Corals can shade themselves when sea temperatures rise by creating their own ‘cloud umbrellas’ – but new research has shown there is a tipping point and it could lead to more bleaching events.
The new paper published in Scientific Reports by a Griffith University team – PhD candidate Rebecca Jackson, Associate Professor Albert Gabric and Dr Roger Cropp – builds upon a previous paper published in AMBIO that shed light on the ability of corals to contribute to aerosol formation in the atmosphere when stressed.
The aerosols are produced when stresses such as high irradiance, high temperatures or low-tide exposure prompt corals to release volatile compounds into the sea that undergo a series of reactions to become aerosols in the air above them, potentially contributing to the formation of low-level clouds.
However, the latest paper has cast a closer eye over that relationship and revealed that if sea temperatures become too high, the ability for corals to protect themselves breaks down.
Jackson said a review of anomalies in the amount of aerosols above the reef during four bleaching events, the two last of which happened in 2016 and 2017, confirmed that the amount of aerosols above the reef increased as temperatures began to rise prior to the bleaching event.
“But this was followed by a sharp decline to below average levels of aerosols after the corals expelled their symbiotic algae and become bleached and no longer released dimethylsulfide (DMS),” Jackson said.
“So this suggests that corals can influence the amount of aerosols and potentially the amount of cloud cover above them – which is the main idea of this ‘cloud feedback’ – but when ocean temperatures become too warm corals stop emitting DMS to the atmosphere and instead use its precursor dimethylsulfoniopropionate (DMSP) as a kind of antioxidant to try and cope with oxidative stress.”
The team studied the aerosol emissions via satellite data from the 2002, 2006, 2016 and 2017 bleaching events that took place from four sites across the Great Barrier Reef, from its far north to as far south as Heron Island.
Jackson said the idea of corals themselves producing DMS, not just their symbiotic algae, to protect themselves is a relatively new school of thought. And in other regions around the world it’s well recognised that phytoplankton produce DMS, but the process is less well associated with coral reefs.
“This discovery could have some serious implications for coral reefs with ongoing ocean warming,” Jackson said.
“If the amount of coral-reef derived aerosol declines, cloud cover could decrease and lead to more extreme sea surface temperatures and potentially more coral bleaching.
“It highlights the importance of future work to understand the complex ocean-atmosphere interactions involved and the role of marine biogenic aerosols in climate over the GBR.”