Page 29 - The World About Us

Coral bleaching
2.1.5
1.1.1

What are the threats to the Great Barrier Reef?

1. Coral bleaching extra acidity decreases the capacity of recover from severe damage. Floods
corals to build their calcium carbonate origina ng in land can wash large
There are many current threats to skeletons, especially when young. amounts of sediment and other
the health of the Great Barrier Reef. The The extra acidity may also affect key pollutants on to the reef. Since the
most significant and widespread is the marine species such as the sea European se lement of Australia, about
threat posed by climate change. As sea bu erfly, a marine snail with a 400km² of coastal mangroves and salt
temperatures rise above the recent marshes have been lost along the
transparent, thin shell which 'flies'
norms, corals begin to suffer thermal through the ocean on an elongated foot. Barrier Reef coastline. The estuarine
stress. This causes the polyps to expel The sea bu erfly is an important mangroves act as an important filter
the zooxanthellae algae from their species in many marine food chains. removing sediments and excess
ssue. It is this algae that gives the nutrients from the river discharge.
polyps their colour, and as we have 3. Pollu on
seen, much of their food. The result is
Run off from agricultural land along
coral bleaching.
the Queensland coast is also threatening
If the water condi ons do not the reef. Thirty five major rivers drain an
improve quickly, the corals will area of 424,000 square kilometres.
eventually starve and die. Many of these rivers carry increased
2017 saw extensive coral bleaching sediment, pes cides and nutrients from
on the Great Barrier Reef. This followed agricultural fer lizers into the sea. These
severe bleaching events in 2016 which extra nutrients can change the
killed an es mated 22% of the Barrier ecological balance on the reef. They
Reef's coral. encourage algal growth and decrease
the ability of corals to establish and fig.79 Crown-of-thorns, coral killer.
grow, especially a er bleaching events.
5. Crown-of-thorns starfish
4. Extreme weather
The crown-of-thorns starfish is a
Coral reefs can cope with extreme
coral predator. There are regular
weather condi ons. A er all, they have
been around for millions of years. outbreaks, where crown-of-thorns
However, the increasing frequency and starfish numbers explode and coral is
rapidly threatened by 'plagues' of
intensity of extreme weather events is
starfish. It is es mated that these
causing extensive damage. Already
starfish may be responsible for the loss
stressed corals may take decades to
of 50% of coral cover on some reefs.
The most recent outbreak started in
2010. It is not en rely clear why these
outbreaks occur. Some suggest it is
fig.78 Coral bleaching.
linked to increased land run-off during
2. Ocean acidifica on storms when the starfish are spawning.
With a large female starfish able to
Another side effect of the mass produce 60 million eggs in a single
burning of fossil fuels that is leading to season, the extra algal blooms produced
climate change, is ocean acidifica on. It by the run-off may provide extra food
is es mated that up to half of all carbon for the growing starfish.
dioxide released into the atmosphere
since the Industrial Revolu on has been Predators of the starfish include the
Triton's trumpet snail and the Titan
dissolved in the oceans. While this has triggerfish. Triton’s trumpets, a large
stopped catastrophic climate change so predatory snail, used to be heavily
far, the oceans have become slightly
more acidic over me. A 19th Century collected for their beau ful shells. Today
ocean pH of 8.3 has dropped by 0.1 to their numbers have yet to fully recover,
8.2 today and could fall as low as 7.8 by so reducing the ability of this predator
to control the numbers of its prey.
2100. While this appears small, the fig.80 Tropical storms threaten reefs.

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Biodiverse ecosystems are under threat from human activity.