding and implementing environment related development projects.

Great Barrier Reef Coral reef system off the east coast of Australia, World Heritage Site

The Great Barrier Reef is the world's largest coral reef system composed of over 2,900 individual reefs and 900 islands stretching for over 2,300 kilometres (1,400 mi) over an area of approximately 344,400 square kilometres (133,000 sq mi). The reef is located in the Coral Sea, off the coast of Queensland, Australia. The Great Barrier Reef can be seen from outer space and is the world's biggest single structure made by living organisms. This reef structure is composed of and built by billions of tiny organisms, known as coral polyps. It supports a wide diversity of life and was selected as a World Heritage Site in 1981. CNN labelled it one of the seven natural wonders of the world. The Queensland National Trust named it a state icon of Queensland.


Coral and microscopic algae have a symbiotic relationship. When water temperatures get too high, the algae leave the coral tissue and the coral begins to starve. Coral Bleaching.jpg
Coral and microscopic algae have a symbiotic relationship. When water temperatures get too high, the algae leave the coral tissue and the coral begins to starve.

The corals that form the great reef ecosystems of tropical seas depend upon a symbiotic relationship with algae-like single-celled flagellate protozoa called zooxanthellae that live within their tissues and give the coral its coloration. The zooxanthellae provide the coral with nutrients through photosynthesis, a crucial factor in the clear and nutrient-poor tropical waters. In exchange, the coral provide the zooxanthellae with the carbon dioxide and ammonium needed for photosynthesis. Negative environmental conditions thwart the coral's ability to provide for the zooxanthellae's needs. To ensure short-term survival, the coral-polyp then expels the zooxanthellae. This leads to a lighter or completely white appearance, hence the term "bleached". [9] As the zooxanthellae provide up to 90% of the coral's energy needs through products of photosynthesis, after expelling, the coral may begin to starve.

Coral reef Outcrop of rock in the sea formed by the growth and deposit of stony coral skeletons

A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in groups.

Ecosystem A community of living organisms together with the nonliving components of their environment

An ecosystem is a community of living organisms in conjunction with the nonliving components of their environment, interacting as a system. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the system through photosynthesis and is incorporated into plant tissue. By feeding on plants and on one-another, animals play an important role in the movement of matter and energy through the system. They also influence the quantity of plant and microbial biomass present. By breaking down dead organic matter, decomposers release carbon back to the atmosphere and facilitate nutrient cycling by converting nutrients stored in dead biomass back to a form that can be readily used by plants and other microbes.

Flagellate cell or organism

A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word flagellate also describes a particular construction characteristic of many prokaryotes and eukaryotes and their means of motion. The term presently does not imply any specific relationship or classification of the organisms that possess flagellae. However, the term "flagellate" is included in other terms which are more formally characterized.

Healthy coral at left and bleached, but still living, coral to right CoralBleaching.jpg
Healthy coral at left and bleached, but still living, coral to right

Coral can survive short-term disturbances, but if the conditions that lead to the expulsion of the zooxanthellae persist, the coral's chances of survival diminish. In order to recover from bleaching, the zooxanthellae have to re-enter the tissues of the coral polyps and restart photosynthesis to sustain the coral as a whole and the ecosystem that depends on it. [10] If the coral polyps die of starvation after bleaching, they will decay. The hard coral species will then leave behind their calcium carbonate skeletons, which will be taken over by algae, effectively blocking coral re-growth. Eventually, the coral skeletons will erode, causing the reef structure to collapse.

Exoskeleton External skeleton of an organism

An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton (endoskeleton) of, for example, a human. In usage, some of the larger kinds of exoskeletons are known as "shells". Examples of animals with exoskeletons include insects such as grasshoppers and cockroaches, and crustaceans such as crabs and lobsters. The shells of certain sponges and the various groups of shelled molluscs, including those of snails, clams, tusk shells, chitons and nautilus, are also exoskeletons. Some animals, such as the tortoise, have both an endoskeleton and an exoskeleton.


Coral bleaching may be caused by a number of factors. While localized triggers lead to localized bleaching, the large scale coral bleaching events of the recent years have been triggered by global warming. Under increased carbon dioxide concentration expected in the 21st century, corals are expected to becoming increasingly rare on reef systems. [11] Coral reefs located in warm, shallow water with low water flow have been more affected than reefs located in areas with higher water flow. [12]

List of triggers

Bleached coral - partially overgrown with algae EL18p-Reunion.jpg
Bleached coral - partially overgrown with algae
Zooplankton Heterotrophic protistan or metazoan members of the plankton ecosystem

Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word zooplankton is derived from the Greek zoon (ζῴον), meaning "animal", and planktos (πλαγκτός), meaning "wanderer" or "drifter". Individual zooplankton are usually microscopic, but some are larger and visible to the naked eye.

