"35% of the northern reef is dead"
High water temperatures at the beginning of 2016 resulted in extreme coral bleaching effects. Only 7% of the 2,300-km reef system was completely spared the 'heat shock' of the hottest summer on record. The northern regions between Cairns and Papua New Guinea suffered particularly badly. "35% of the northern reefs we have inspected are dead,” concluded Professor Terry Hughes, director of the ARC Coral Reef Studies Centre at James Cook University, in June following his observations in the region. He found that the corals in the south had recovered distinctly better: “There, only about 5% has been permanently destroyed.”
The causes of the worst coral bleaching seen to date can be found in an El Niño year and the higher water temperatures produced by global warming. The heat initially affects the tiny algae, zooxanthellae, that live symbiotically with the corals. Under heat stress they produce toxins which cause the mollusks to reject them. Without the zooxanthellae, however, the coral reefs lose their color and turn white. If the temperature subsequently drops, new algae can colonize the reefs and the coral may survive. Long-term studies following similar bleaching events in 1998 and 2002 showed that some corals develop resistance to the temperature stress and learn, so to speak, to cope with it. However, constant rises in temperature increase the risk of permanent damage.
On the Opal Reef east of Port Douglas, small waves lap over the outboard steps of the Calypso Ten. “Normally the water would only be 22° by now in June,” says Lisa MacLeod, who has just spent a good 1,000 hours at sea for her captain's license. She is now standing barefoot on the stern deck keeping an eye on the divers. “But we are still registering 26° here.” Pleasant for the snorkelers flippering away from the catamaran – not so nice for the corals, which need cooler water to regenerate. The fish population at the spot the divers call 'Bashful Bommie' where the boat is anchored so far appears hardly diminished. Some of the snorkelers follow Rhiannon Percival over the broad plate corals to an enormous giant clam and wait for its corrugated shell to open. The diving instructor explains to the astonished party how the clam filters suspended matter though its turquoise-patterned mantle lobes. Escorted by a respect-demanding Napoleon fish, the snorkelers swim back to the boat.