But hard coral cover does not tell the whole story of a reef.
Changes are often given as relative increases or declines: the percentage gained or lost from the previous level of coral cover – a percentage of a percentage.
Hard coral cover is the proportion of the reef substrate – the sea floor – that is covered by living hard corals, expressed as a percentage.
A coral cover of 25 per cent means a quarter of the surveyed reef area is covered in living hard coral.
When temperatures over reefs rise, corals become stressed and expel the zooxanthellae that give them their colour, leaving them bleached white.
Scientists often use it because it is the easiest measure for monitoring teams and does not require detailed taxonomic or species knowledge. But hard coral cover does not tell the whole story of a reef. Changes in coral species or a decline in reef fish can also signal degraded reef health.
Changes are often given as relative increases or declines: the percentage gained or lost from the previous level of coral cover – a percentage of a percentage. If cover falls from 40 per cent to 30 per cent, that is a drop of 10 percentage points, but a relative decline of 25 per cent.
Hard coral cover is the proportion of the reef substrate – the sea floor – that is covered by living hard corals, expressed as a percentage. A coral cover of 25 per cent means a quarter of the surveyed reef area is covered in living hard coral.
What is a coral and why does it bleach?
Are corals animals, plants or rock? In a way, all three.
The coral itself is an animal, made up of tiny polyps that use tentacles to catch food from the water. But most of its energy comes from a symbiotic relationship with photosynthesising algae called zooxanthellae, which live inside its tissues. As the polyps secrete calcium carbonate to build protective skeletons, they construct the massive, rock-like structures that form reefs.
When temperatures over reefs rise, corals become stressed and expel the zooxanthellae that give them their colour, leaving them bleached white.
Bleached corals are not dead. But if temperatures stay high for too long, they can starve, since they get more than 90 per cent of their energy from zooxanthellae.
Heat is the main stress on reefs, but not the only one. The report notes that heat stress is “compounded by local and regional-scale environmental and anthropogenic stressors”, including “often chronic pressures from pollution, coastal development, and unsustainable resource use”.
It adds: “Increased sedimentation, eutrophication, coastal acidification, and overfishing can have cumulative effects on key drivers of recovery, including reduced coral recruitment and diversity, and in increased macroalgal abundance, coral disease and coral predation.”
