17 Apr Coral Watch: Bleaching Observed in Lease Islands
In early April this year, Professor Terry Hughes of James Cook University announced that the Great Barrier Reef (GBR) was experiencing its third coral bleaching event in five years. For the first time, he said, bleaching was affecting all three regions of the GBR in Australia.
At about the same time that Professor Hughes was conducting the aerial surveys that led to this conclusion, CTC’s scientific team was observing bleached corals some 2,000 kilometers away. In the Lease Islands, Maluku, a reef health monitoring (RHM) survey revealed that 2–15% of hard coral cover was fully or partially bleached with an average of 7% across all monitoring sites. Soft corals and anemones were also affected.
According to NOAA’s Coral Reef Watch, Maluku began to experience unusually high sea surface temperatures in late November. Temperatures peaked in December at about 31°C before subsiding somewhat, leveling out at values still above the maximum monthly average. By the time of our survey in mid-March, corals had been experiencing heat stress for several months. However, conditions were improving, reflected by a falling value for degree heating weeks (DHW), a cumulative measure of the intensity and duration of heat stress. Between December and March, DHW exceeded the threshold where bleaching is likely but avoided topping the threshold where severe, widespread coral bleaching and mortality are predicted. With luck, water temperatures will continue to fall as the southeast monsoon develops.
Corals can recover from bleaching if the intensity and duration of heat stress are not too extreme.
Lease Islands reefs have proven resilient to stressors over hundreds or thousands of years, as suggested by the monumental size of some of their coral colonies. During our RHM survey, we found a diversity of corals that rivaled, and in some cases surpassed, that of the Banda Islands, no small ecological footnote. Reef sharks, trevally, Napoleon wrasse and hawksbill turtles all made cameo appearances. We hope to return and find the signs of bleaching gone. With the climate changing at a pace ten times faster than at any other time in the last 65 million years, however, it may not be long before bleaching returns.
Photos by: Andreas Muljadi and Purwanto/CTC; DHW chart from NOAA Coral Reef Watch