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Sri Lanka: One Island Two Nations
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Sri Lanka: One Island Two Nations
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Thiranjala Weerasinghe sj.- One Island Two Nations
?????????????????????????????????????????????????Sunday, April 7, 2019
Coral reproduction on the Great Barrier Reef falls 89% after repeated bleaching
Morgan Pratchett-April 3, 2019 3.17pm EDT
The severe and repeated bleaching of the Great Barrier Reef has not only damaged corals, it has reduced the reef’s ability to recover.
Our research, published today in Nature,
found far fewer baby corals are being produced than are needed to
replace the large number of adult corals that have died. The rate at
which baby corals are settling on the Great Barrier Reef has fallen by
nearly 90% since 2016.
While coral does not always die after bleaching, repeated bleaching has killed large numbers of coral. This new research has negative implications for the Reef’s capacity to recover from high ocean temperatures.
How coral recovers
Most corals reproduce by “spawning”:
releasing thousands of tight, buoyant bundles with remarkable
synchronisation. The bundles burst when they hit the ocean surface,
releasing eggs and/or sperm. Fertilised eggs develop into larvae as they
are moved about by ocean currents. The larvae settle in new places,
forming entirely new coral colonies. This coral “recruitment” is
essential to reef recovery.
The research team, led by my colleague Terry Hughes from the ARC Centre
of Excellence for Coral Reef Studies, measured rates of coral
recruitment by attaching small clay tiles to the reef just before the
predicted mass spawning each year. These settlement panels represent a
standardised habitat that allows for improved detection of the coral
recruits, which are just 1-2mm in size.
Almost 1,000 tiles were deployed across 17 widely separated reefs after
the recent mass bleaching, in late 2016 and 2017. After eight weeks they
were collected and carefully inspected under a microscope to count the
number of newly settled coral recruits. Resulting estimates of coral
recruitment were compared to recruitment rates recorded over two decades
prior to the recent bleaching.
Rates of coral recruitment recorded in the aftermath of the recent coral
bleaching were just 11% of levels recorded during the preceding
decades. Whereas there were more than 40 coral recruits per tile before
the bleaching, there was an average of just five coral recruits per tile
in the past couple of years.
Reef resilience
The Great Barrier Reef (GBR) is the world’s largest reef system. The
large overall size and high number of distinct reefs provides a buffer
against most major disturbances. Even if large tracts of the GBR are
disturbed, there is a good chance at least some areas will have healthy
stocks of adult corals, representing a source of new larvae to enable
replenishment and recovery.
Larvae produced by spawning corals on one reef may settle on other
nearby reefs to effectively replace corals lost to localised
disturbances.
It is reassuring there is at least some new coral recruitment in the
aftermath of severe bleaching and mass mortality of adult corals on the
GBR. However, the substantial and widespread reduction of regrowth
indicates the magnitude of the disturbance caused by recent heatwaves.
Declines in rates of coral recruitment were greatest in the northern parts of the GBR. This is where bleaching was most pronounced in 2016 and 2017,
and there was the greatest loss of adult corals. There were much more
moderate declines in coral recruitment in the southern GBR, reflecting
generally higher abundance of adults corals in these areas. However,
prevailing southerly currents (and the large distances involved) make it
very unlikely coral larvae from southern parts of the Reef will drift
naturally to the hardest-hit northern areas.
It is hard to say how long it will take for coral assemblages to recover
from the recent mass bleaching. What is certain is low levels of coral
recruitment will constrain coral recovery and greatly increase the
recovery time. Any further large-scale developments with also greatly
reduce coral cover and impede recovery.
Reducing carbon emissions
This study further highlights the vulnerability of coral reefs to
sustained and ongoing global warming. Not only do adult corals bleach
and die when exposed to elevated temperatures, this prevents new coral
recruitment and undermines ecosystem resilience.
The only way to effectively redress global warming is to immediately and
substantially reduce global carbon emissions. This requires that all
countries, including Australia, renew and strengthen their commitments
to the Paris Agreement on climate change.
While further management is required to minimise more direct human pressure on coral reefs – such as sediment run-off and pollution – all these efforts will be futile if we do not address global climate change.