22 July, 2021
Reef in recovery but still under pressure
AFTER a series of severe and widespread disturbances over the last decade, the Great Barrier Reef is currently in a recovery window with coral cover rising in all three regions.
Published this week, the Australian Institute of Marine Science’s (AIMS) Long-Term Monitoring Program - Annual Summary Report on Coral Reef Condition for 2020/21 shows after a year’s reprieve, with no major pressures from heat stress or cyclones, widespread recovery is underway.
AIMS CEO Dr Paul Hardisty said AIMS’ 35-year history of monitoring the Reef shows increases in coral cover are expected during periods of low disturbance.
“Coral reefs can recover from disturbances if given enough time and the Reef has been given a breather over the last year,” he said.
“Another year like this will continue the Reef’s recovery but the increasing prominence of climate related extreme weather events and crown-of-thorns starfish outbreaks is causing more severe and frequent pressures, giving the Reef fewer opportunities like this to recover.”
Since 2009, the Great Barrier Reef has been hit hard with three mass coral bleaching events, the fourth wave of crown-of-thorns starfish outbreaks and 17 cyclones that potentially exposed reefs to damaging waves. Hard coral cover dipped to a record low in all regions at different times during this period.
AIMS monitoring program team leader Dr Mike Emslie said 127 reefs were surveyed in 2021, with 81 of those reefs previously surveyed in the last two years. Over this period, 69 of these reefs had increased in hard coral cover across the Northern, Central and Southern regions of the Great Barrier Reef.
Surveys showed much of the increase was driven by fast growing Acropora corals, known as table and branching corals, which are common to many parts of the Great Barrier Reef.
“We found once these dominant corals re-established after disturbances, they hit a period of exponential growth which has led to the increases we see this year,” Dr Emslie said.
“However, while they are fast to grow, they are often the first to go – they are susceptible to cyclones, coral bleaching and are the favourite food for crown-of-thorns starfish.
“Because of these vulnerabilities and likelihood of more climate-related severe weather events, future disturbances may result in rapid decline on these reefs.”
Each region of the Reef has been affected by a different set of disturbances over the last decade, and the path to recovery for each region is unique.
While conditions vary in the Northern Great Barrier Reef, region-wide hard coral cover has continued to increase to 27% (classified as ‘moderate’) this year. This is an improvement from the lowest levels (13%) recorded by AIMS in 2017 following cumulative effects of severe tropical cyclones in 2014 and 2015, crown-of-thorns starfish outbreaks, and the 2016 and 2017 mass coral bleaching events.
For the Central region, hard coral cover reached its lowest recorded level in 2012 (11%), following the impact of Severe Tropical Cyclone Yasi. The central region now has an average hard coral cover of 26% (classified as ‘moderate’).
In the Southern region, the lowest coral cover level was recorded in 2011 (12%) following impacts from Severe Tropical Cyclone Hamish. This year’s data shows average hard coral cover was estimated at 39% (classified as ‘high’).
The Southern Great Barrier Reef, which escaped the bleaching events of 2016 and 2017, was also affected by the mass bleaching 2020, but minimal coral mortality was observed on the survey reefs.
Dr Hardisty said the Great Barrier Reef remains under continued pressure from climate change and requires help to survive into the future.
“The Reef has shown its ability to recover after disturbances before, but such resilience has limits,” he said.
“Continued long-term monitoring to understand how the Reef responds to disturbances is critical to its protection, along with a reduction in global emissions, and continued good environmental management.”