Overfishing the act whereby fish stocks are depleted to unacceptable levels, regardless of water body size

Overfishing is the removal of a species of fish from a body of water at a rate that the species cannot replenish in time, resulting in those species either becoming depleted or very underpopulated in that given area. Overfishing has spread all over the globe and has been present for centuries.

Photosynthetically active radiation

Photosynthetically active radiation, often abbreviated PAR, designates the spectral range of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. This spectral region corresponds more or less with the range of light visible to the human eye. Photons at shorter wavelengths tend to be so energetic that they can be damaging to cells and tissues, but are mostly filtered out by the ozone layer in the stratosphere. Photons at longer wavelengths do not carry enough energy to allow photosynthesis to take place.

Mass bleaching events

Bleached Acropora coral (foreground) and normal colony (background), Keppel Islands, Great Barrier Reef Keppelbleaching.jpg
Bleached Acropora coral (foreground) and normal colony (background), Keppel Islands, Great Barrier Reef

Elevated sea water temperatures are the main cause of mass bleaching events. [32] Sixty major episodes of coral bleaching have occurred between 1979 and 1990, [33] [34] with the associated coral mortality affecting reefs in every part of the world. In 2016, the longest coral bleaching event was recorded. [35] The longest and most destructive coral bleaching event was because of the El Niño that occurred from 2014–2017. [36] During this time, over 70% of the coral reefs around the world have become damaged. [36]

Factors that influence the outcome of a bleaching event include stress-resistance which reduces bleaching, tolerance to the absence of zooxanthellae, and how quickly new coral grows to replace the dead. Due to the patchy nature of bleaching, local climatic conditions such as shade or a stream of cooler water can reduce bleaching incidence. [37] Coral and zooxanthellae health and genetics also influence bleaching. [37]

Large coral colonies such as Porites are able to withstand extreme temperature shocks, while fragile branching corals such Acropora are far more susceptible to stress following a temperature change. [38] Corals consistently exposed to low stress levels may be more resistant to bleaching. [39] [40]

Scientists believe that the oldest known bleaching was that of the Late Devonian (Frasnian/Famennian), also triggered by the rise of sea surface temperatures. It resulted in the demise of the largest coral reefs in the Earth's history. [41]

According to Clive Wilkinson of Global Coral Reef Monitoring Network of Townsville Australia ,in 1998 the mass bleaching event occurred the indian ocean region worst affected by it due to rising of temperature of sea by 2℃ to normal temperature level coupled by strong El nino event in 1997-1998.


Two images of the Great Barrier Reef showing that the warmest water (top picture) coincides with the coral reefs (lower picture), setting up conditions that can cause coral bleaching GBReef TempChlorophyll 200602.jpg
Two images of the Great Barrier Reef showing that the warmest water (top picture) coincides with the coral reefs (lower picture), setting up conditions that can cause coral bleaching

In the 2012–2040 period, coral reefs are expected to experience more frequent bleaching events. The Intergovernmental Panel on Climate Change (IPCC) sees this as the greatest threat to the world's reef systems. [42] [43] [44] [45] Coral reefs worldwide were lost by 19%, and 60% of the remaining reefs are at immediate risk of being lost. There are a few ways to tell the impacts of coral bleaching on reefs. First by the coral cover, the more coral that is covering the ground the less of an impact bleaching had. Second, coral abundance, which is the number of different living species on the coral reef.

Pacific Ocean

Great Barrier Reef

The Great Barrier Reef along the coast of Australia experienced bleaching events in 1980, 1982, 1992, 1994, 1998, 2002, 2006, 2016 and 2017. [45] [46] Some locations suffered severe damage, with up to 90% mortality. [47] The most widespread and intense events occurred in the summers of 1998 and 2002, with 42% and 54% respectively of reefs bleached to some extent, and 18% strongly bleached. [48] [49] However coral losses on the reef between 1995 and 2009 were largely offset by growth of new corals. [50] An overall analysis of coral loss found that coral populations on the Great Barrier Reef had declined by 50.7% from 1985 to 2012, but with only about 10% of that decline attributable to bleaching, and the remaining 90% caused about equally by tropical cyclones and by predation by crown-of-thorns starfishes. [51] A global mass coral bleaching has been occurring since 2014 because of the highest recorded temperatures plaguing oceans. These temperatures have caused the most severe and widespread coral bleaching ever recorded in the Great Barrier reef. The most severe bleaching in 2016 occurred near Port Douglas. In late November 2016 surveys of 62 reefs showed that long term heat stress from climate change caused a 29% loss of shallow water coral. The highest coral death and reef habitat loss was inshore and mid-shelf reefs around Cape Grenville and Princess Charlotte Bay. [52] The IPCC's moderate warming scenarios (B1 to A1T, 2 °C by 2100, IPCC, 2007, Table SPM.3, p. 13 [53] ) forecast that corals on the Great Barrier Reef are very likely to regularly experience summer temperatures high enough to induce bleaching. [48]


