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mortality data from October-November because corals
can survive a bleaching event, and many of the corals
on the GBR did just that.
Coral bleaching occurs when corals are stressed, in
this case from high water temperatures. Corals have
a mutualistic symbiotic relationship with an algae
called zooxanthellae that lives within the coral tissue.
Zooxanthellae use sunlight to photosynthesize food;
they share this food with their coral hosts. In turn, the
zooxanthellae have a safe place to live, and they receive
“fertilizer” via coral waste. While corals do feed on
their own, they rely on zooxanthellae to get sufficient
nourishment. Zooxanthellae also give corals their
color; without the algae, corals are actually clear (think
of jellyfish, which are relatives of coral polyps). When
subject to stress factors, the relationship between the
coral and algae is damaged; without the algae, the coral
tissue appears white, or “bleached.” If the temperature
goes back into the preferred range within a given
period, the algae will return, giving back corals their
color and, more important, the food they need to
survive. The coral can live only for so long without the
zooxanthellae before they starve to death. The overall
mortality numbers, therefore, should be the focus,
but it is important to note that a coral that survives
bleaching tends to be weaker and more vulnerable
to diseases, has a slower growth rate and does not
reproduce for one to two years.
During the follow-up surveys in November,
scientists saw a lower than expected mortality rate
on offshore far-north reefs. Considering 81 percent
of the northern region was bleached in March, only
26 percent mortality in an area within this region is
indeed good news. Coral CoE director Terry Hughes
suggested that upwelling from the nearby Coral Sea
cooled the water in this area.
While some corals did bleach, the mortality in the
central and southern regions of the reef, the areas
most visited by tourists, is so low that it will not likely
be apparent to most visitors. In the central region, 33
percent of the corals had bleached, but only 6 percent
of the corals died. At this point, the zooxanthellae have
returned to the majority of the corals that bleached,
giving the coral its color once again. If diving the
GBR is on your bucket list, you are in luck. A weather
anomaly spared the reef from much greater damage.
A cyclone that passed over Fiji in February 2016
transitioned to a rain depression that hit the southern
coast of Queensland and cooled the southern half of
the GBR, sparing that section the extent of bleaching
and mortality found further north.
WHAT CAUSED THE 2016 BLEACHING?
Corals are extremely particular in terms of the
environmental parameters in which they can survive.
Stable environmental conditions, specifically in regard
to water temperature, salinity and low nutrient levels,
are imperative for a healthy coral reef. Corals have a
preferred temperature range in which they can thrive. If
the water gets too warm or too cold, the corals stress.
Our oceans are warming, and human-induced global
climate change is to blame. As we add greenhouse gases
to the atmosphere, the Earth is getting warmer. The
oceans absorb more heat as a result, and we are seeing
a steady increase in sea surface temperature
.
The U.S.
National Oceanic and Atmospheric Administration
(NOAA) indicates that sea surface temperature has
increased at a rate of 0.13°F per decade since 1901.
While this increase may seem slight, for animals such as
corals, which have a 10-degree (°F) temperature window
in which they can survive, a degree or even less can be
the difference between life and death.
According to the Australian Bureau of Meteorology,
in 2016 the GBR recorded its hottest average sea
surface temperatures for February, March, April, May
and June since records began in 1900. In 2015 and
2016 the waters off Australia experienced an El Niño
event in addition to the warming ocean temperatures.
El Niño is a climate pattern that describes the unusual
warming of sea surface waters in the eastern tropical
Pacific Ocean. El Niño events can occur every two to
seven years, but they are not regular or predictable,
and their intensity can vary. This combination of El
Niño and warmer seas precipitated the 2016 mass
coral bleaching event.
WILL MASS CORAL BLEACHING HAPPEN AGAIN?
In 1997 and 1998 an El Niño and bleaching event
occurred; it was the first mass bleaching event on
the GBR. Another mass bleaching event on the GBR
occurred in 2002. Both events resulted in 50-60
percent bleaching, but less than 5 percent of the reefs
experienced high coral mortality. A 2016 AIMS study
conducted between 2012 and 2015 found that these
areas not only recovered, but there was also an overall
19 percent increase in coral cover in the southern
GBR. The reefs have proven resilient in the past, and
the hope is that the northern corals affected in 2016
will follow suit and return to predisturbance status in
the next 10-15 years.
Bleaching, of course, is not unique to the GBR.
In 2015 a study was published that examined the
resilience of corals in the Seychelles affected by the
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WINTER 2017
WATER PLANET
CORAL BLEACHING