Coral bleaching

Coral bleaching is the whitening of corals, due to stress-induced expulsion or death of symbiotic unicellular algae or due to the loss of pigmentation within the algae^[1]. The corals that form the structure of the great reef ecosystems of tropical seas depend on a symbiotic relationship with photosynthesizing unicellular algae called zooxanthellae that live within their tissues. Zooxanthellae give coral its coloration, depending on the particular clade. Under stress, corals may expel their zooxantheallae, which leads to a lighter or completely white appearance, hence the term "bleached".^[2]

Bleached corals

Healthy corals

Once bleaching begins, it tends to continue even without continuing stress. If the coral colony survives the stress period, zooxanthellae often require weeks to months to return to normal density ^[3]. The new residents may be of a different species. Some species of zooxanthellae and corals are more resistant to stress than other species.

Causes of Coral Bleaching

Coral bleaching is a vivid sign of corals responding to stress which can be induced by any of:

• increased (most commonly) or reduced water temperatures^[4][5]
• increased solar irradiance (photosynthetically active radiation and ultraviolet band light)^[6]
• changes in water chemistry (in particular acidification)^[7][8]
• starvation caused by a decline in zooplankton.^[9]
• increased sedimentation (can be contributed to silt runoff)
• pathogen infections
• changes in salinity
• wind^[5]
• low tide air exposure^[5]

Temperature change

Temperature change is the most common cause of coral bleaching.^[4]

Large coral colonies such as Porites lobata are able to withstand extreme temperature shocks, while fragile branching corals, such as Acropora spp. are far more susceptible to thermal stress following a temperature event ^[10]. Recent research showed that corals consistently exposed to low stress levels may be more bleaching resistant.

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. Coral and zooxanthellae health and genetics also influence bleaching.^[11]

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 degree Celsius or more above the long-term monthly average. Some argue this system detected the massive 1998 bleaching event that was worldwide.^[12][13] This corresponded to an El Niño event. At the same time, NOAA Coral Bleaching "Hotspot" program uses a 50k satellite resolution at nighttime, which some argue covers too large of a spatial area and does not incorporate the maximum sea surface temperatures occurring usually around height of daytime, noon.

Changes in ocean chemistry

Increasing acidification and infection^[14] likely exacerbate the bleaching effects of thermal stress.

Infectious disease

Bioerosion (coral damage) such as this may be caused by coral bleaching.^[15]

In 1996, Kushmaro, et al.^[16] report that the bleaching agent of Oculina patagonica in the Mediterranean Sea was infectious bacteria attacking the algae. The agent was later identified as Vibrio shiloi. 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.^[14][17] Then V. shiloi penetrates the coral's epidermis, multiplies there, and finally transforms into a “viable but non-culturable” (VBNC) state, producing both heat-stable and heat-sensitive toxins, affecting 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 ^[18]. The main hypothesis for the emerged resistance is the presence of symbiotic communities of bacteria living with the corals. One species capable of lysing V. shiloi has gained prominence. This hypothetical bacteria has not been identified.

Impact

In the 2010-2040 period coral reefs will become highly susceptible to more frequent bleaching events. The IPCC sees this as the greatest threat to the world's reef systems.^{[19][20][21][22]}

Great Barrier Reef

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.

The Great Barrier Reef (GBR) along the coast of Australia experienced bleaching events in 1980, 1982, 1992, 1994, 1998, 2002, and 2006.^[22] While most areas recovered with relatively low levels of coral death, some locations suffered severe damage, with up to 90% of corals killed.^[7]

The most widespread and intense events occurred in the summers of 1998 and 2002, affecting about 42% and 54% of reefs, respectively.^[23][24]

The IPCC's Moderate warming scenarios (B1 to A1T, 2°C by 2100, IPCC, 2007, Table SPM.3, p. 13^[25]) forecast that corals on the GBR are very likely to regularly experience summer temperatures high enough to induce bleaching.^[23]

Other Areas

Other 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.

Evidence from extensive research in the 1970’s of thermal tolerance in Hawaiian corals and of oceanic warming led researchers in 1990 to predict mass occurrences of coral bleaching throughout Hawaii. Major bleaching occurred in 1996 and in 2002.^[26]

Coral in the south Red Sea does not bleach despite summer water temperatures up to 34°C.

Significant bleaching occurred in the Mediterranean Sea in 1996.

See also

Ove Hoegh-Guldberg (biologist)

Notes and references

External links

• Great Barrier Reef Marine Park Authority information on bleaching.
• ReefBase: a global information system on coral reefs.
• More details on coral bleaching, causes and effects.
• Travellers Impressions
• The Link between Overfishing and Mass Coral Bleaching
• Discussion on Overfishing and Coral Bleaching
• Social & Economic Costs of Coral Bleaching from "NOAA Socioeconomics" website initiative
• Microdocs: Coral bleaching

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