Major bleaching occurred in Hawaiian coral reefs in 1996 and in 2002. [54] In 2014, biologists from the University of Queensland observed the first mass bleaching event, and attributed it to The Blob. [55] In 2014 and 2015, a survey in Hanauma Bay Nature Preserve on Oahu found 47% of the corals suffering from coral bleaching and close to 10% of the corals dying. [56] In 2014 and 2015, 56% of the coral reefs of the big island were affected by coral bleaching events. During the same period, 44% of the corals on west Maui were effected. [57] On January 24, 2019, scientists with The Nature Conservancy found that the reefs had begun to stabilize nearly 4 years after the last bleaching event. [58]

Jarvis Island

Eight severe and two moderate bleaching events occurred between 1960 and 2016 in the coral community in Jarvis Island, with the 2015-16 bleaching displaying the unprecedented severity in the record. [59]


According to a 2017 Japanese government report, almost 75% of Japan's largest coral reef in Okinawa has died from bleaching. [60]

Indian Ocean

Coral reef provinces have been permanently damaged by warm sea temperatures, most severely in the Indian Ocean. Up to 90% of coral cover has been lost in the Maldives, Sri Lanka, Kenya and Tanzania and in the Seychelles during the massive 1997–98 bleaching event.


More than 60% of the coral in the Maldives has suffered from bleaching in 2016. [61]


Thailand experienced a severe mass bleaching in 2010 which affected 70% of the coral in the Andaman Sea. Between 30% and 95% of the bleached coral died. [62]


In 2017 there was a study done on two islands in Indonesia to see how their coral cover was. One of the places was Melinjo Islands and the other was Saktu Islands. In Saktu Island the lifeform conditions were categorized as bad, with an average coral cover of 22.3%. In Melinjo Islands the lifeform conditions were categorized as bad, with an average coral cover of 22.2%.

Atlantic Ocean

United States

In South Florida, a 2016 survey of large corals from Key Biscayne to Fort Lauderdale found that about 66% of the corals were dead or reduced to less than half of their live tissue. [63]


The first recorded mass bleaching event that took place in the Belize Barrier Reef was in 1998, where sea level temperatures reached up to 31.5 °C (88.7 °F) from 10 August to 14 October. For a few days, Hurricane Mitch brought in stormy weather on 27 October but only reduced temperatures by 1 degree or less. During this time period, mass bleaching in the fore-reef and lagoon occurred. While some fore reef colonies suffered some damage, coral mortality in the lagoon was catastrophic.

The most prevalent coral in the reefs Belize in 1998 was the lettuce coral, Agaricia tenuifolia . On 22 and 23 October, surveys were conducted at two sites and the findings were devastating. Virtually all the living coral was bleached white and their skeletons indicated that they had died recently. At the lagoon floor, complete bleaching was evident among A. tenuifolia. Furthermore, surveys done in 1999 and 2000 showed a near total mortality of A. tenuifolia at all depths. Similar patterns occurred in other coral species as well. Measurements on water turbidity suggest that these mortalities were attributed to rising water temperatures rather than solar radiation.


Hard coral cover on reefs in the Caribbean have declined by an estimated 80%, from an average of 50% cover in the 1970s to only about 10% cover in the early 2000s. [64] A 2013 study to follow up on a mass bleaching event in Tobago from 2010 showed that after only 1 year, the majority of the dominant species declined by about 62% while coral abundance declined by about 50%. However, between 2011 and 2013, coral cover increased for 10 of the 26 dominant species but declined for 5 other populations. [65]

Other areas

Coral in the south Red Sea does not bleach despite summer water temperatures up to 34 °C (93 °F). [39] [66] Coral bleaching in the Red Sea is more common in the northern section of the reefs, the southern part of the reef has been plagued by coral eating starfish, dynamite fishing and human impacts on the environment. In 1988 there was a massive bleaching event that affected the reefs in Saudi Arabia and in Sudan, the southern reefs were more resilient and affected them very little. Previously it was thought that the North suffers more from coral bleaching but they show a fast turnover of coral and the southern reef was thought to not suffer from bleaching as harshly, they show more consistency. However, new research shows where the south reef should be bigger and healthier than the north it was not. This is believed to be because of major disturbances in recent history from bleaching events, and coral eating starfish. [67] In 2010, coral bleaching occurred in Saudi Arabia and Sudan, where the temperature rose 10 to 11 degrees. Certain taxa experienced 80% to 100% of their colonies bleaching, while some showed on average 20% of that taxa bleaching. [68]

