30% of coral died in 'catastrophic' 2016 heatwave

Great Barrier Reef bleaching 2016–17

"We're now at a point where we've lost close to half of the corals in shallow-water habitats across the northern two-thirds of the Great Barrier Reef due to back-to-back bleaching over two consecutive years", said Sean Connolly, also with the center for coral reef studies at James Cook University. This was followed by another mass bleaching the following year. The south, in particular, escaped much of the bleaching in 2016 and 2017.

For Dr Heron, the increasing frequency of marine heatwaves on the Great Barrier Reef is a pressing issue.

The study was conducted by researchers at James Cook University and Lancaster University, in the United Kingdom, who examined 16 reefs off Lizard Island, in the northern section of the Great Barrier Reef.

"The coral die-off has caused radical changes in the mix of coral species on hundreds of individual reefs, where mature and diverse reef communities are being transformed into more degraded systems, with just a few tough species remaining", he said.

Professor Terry Hughes, Director of the ARC Centre of Excellence for Coral Reef Studies, said: "When corals bleach from a heatwave, they can either survive and regain their colour slowly as the temperature drops, or they can die".

First, aerial surveys from planes generated thousands of videos of the reef. These were combined with underwater surveys at over 100 locations.

Finally, teams of divers took samples of corals and investigated their physiology in the laboratory.

That's because corals take a long time to recover after bleaching events - the fastest ones might take a decade, while the slower ones take centuries. Not before yet another bleaching event occurs.

We measure this accumulated heat stress in "degree heating weeks" (DHW) - the number of degrees above the average summer maximum, multiplied by the number of weeks. The reef is unlikely to avoid additional bleaching events before damaged coals can recover.

Global coral bleaching had been observed just twice, in 1998 and 2002, before the extreme 2016 incident.

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If warming continues apace: "Then it's game over".

Coral bleaching occurs when corals expel the tiny algae (called zooxanthellae) that live inside them and provide their food.

This study shows clearly that the structure of coral communities in the northern sector of the reef has changed dramatically, with a predominant loss of branching corals.

Still, most fish depend on branching corals as a nursery - the type of corals that suffered high mortality in the heatwave. This loss of hiding places is one of the reasons for the reduction of fish populations following severe bleaching events.

The study is unique because it tests the emerging framework for the International Union for Conservation of Nature (IUCN) Red List of Ecosystems, which seeks to classify vulnerable ecosystems as Safe, Threatened or Endangered. These cover the requirements for such a listing. Replacement of lost corals through recruitment relies on healthy coral larvae arriving and finding suitable settlement substrate. Overall, these events have affected every part of the reef.

"In the northern, 700-km-long section of the Great Barrier Reef in which the heat exposure was the most extreme, 50.3 percent of the coral cover on reef crests was lost within eight months". We can save this reef, but the time to act is now.

The scientists said their research underscores the need for further risk assessment into the collapse of reef ecosystems, especially if global action on climate change fails to limit warming to a 1.5C to 2C increase on pre-industrial levels.

The paper says extreme weather events, caused by man-made climate change, are "rapidly emerging as major contemporary threats to nearly all ecosystems", and suggests that while not all corals will be killed off, the shape and variety will be affected.

Prof Jörg Wiedenmann from the University of Southampton's Coral Reef Laboratory agrees that, if present conditions continue, significant change lies ahead.

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