Marine heatwaves kill corals quicker than previously thought

Australia’s Great Barrier Reef, a World Heritage site, is one of the most iconic coral reefs. But it’s in serious trouble due to bleaching: a new Australian Government report has officially downgraded the reef’s outlook from poor to very poor.

Coral bleaching is caused by global heating and leads corals to expel vital algae that live in its tissues, resulting ultimately in the death of these animals.

Now, according to scientists, there is an even more deadly threat—marine heatwaves—which destroy corals much more rapidly than previously thought.

A new study indicates that marine heatwaves should be considered a distinct biological phenomenon from bleaching events on coral reefs.

What happens is that in warmer water, within days, corals attract bacteria which speed up the breakdown of corals, says the study.

The New Caledonia barrier reef is also a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage site and the second largest in the world after the Great Barrier Reef. A study carried out there by the Institut de Recherche pour le Developpement and supported by UNEP, is trying to understand resilience to coral bleaching.

Corals there have been observed to glow in luminescent colours just before they die, a phenomenon that has sparked a Glowing Campaign in which UNEP has partnered with The Ocean Agency, among other leading ocean conservation organizations.

The campaign highlights the fluorescing phenomenon that some corals experience to protect themselves during extreme ocean heatwaves and encourages designers to use three newly created “glowing colours” in their art to draw attention to the issue.

Nutrient and plastic pollution

Global heating is not the only stressor of corals and the marine environment. Pollution is also a major cause of the unprecedented levels of decline in reef health and coral cover across the globe.

Plastic makes up a sizeable proportion of marine pollution. It can be found in all the world’s oceans, but it is thought to be in highest concentration in coastal areas and reef environments, where vast amounts of litter flow in from land-based sources.

A 2019 UNEP study Plastics and shallow water coral reefs. Synthesis of the science for policy-makers identifies a number of knowledge gaps that need to be addressed in order to strengthen the scientific evidence base for action on marine plastics that impact coral reefs, and to achieve the targets set by the global community.

These gaps include understanding the impacts of leaching chemicals from plastics in coral reef environments and exploring the level of risk microplastics have on reef organisms.

“Our current understanding is that although they are significantly damaging, plastics are not the number one threat to coral reefs,” says Grimsditch. “Nutrient and wastewater pollution can cause more damage to coral reef ecosystems by promoting the growth of macroalgae (seaweed) and coral diseases… throwing the whole system off balance.”

As UNEP’s 2017 report, Wastewater pollution on coral reefs points out, “more than 80 per cent of marine pollution originates from land-based wastewater and sediment and nutrients delivered via waterways.”

UNEP’s Clean Seas campaign addresses the root-cause of marine litter by targeting the production and consumption of non-recoverable and single-use plastic. 

UNEP's Wild for Life campaign raises awareness for the plight of coral species threatened by illegal trade and other pressures. Indonesian singer songwriter Raisa Andriana champions coral.

The UN Climate Action Summit takes place in New York City on 23 September 2019 to increase ambition and accelerate action on the global climate emergency and support the rapid implementation of the Paris Climate Change Agreement. The 2019 UN Climate Action Summit is hosted by UN Secretary-General António Guterres.

For more information, please contact Gabriel Grimsditch