Economic and political impact

According to Brian Skoloff of The Christian Science Monitor , "If the reefs vanished, experts say, hunger, poverty and political instability could ensue." [69] Since countless sea life depend on the reefs for shelter and protection from predators, the extinction of the reefs would ultimately create a domino effect that would trickle down to the many human societies that depend on those fish for food and livelihood. There has been a 44% decline over the last 20 years in the Florida Keys, and up to 80% in the Caribbean alone. [70]

Coral reefs provide various ecosystem services, one of which is being a natural fishery, as many frequently consumed commercial fish spawn or live out their juvenile lives in coral reefs around the tropics. [71] [72] [73] Thus, reefs are a popular fishing site and are an important source of income for fishers, especially small, local fisheries. [73] As coral reef habitat decreases due to bleaching, reef associated fish populations also decrease, which affects fishing opportunities. [71] A model from one study by Speers et al. calculated direct losses to fisheries from decreased coral cover to be around $49 – $69 billion, if human societies continue to emit high levels of greenhouse gases. [71] But, these losses could be reduced for a consumer surplus benefit of about $14 – $20 billion, if societies chose to emit a lower level of greenhouse gases instead. [71] These economic losses also have important political implications, as they fall disproportionately on developing countries where the reefs are located, namely in Southeast Asia and around the Indian Ocean. [71] [73] [74] It would cost more for countries in these areas to respond to coral reef loss as they would need to turn to different sources of income and food, in addition to losing other ecosystem services such as ecotourism. [72] [74] A study completed by Chen et al. suggested that the commercial value of reefs decreases by almost 4% every time coral cover decreases by 1% because of losses in ecotourism and other potential outdoor recreational activities. [72]

Coral reefs also act as a protective barrier for coastlines by reducing wave impact, which lowers the damage from storms, erosions, and flooding. Countries that lose this natural protection will lose more money because of the increased susceptibility of storms. This indirect cost, combined with the lost revenue in tourism, will result in enormous economic effects. [9]

Monitoring reef sea surface temperature

The US National Oceanic and Atmospheric Administration (NOAA) monitors for bleaching "hot spots", areas where sea surface temperature rises 1 °C or more above the long-term monthly average. The "hot spots" are the location in which thermal stress is measured and with the development of Degree Heating Week (DHW), the coral reef's thermal stress is monitored. [75] [76] Global coral bleaching is being detected earlier due to the satellite remote sensing the rise of sea temperatures. [75] [77] It is necessary to monitor the high temperatures because coral bleaching events are affecting coral reef reproduction and normal growth capacity, as well as it weakening corals, eventually leading to their mortality. [77] This system detected the worldwide 1998 bleaching event, [78] [79] that corresponded to the 1997–98 El Niño event. [80] Currently, 190 reef sites around the globe are monitored by the NOAA, and send alerts to research scientists and reef managers via NOAA Coral Reef Watch (CRW) website. [81] By monitoring the warming of sea temperatures, the early warnings of coral bleaching, alerts reef managers to prepare and draw awareness to future bleaching events. [81] The first mass global bleaching events were recorded in 1998 and 2010, which was when the El Niño caused the oceans temperatures to rise and worsened the corals living conditions. [36] The 2014–2017 El Niño was recorded to be the longest and most damaging to the corals, which harmed over 70% of our coral reefs. [36] Over two thirds of the Great Barrier Reef have been reported to be bleached or dead. [36]

Changes in ocean chemistry

Increasing ocean acidification due to rises in carbon dioxide levels exacerbates the bleaching effects of thermal stress. Acidification affects the corals' ability to create calcareous skeletons, essential to their survival. [82] This is because ocean acidification decreases the amount of carbonate ion in the water, making it more difficult for corals to absorb the calcium carbonate they need for the skeleton. As a result, the resilience of reefs goes down, while it becomes easier for them to erode and dissolve. [83] In addition, the increase in CO2 allows herbivore overfishing and nutrification to change coral-dominated ecosystems to algal-dominated ecosystems. [84] A recent study from the Atkinson Center for a Sustainable Future found that with the combination of acidification and temperature rises, the levels of CO2 could become too high for coral to survive in as little as 50 years. [82]

Infectious disease

Infectious bacteria of the species Vibrio shiloi are the bleaching agent of Oculina patagonica in the Mediterranean Sea, causing this effect by attacking the zooxanthellae. [85] [86] [87] V. shiloi is infectious only during warm periods. Elevated temperature increases the virulence of V. shiloi, which then become able to adhere to a beta-galactoside-containing receptor in the surface mucus of the host coral. [86] [88] V. shiloi then penetrates the coral's epidermis, multiplies, and produces both heat-stable and heat-sensitive toxins, which affect zooxanthellae by inhibiting photosynthesis and causing lysis.

