A study with a new assessment of coral health is published in the journal Proceedings of the Royal Society B. Scientists at James Cook University in Queensland examined how coral communities and coral sizes changed across the length of the reef from 1995 to 2017, and found declines in small, medium and large large corals both in deep water and in shallow water "in almost all species".
The reef, which is a UNESCO World Heritage Site, was particularly hard hit by record temperatures in 2016 and 2017, which caused massive coral bleaching.
Bleaching occurs when overheating or too much light causes stress on the corals. As a result, they displace the symbiotic algae living in their pores, and this makes the coral white. This does not always kill the corals: they may regenerate over time, but it makes them much more vulnerable. In 2016, a third of the world's coral reefs were bleached. Only 10% of the corals of the Great Barrier Reef have escaped this. The same thing happened a year later.
Changing the size of corals affects their survival and reproduction. As the authors of the study write, “There are millions of small 'baby' corals in the living coral population, as well as many adult 'mothers' who produce most of the larvae. Our results indicate that the Great Barrier Reef's ability to regenerate, or resilience, is at risk compared to past years because there are fewer “children” and fewer large breeding adults now. We used to think that the Great Barrier Reef is already protected by its size, but our results show that even the largest and relatively well protected reef system in the world is increasingly at risk and declining”.
Scientists believe the reefs are affected by the climate crisis, causing the ocean to warm. “There is no time to waste — we must dramatically reduce greenhouse gas emissions as soon as possible”, - the study authors said.