‘It’s not supposed to be white’: one of the Great Barrier Reef’s healthiest reefs succumbs to bleaching

Through a snorkel mask, the corals struggling for survival in the heat are easy to spot. Some have turned white while others are pouring out a fluorescent pigment into their flesh - it's spectacular, but it's also a sign of a coral in deep distress.

This is John Brewer Reef about 70km off Townsville in Queensland - the centre of a widespread coral bleaching event. For hundreds of kilometres in either direction, reefs are going through the same battle.

As we float over one of about 3,000 individual reefs that make up the vast Great Barrier Reef on Sunday afternoon, two scientists are touching down in Brisbane for a 10-day United Nations monitoring mission.

Just to the south, there's another mission. Scientists are in the air, surveying the bleaching from above in a series of government-chartered flights that, by the middle of this week, will have traversed the entire length of the 2,300-km reef.

As the Guardian enters the water, the first thing Dr Adam Smith does is glance at his dive watch. It tells him the water is 29C. That is warm.

Staghorn corals clinging to the reef edge come into view and almost all are partly or entirely white.

Thousands of colourful reef fish, some graceful sharks, and lots of still-healthy corals share the space with the strugglers. Soft corals nestled in the gaps on the top of the reef are white.