During the summer of 2003, coral reefs in the Mediterranean Sea appeared to gain resistance to the pathogen, and further infection was not observed. [89] The main hypothesis for the emerged resistance is the presence of symbiotic communities of protective bacteria living in the corals. The bacterial species capable of lysing V. shiloi had not been identified as of 2011.

Coral adaptation

In 2010, researchers at Penn State discovered corals that were thriving while using an unusual species of symbiotic algae in the warm waters of the Andaman Sea in the Indian Ocean. Normal zooxanthellae cannot withstand temperatures as high as was there, so this finding was unexpected. This gives researchers hope that with rising temperatures due to global warming, coral reefs will develop tolerance for different species of symbiotic algae that are resistant to high temperature, and can live within the reefs. [90] [91] In 2010, researchers from Stanford University also found corals around the Samoan Islands that experience a drastic temperature increase for about four hours a day during low tide. The corals do not bleach or die regardless of the high heat increase. Studies showed that the corals off the coast of Ofu Island near America Samoa have become trained to withstand the high temperatures. Researchers are now asking a new question: can we condition corals, that are not from this area, in this manner and slowly introduce them to higher temperatures for short periods of time and make them more resilient against rising ocean temperatures. [92]

Recovery and macroalgal regime shifts

After corals experience a bleaching event to increased temperature stress some reefs are able to return to their original, pre-bleaching state. [93] [94] Reefs either recover from bleaching, where they are recolonized by zooxanthellae, or they experience a regime shift, where previously flourishing coral reefs are taken over by thick layers of macroalgae. [95] This inhibits further coral growth because the algae produces antifouling compounds to deter settlement and competes with corals for space and light. As a result, macroalgae forms stable communities that make it difficult for corals to grow again. Reefs will then be more susceptible to other issues, such as declining water quality and removal of herbivore fish, because coral growth is weaker. [11] Discovering what causes reefs to be resilient or recover from bleaching events is of primary importance because it helps inform conservation efforts and protect coral more effectively.

Corals have shown to be resilient to short-term disturbances. Recovery has been shown in after storm disturbance and crown of thorns starfish invasions. [93] Fish species tend to fare better following reef disturbance than coral species as corals show limited recovery and reef fish assemblages have shown little change as a result of short-term disturbances. [93] In contrast, fish assemblages in reefs that experience bleaching exhibit potentially damaging changes. One study by Bellwood et al. notes that while species richness, diversity, and abundance did not change, fish assemblages contained more generalist species and less coral dependent species. [93] Responses to coral bleaching are diverse between reef fish species, based on what resources are affected. [96] Rising sea temperature and coral bleaching do not directly impact adult fish mortality, but there are many indirect consequences of both. [96] Coral-associated fish populations tend to be in decline due to habitat loss; however, some herbivorous fish populations have seen a drastic increase due to the increase of algae colonization on dead coral. [96] Studies note that better methods are needed to measure the effects of disturbance on the resilience of corals. [93] [97]

The lemon damselfish ( Pomacentrus moluccensis ) is a coral associated species that has been shown to decline dramatically following coral bleaching. Pomacentrus moluccensis2.jpg
The lemon damselfish ( Pomacentrus moluccensis ) is a coral associated species that has been shown to decline dramatically following coral bleaching.

Until recently, the factors mediating the recovery of coral reefs from bleaching were not well studied. Research by Graham et al. (2005) studied 21 reefs around Seychelles in the Indo-Pacific in order to document the long-term effects of coral bleaching. [94] After the loss of more than 90% of corals due to bleaching in 1998 around 50% of the reefs recovered and roughly 40% of the reefs experienced regime shifts to macroalgae dominated compositions. [94] After an assessment of factors influencing the probability of recovery, the study identified five major factors: density of juvenile corals, initial structural complexity, water depth, biomass of herbivorous fishes, and nutrient conditions on the reef. [94] Overall, resilience was seen most in coral reef systems that were structurally complex and in deeper water. [94]

The ecological roles and functional groups of species also play a role in the recovery of regime shifting potential in reef systems. Coral reefs are affected by bioeroding, scraping, and grazing fish species. Bioeroding species remove dead corals, scraping species remove algae and sediment to further future growth, grazing species remove algae. [99] The presence of each type of species can influence the ability for normal levels of coral recruitment which is an important part of coral recovery. [99] Lowered numbers of grazing species after coral bleaching in the Caribbean has been likened to sea-urchin-dominated systems which do not undergo regime shifts to fleshy macroalgae dominated conditions. [95]

