Coral bleaching is the movement of the symbiotic algae zooxanthellae out of the coral polyp. It is termed bleaching as the colouration is due to the different pigments of algae and when that leaves, a lack of colour is apparent and the calcium carbonate skeleton is visible.  The zooxanthellae provide the polyp with its energy and also aim with nutrient recycling.

bleached coral on Great barrier reef white in colour

Figure 6: A bleached branching species of coral at the Great Barrier Reef

There could be several reasons why the corals bleach:

  • unfavorable conditions in the host cell due to cell shrinkage or photosystem damage
  • zooxanthellae stop translocating; so they’re not necessary to the corals therefore they are rejected
  • The host experiences intracellular damage, when the algae leaves the light is reflected within the cell damaging it from the inside and causing toxic amounts of O2 to be produced.

If the warming event time period is short the corals may recover the zooxanthellae. If the period is long the corals may never recover causing mortality on the reef or have serious effects in other ways such as reproduction (Wilkinson & Souter 2008). Corals are thought to have seasonal thermal thresholds; decreasing in the winter and rising again in the summer. Above these thresholds is when the bleaching occurs (Weeks et al. 2008)

A mass bleaching event occurred after the 1997-1998 El Niño–Southern Oscillation (ENSO). A study by Baker et al. (2004)  revealed that more corals had a higher percentage of Clade D. Clade D is a form of algae symbiont that the coral contains which could be more tolerable to warmer temperatures. It seems that corals can expel one type of symbiont and exchange it for a ‘better’ more adapted strain.  The corals that contained this strain of algae weren’t affected by the bleaching event.  This could be an extremely rapid adaptive measure to cope with sudden temperature changes in the upper levels of the sea surface (Baker et al. 2004). Corals containing Clade C seemed to not cope with the temperature and as a result bleached.

When corals bleach the ecosystem as a whole becomes severely less productive as the primary producers are producing less oxygen for the water and less food available for the fish and other organisms that live there. Once there is a decrease in fish on the reefs there will be a decrease in the value due to lack of fishing and recreational diving that can occur.

Bleaching events like the one after the 1997 ENSO makes the corals more vulnerable to other threats and disturbances therefore decreasing the resilience and ability to recover.

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