According to
the Australian Academy of Science:
Imagine you are on a beautiful tropical island on the Great
Barrier Reef. You put on your mask, fins and snorkel and jump into
the water. But instead of the famous brightly coloured coral,
brittle white branches spread as far as the eye can see. The coral
is dead.
As the world's oceans become warmer, this scenario is taking
place more frequently, on the Great Barrier Reef and on coral reefs
all over the world. Coral bleaching has never been more common or
widespread, prompting the question: are rising sea temperatures
killing the reefs?
Corals are animals
The ancient Greeks mistakenly
believed that corals were plants. Corals are actually animals,
related to anemones and jellyfish. Corals consist of a limestone
structure filled with thousands of small animals called polyps.
Each polyp has a skeleton cup, tentacles with stinging cells, a
mouth and a stomach. The tiny tentacles snatch at passing plankton
for food, but for their main course, reef-building corals have
devised a much more ingenious method to get fed.
Algae called zooxanthellae live within each coral. In return for a
safe sunny home, the zooxanthellae eat the nitrogen waste that the
coral produces (nitrogen is very good for algal growth) and, like
all plants, algae turn sunlight into sugars by the process of
photosynthesis. The sugars produced by the zooxanthellae make up 98
per cent of the coral's food. So, without having to do any work at
all, the coral is kept clean and well fed, and the zooxanthellae
with their brilliant reds, oranges and browns give corals their
colour.
Importance of coral reefs
Coral reefs are important for many
different reasons. Apart from protecting the shoreline from the
damaging effects of the ocean, they provide habitats and shelter
for many organisms and are the source of nitrogen and other
nutrients essential for the food chain. This is why hundreds of
thousands of marine species live in reefs. Many fisheries depend on
the fish that spend the first part of their life in coral reefs,
before making their way out to the open ocean.
The Great Barrier Reef is especially
important to the Australian economy and generates 1.5 billion
dollars every year from fishing and tourism. In addition, the study
of coral reefs can provide a history of past climates.
Rising sea temperatures and coral bleaching
Over the past one hundred years, the
temperature of sea water in many tropical areas has been rising.
For example, the Australian Institute of Marine Science has
collated data showing that 2002 was the warmest year for water
temperatures off northeast Australia since 1870. And there are
predictions of a sea temperature rise of up to 1ºC within 50
years.
Rising water temperatures block the photosynthetic reaction that
converts carbon dioxide into sugar. This results in a build-up of
products that poison the zooxanthellae. To save itself, the coral
spits out the zooxanthellae and some of its own tissue, leaving the
coral a bleached white. The bleached coral can recover, but only if
cooler water temperatures return and the algae are able to grow
again. Without the zooxanthellae, the coral slowly starves to
death.
Other causes of coral bleaching
Apart from heat stress, other causes
of coral bleaching may include:
- increased exposure to ultraviolet (UV) radiation;
- large amounts of storm water from heavy rains flooding the
reef;
- the exposure of coral to certain chemicals or diseases;
- sediments such as sand or dirt covering the coral;
- excess nutrients such as ammonia and nitrate from fertilisers
and household products entering the reef ecosystem. (The nutrients
might increase the number of zooxanthellae in the coral, but it is
possible that the nutrient overload increases the susceptibility of
coral to diseases.)
- Often coral reefs are exposed to a combination of these
factors.
Rate of coral bleaching
Coral bleaching is a natural
process. For thousands of years, fishermen have noticed mysterious
whitening of the reefs. What is not natural is the rate at which
coral bleaching is occurring. In 1998, the worst coral bleaching in
700 years struck the Great Barrier Reef, followed by an even worse
bleaching only 4 years later. Massive areas of corals were affected
all over the world. In Australia alone, the 2002 bleaching saw
nearly 60 per cent of the reef suffer bleaching and, in the worst
areas, 90 per cent of the coral was bleached.
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