There is always the possibility of unobservable changes, or cryptic losses or resilience, in a coral community's ability to perform ecological processes. [93] [99] These cryptic losses can result in unforeseen regime changes or ecological flips. [93] More detailed methods for determining the health of coral reefs that take into account long-term changes to the coral ecosystems and better-informed conservation policies are necessary to protect coral reefs in the years to come. [93] [94] [97] [99]

Rebuilding coral reefs

Research is being done to help slow down the mortality rate of corals. Worldwide Projects are being completed to help replenish and restore our coral reefs. The population of corals is rapidly declining, so scientists are doing experiments in coral growth and research tanks to help replenish the population of corals. [36] These research tanks mimic the coral reefs natural environment in the ocean. [36] They are growing corals in these tanks to use for their experiments, so no more corals are being harmed or taken from the ocean. [36] They are also transplanting the successfully grown corals from the research tanks and putting them into the areas of the ocean where the reefs are dying out. [36] An experiment is being done in some coral growth and research tanks by Ruth Gates and Madelaine Van Oppen. [36] They are trying to make "super corals" that can withstand some of the environmental factors that the corals are currently dying from. [36] Van Oppen is also working on developing a type of algae that will have a symbiotic relationship with corals and can withstand water temperature fluctuations for long periods of time. [36] This project may be helping to replenish our reefs, but the growing process of corals in research tanks is very time consuming. [36] It can take at least 10 years for the corals to fully grow and mature enough to where they will be able to breed. [36]

Economic value of coral reefs

Coral reefs provide shelter to an estimated quarter of all ocean species. [100] Experts estimate that coral reef services are worth up to $1.2 million per hectare which translates to an average of $172 billion per year. [101] The benefits of coral reefs include providing physical structures such as coastal shoreline protection, biotic services within and between ecosystems, biogeochemical services such as maintaining nitrogen levels in the ocean, climate records, and recreational and commercial (tourism) services. [102] Coral reefs are one of the best marine ecosystems to use to as a food source. [31] The coral reefs are also the perfect habitat for rare and economically important species of tropical fish, as they provide the perfect area for fish to breed and create nurseries in. [31] If the populations of the fish and corals in the reef are high, then we can use the area as a place to gather food and things with medicinal properties, which also helps create jobs for people who can collect these specimens. [31] The reefs also have some cultural importance in specific regions around the world. [31]

Cost benefit analysis of reducing loss of coral reefs

In 2010, the Convention on Biological Diversity's (CBD) Strategic Plan for Biodiversity 2011–2020 created twenty distinct targets for sustainable development for post-2015. Target 10 indicates the goal of minimizing "anthropogenic pressures on coral reefs". [103] Two programs were looked at, one that reduces coral reef loss by 50% that has a capital cost of $684 million and a recurrent cost of $81 million. The other program reduces coral reef loss by 80% and has a capital cost of $1,036 million with recurring costs of $130 million. CBD acknowledges that they may be underestimating the costs and resources needed to achieve this target due to lack of relevant data but nonetheless, the cost-benefit analysis shows that the benefits outweigh the costs by a great enough amount for both programs (Benefit Cost Ratio of 95.3 and 98.5) that "there is ample scope to increase outlays on coral protection and still achieve a benefit to cost ratio that is well over one". [103]


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Zooxanthellae are single-celled dinoflagellates that are able to live in symbiosis with diverse marine invertebrates including corals, jellyfish, and nudibranchs. Most known zooxanthellae are in the genus Symbiodinium but some are known from the genus Amphidinium, and other taxa, as yet unidentified, may have similar endosymbiont affinities. Another group of unicellular eukaryotes that partake in similar endosymbiotic relationships in both marine and freshwater habitats are green algae zoochlorellae.

Southeast Asian coral reefs Marine ecosystem

Southeast Asian coral reefs have the highest levels of biodiversity for the world's marine ecosystems. They serve many functions, such as forming the livelihood for subsistence fishermen and even function as jewelry and construction materials. Coral reefs are developed by the carbonate-based skeletons of a variety of animals and algae. Slowly and overtime, the reefs build up to the surface in oceans. Coral reefs are found in shallow, warm salt water. The sunlight filters through clear water and allows microscopic organisms to live and reproduce. The Indian Ocean holds 60% of the world's coastal reefs, 25% are in the Pacific and 15% are in the western Atlantic. There are coral reefs in the Persian Gulf, Madagascar, the Philippines, Hawaiian Islands and off Southeast Asia. Coral reefs have been preserved and identified in rocks over 400 million years old. Coral reefs are actually composed of tiny, fragile animals known as coral polyps. Coral reefs are significantly important because of the biodiversity. Although the number of fish are decreasing, the remaining coral reefs contain more unique sea creatures. The variety of species living on a coral reef is greater than anywhere else in the world. An estimation of 70-90% of fish caught are dependent on coral reefs in Southeast Asia and reefs support over 25% of all known marine species. However, those sensitive coral reefs are facing detrimental effects on them due to variety of factors: overfishing, sedimentation and pollution, bleaching, and even tourist-related damage.

