PARIS (AFP) – A sharp rise in sea temperature can inflict potentially catastrophic “bleaching” on corals, but research published Wednesday identified factors that may render some reefs more resilient than others.
Five conditions can determine whether or not a reef is doomed after bleaching – episodes that threaten a valuable source of biodiversity, tourism and fishing, scientists reported in the journal Nature.
“Water depth, the physical structure of the reef before disturbance, nutrient levels, the amount of grazing by fish and survival of juvenile corals could help predict reef recovery,” said Nicholas Graham at Australia’s James Cook University, who headed the probe.
Bleaching occurs when reef symbiosis – the mutually beneficial relationship between two organisms that inhabit corals – is disrupted by a surge in ocean warming, although there can also be other causes.
One of the worst episodes of mass bleaching, which affected reefs in 60 tropical countries, took place in 1998, a year of an exceptionally strong El Nino weather pattern.
Corals depend on single-cell algae called dinoflagellates that live in vast colonies on their surface.
The dinoflagellates feed on nitrogen, phosphorus and other nutrients provided by the coral, and use light to transform this food into energy.
The photosynthesis also releases energy into the tissues of the coral, enabling it to build the calcium skeleton which houses the dinoflagellates.
When corals come under stress, such as from significantly warmer seas, they expel the dinoflagellates. The corals turn visibly pale, as the algae have the pigments which give the skeletons their distinctive colour.