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What are the causes of coral bleaching?
Distribution of coral bleaching events, 1998 |
A high concentration of zooxanthellae is vital for coral growth, as healthy corals contain millions of zooxanthellae per square centimeter of live tissue (Glynn 2006). Consequently, reefs can only grow where these dinoflagellates thrive. Zooxanthellae have a limited tolerance zone for certain conditions, including water temperature and ultra-violet light. If these conditions are altered and zooxanthellae are unable to photosynthesize, coral cannot survive. Corals live in water between twenty-five and twenty-nine degrees Celsius and are typically found in shallow depths where zooxanthellae can receive sufficient sunlight to photosynthesize (Brown 1996). Numerous lab experiments have shown that a drop or raise of just two degrees Celsius for a prolonged period of time can incite bleaching, as the slight change in temperature disrupts enzymes in zooxanthellae that are necessary in photosynthesis and thus the zooxanthellae cannot provide adequate nutrition for the coral (Brown and Ogden 1993). Transitively, a reduction in zooxanthellae levels in coral leaves it vulnerable to ultra-violet radiation; zooxanthellae produce amino acids that absorb and block damaging ultra-violet light. Therefore, when zooxanthellae concentrations are low, there is a deficiency in absorbing compounds and the coral is bleached (Gleason 1993). |
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Reefs are a particularly heterogeneous environment both spatially and temporally, and thus it is difficult to denote a specific factor responsible for bleaching in nature with certainty in the lab (Brown 1997). However, studies have now shown a correlation between elevated ocean temperatures and increased coral bleaching in the Caribbean, Pacific, and Indian Oceans. Bleaching seems to occur when the ocean exceeds a threshold temperature that is greater than the typical seasonal variation. For example, extensive bleaching occurred in Thailand in 1991 and 1995 and French Polynesia in 1991 and 1994, which coincided with increased temperatures in both regions above the typical maximum (see figure above) (Brown 1996). In addition, fifty reefs that have been monitored since 1870 showed the greatest bleaching in recorded history during the El Nino Southern Oscillation (ENSO) of 1997-1998, further providing field evidence for the correlation between increased temperature and coral bleaching (see figure to left) (AIMS 2005). Ultra-violent radiation alone has also been shown in both the field and lab to induce coral bleaching, and this is of particular concern because global warming causes changes in tropical weather patterns which results in less cloud cover and thus more radiation (Brown 1996). In fact, reduced cloud cover may affect solar radiation in many locations more dramatically than ozone depletion (Bachelet et al. 1991). Therefore, global warming may damage reefs by two separate mechanisms. |
| Due to the sensitivity of zooxanthellae to temperature and ultra-violet radiation, anthropogenic impacts affecting these conditions have induced bleaching on a global scale. Over the past century, there has been a steady increase in coral bleaching (attached chart) which is strong evidence that exponential coral reef decline is related to anthropogenic impacts (AIMS 2005). Global warming due the emission of Greenhouse gasses has increased water temperatures worldwide, and ozone depletion and depleted cloud cover in tropical regions has increased the amount of ultra-violet radiation that reaches Earth (Glynn 1996). Coral reefs are currently at a very vulnerable state after the ENSO that occurred in 1997-1998, which caused severe coral bleaching in over fifty countries due to dramatic temperature changes (Wilkinson 2005). As average temperatures continue to rise as predicted, natural variations in ocean temperature, even due by minor ENSO events which happen every few years, may cause the ocean temperature to exceed the bleaching threshold for coral. Currently, sixty percent of coral reefs are at risk, and it is predicted that ninety-five percent of the Great Barrier Reef will be bleached by 2050 (AIMS 2005). Since coral is at a particularly unstable state, coral reef conservation is more important now than ever, as anthropogenic pollution may likely increase due to an increasing human population. |
