There are roughly 400 species of corals on the Great Barrier Reef, and most of them have a symbiotic relationship with zooxanthellae algae – a microscopic plant that lives inside the tissue of the coral in exchange for providing the coral with up to 90% of its energy requirements.
Like most plants, zooxanthellae photosynthesize – using sunlight to turn water and carbon dioxide into carbohydrates (sugars) to feed themselves and the corals. This allows corals to grow much more quickly than they could if they relied on plankton for food. The skeletons of most corals are white – it’s the zooxanthellae living inside the tissue of the coral polyps that gives corals the browns, greens, purples and other colours we see out on the reef.
Zooxanthellae (“sounds like jelly”!) are fussy about their living conditions. They can’t survive in cold deep water, which is why corals reefs are found mainly in the tropics (although some corals that can survive in cold deep waters – for example off Scotland and Norway – they don’t have a relationship with zooxanthellae and grow too slowly to build the kind of reef systems we have along the Great Barrier Reef).
Unfortunately zooxanthellae don’t like hot water either. If the sea temperature becomes too warm, the algae develops heat-stress, and stops producing carbohydrates. However, it begins to produce toxic waste products which poison the coral until it rejects the zooxanthellae – expelling it into the open water. When this happens, the coral turns white, and this is known as ”coral bleaching“.
Bleached coral isn’t necessarily dead. If the sea temperature drops relatively quickly, the coral will survive and within a few months will have fully recovered its zooxanthellae.
However, if that sea temperature stays too high for too long (it’s impossible to generalise about how high and for how long because it depends on many variables, such as the particular strain of zooxanthellae, the species of coral and it’s geographic location, etc) the coral may not be able to feed itself effectively and the polyps will eventually die, leaving the dead, white, calcium-carbonate skeleton behind.
The dead coral will eventually become covered with other types of algae, leading to changes in the species of fish and other animals living on the reef. The number of algae-eating species (e.g. surgeonfish, sea urchins) on the reef may increase, while coral dependent species (like butterflyfish) may decline, alongside an overall decline in species and even local extinctions.
Bleaching can also occur (mainly on inshore reefs) after particularly heavy rainfall (for example during a cyclone), when a sharp drop in salinity causes stress in the algae. And other animals, like anemones and clams, nudibranchs and jellyfish, some of which also have a symbiotic relationship with zooxanthellae, can also bleach.
In 1998 and 2006 severe bleaching occurred around the world, caused by warmer than average sea temperatures linked to El Niño events. These widescale events are known as “mass bleaching“. Until 1979 only 3 mass bleaching events ahd been recorded around the world. In 2002 alone, 400 events mass bleaching events were recorded (UNEP) and by 2008 mass bleaching had occured in every coral reef region of the world.
In 1998 and 2002 the worst bleaching ever experienced along the Great Barrier Reef (GBR) was recorded. In 2002 55% of corals on the GBR were affected (GBRMPA). Between 1871-2005, water temperature along the GBR rose 0.4°C , and is predicted to rise 1.1-1.2°C by 2050. Scientists are concerned that the reef will experience more frequent and severe episodes of mass bleaching.
There are many factors that affect resistance and recovery from coral bleaching, and while some reefs may recover within a decade or two, others may never recover. The more stresses a reef is exposed to – for example, storm damage, pollutants, destructive fishing methods, the effects of ocean acidification – the less resistance and resilience it is likely to have to bleaching.
The Great Barrier Reef Marine Park Authority predicts a low risk of bleaching along the GBR for the summer of 2009-2010, however conditions are consistent with a weak El Niño event, and this has been associated with a higher likelihood of bleaching in the past.