White band disease coral disease

White band disease is a coral disease that affects acroporid corals and is distinguishable by the white band of dead coral tissue that it forms. The disease completely destroys the coral tissue of Caribbean acroporid corals, specifically elkhorn coral and staghorn coral. The disease exhibits a pronounced division between the remaining coral tissue and the exposed coral skeleton. These symptoms are similar to white plague, except that white band disease is only found on acroporid corals, and white plague has not been found on any acroporid corals. It is part of a class of similar disease known as "white syndromes", many of which may be linked to species of Vibrio bacteria. While the pathogen for this disease has not been identified, Vibrio carchariae may be one of its factors. The degradation of coral tissue usually begins at the base of the coral, working its way up to the branch tips, but it can begin in the middle of a branch.

Ocean acidification The ongoing decrease in the pH of the Earths oceans, caused by the uptake of carbon dioxide, and its expected impacts

Ocean acidification is the ongoing decrease in the pH of the Earth's oceans, caused by the uptake of carbon dioxide (CO2) from the atmosphere. Seawater is slightly basic (meaning pH > 7), and ocean acidification involves a shift towards pH-neutral conditions rather than a transition to acidic conditions (pH < 7). An estimated 30–40% of the carbon dioxide from human activity released into the atmosphere dissolves into oceans, rivers and lakes. To achieve chemical equilibrium, some of it reacts with the water to form carbonic acid. Some of the resulting carbonic acid molecules dissociate into a bicarbonate ion and a hydrogen ion, thus increasing ocean acidity (H+ ion concentration). Between 1751 and 1996, surface ocean pH is estimated to have decreased from approximately 8.25 to 8.14, representing an increase of almost 30% in H+ ion concentration in the world's oceans. Earth System Models project that, within the last decade, ocean acidity exceeded historical analogues and, in combination with other ocean biogeochemical changes, could undermine the functioning of marine ecosystems and disrupt the provision of many goods and services associated with the ocean beginning as early as 2100.

<i>Porites</i> genus of cnidarians

Porites is a genus of stony coral; they are SPS corals. They are characterised by a finger-like morphology. Members of this genus have widely spaced calices, a well-developed wall reticulum and are bilaterally symmetrical. Porites, particularly Porites lutea, often form microatolls. Corals of the genus Porites also often serve as hosts for Christmas tree worms.

The Great Barrier Reef, the world's largest reef system, stretching along the East coast of Australia from the northern tip down to the town of Bundaberg, is composed of roughly 2,900 individual reefs and 940 islands and cays that stretch for 2,300 kilometres (1,616 mi) and cover an area of approximately 344,400 square kilometres (133,000 sq mi). The reef is located in the Coral Sea, off the coast of Queensland in northeast Australia. A large part of the reef is protected by the Great Barrier Reef Marine Park.

<i>Symbiodinium</i> genus of dinoflagellates

Symbiodinium is a genus that encompasses the largest and most prevalent group of endosymbiotic dinoflagellates known. These unicellular algae commonly reside in the endoderm of tropical cnidarians such as corals, sea anemones, and jellyfish, where the products of their photosynthetic processing are exchanged in the host for inorganic molecules. They are also harbored by various species of sponge, flatworms, mollusks such as the giant clams, foraminifera (soritids), and some ciliates. Generally, these dinoflagellates enter the host cell through phagocytosis, persist as intracellular symbionts, reproduce, and disperse to the environment. The exception is in most mollusks, where Symbiodinium are intercellular. Cnidarians that are associated with Symbiodinium occur mostly in warm oligotrophic (nutrient-poor), marine environments where they are often the dominant constituents of benthic communities. These dinoflagellates are therefore among the most abundant eukaryotic microbes found in coral reef ecosystems.

Ove Hoegh-Guldberg (biologist) Director of the Global Change Institute at the University of Queensland

Ove Hoegh-Guldberg, is the inaugural Director of the Global Change Institute at the University of Queensland, and the holder of a Queensland Smart State Premier fellowship (2008–2013). He studies climate change and coral reefs. Hoegh-Guldberg has appeared on television, including two Australian Story series profiling his life and work, and radio, and maintains a blog on coral reefs, politics and the environment.

Coral Triangle A roughly triangular area of the tropical marine waters of Indonesia, Malaysia, Papua New Guinea, Philippines, Solomon Islands and Timor-Leste

The Coral Triangle is a geographical term so named as it refers to a roughly triangular area of the tropical marine waters of Indonesia, Malaysia, Papua New Guinea, Philippines, Solomon Islands and Timor-Leste that contain at least 500 species of reef-building corals in each ecoregion. This region encompasses portions of two biogeographic regions: the Indonesian-Philippines Region, and the Far Southwestern Pacific Region. The Coral Triangle is recognized as the global centre of marine biodiversity and a global priority for conservation. It is also called the "Amazon of the seas" and covers 5.7 million square kilometres (2,200,000 sq mi) of ocean waters. Its biological resources sustain the lives of over 120 million people. According to the Coral Triangle Knowledge Network, about $3 billion in fisheries exports and another $3 billion in coastal tourism revenues are derived as annual foreign exchange income in the region.

Coral reef protection Modifying human activities to reduce impact on coral reefs.

Coral reef protection is the process of modifying human activities to avoid damage to healthy coral reefs and to help damaged reefs recover. The key strategies used in reef protection include defining measurable goals and introducing active management and community involvement to reduce stressors that damage reef health. One management technique is to create Marine Protected Areas (MPAs) that directly limit human activities such as fishing.

Fisheries and climate change

Rising ocean temperatures and ocean acidification are radically altering aquatic ecosystems. Climate change is modifying fish distribution and the productivity of marine and freshwater species. This has impacts on the sustainability of fisheries and aquaculture, on the livelihoods of the communities that depend on fisheries, and on the ability of the oceans to capture and store carbon. The effect of sea level rise means that coastal fishing communities are in the front line of climate change, while changing rainfall patterns and water use impact on inland (freshwater) fisheries and aquaculture. The full relationship between fisheries and climate change is difficult to explore due to the context of each fishery and the many pathways that climate change affects.

Environmental issues with coral reefs

Human impact on coral reefs is significant. Coral reefs are dying around the world. Damaging activities include coral mining, pollution, overfishing, blast fishing, the digging of canals and access into islands and bays. Other dangers include disease, destructive fishing practices and warming oceans. Factors that affect coral reefs include the ocean's role as a carbon dioxide sink, atmospheric changes, ultraviolet light, ocean acidification, viruses, impacts of dust storms carrying agents to far-flung reefs, pollutants, algal blooms and others. Reefs are threatened well beyond coastal areas. Climate change, such as warming temperatures, causes coral bleaching, which if severe kills the coral.

The resilience of coral reefs is the biological ability of coral reefs to recover from natural disturbances such as storms and bleaching episodes. Resilience refers to the ability of biological or social systems to overcome pressures and stresses by maintaining key functions through resisting or adapting to change. Reef resistance measures how well coral reefs tolerate changes in ocean chemistry, sea level, and sea surface temperature. Reef resistance and resilience are important factors in coral reef recovery from the effects of ocean acidification. Natural reef resilience can be used as a recovery model for coral reefs and an opportunity for management in marine protected areas (MPAs).

Effects of global warming on oceans

Effects of global warming on oceans provides information on the various effects that global warming has on oceans. Global warming can affect sea levels, coastlines, ocean acidification, ocean currents, seawater, sea surface temperatures, tides, the sea floor, weather, and trigger several changes in ocean bio-geochemistry; all of these affect the functioning of a society.

Anomastraea is a monotypic genus of corals in the family Coscinaraeidae.. It is represented by a single species, the crisp pillow coral.

Symbiodinium trenchi is an endosymbiotic dinoflagellate, a unicellular alga which commonly resides in the tissues of tropical corals. It has a greater tolerance to fluctuations in water temperatures than do other species in the genus. It was named for the marine biologist R. K. Trench.

Impacts of ocean acidification on the Great Barrier Reef Threat to the reef which reduces the viability and strength of reef-building corals

Ocean acidification threatens the Great Barrier Reef by reducing the viability and strength of coral reefs. The Great Barrier Reef, considered one of the seven natural wonders of the world and a biodiversity hotspot, is located in Australia. Similar to other coral reefs, it is experiencing degradation due to ocean acidification. Ocean acidification results from a rise in atmospheric carbon dioxide, which is taken up by the ocean. This process can increase sea surface temperature, decrease aragonite, and lower the pH of the ocean.

Anthony Larkum

Anthony W D Larkum is a plant scientist and academic based in Sydney. He is Professor Emeritus of Plant Sciences at the University of Sydney and Adjunct Professor at the University of Technology Sydney (UTS).