14 % drop in coral growth seen in the Great Barrier Reef

The effects of global warming have shown up in a definite way in the
Great Barrier Reef of Australia. A paper in the latest issue of the
Science journal notes unprecedented effects of increased CO{-2} on the
Great Barrier Reef. Scientists found the rate at which corals were
able to build skeletons dropped by 14 per cent during the p eriod of
study — 1990 to 2005. Coral reefs are considered as the rain forests
of the ocean as they support great biodiversity. Any drop in growth of
the reefs of the Great Barrier Reef is hence worrying. What makes the
study significant is that scientists studied 328 colonies from 69
reefs, and the duration of study was 15 years. "…This study shows that
the effects are probably large-scale in extent and that the observed
changes are unprecedented within the past 400 years."The growth of
coral reefs depends on their ability to build skeletons. Skeletons are
built by calcification of calcium carbonate (CaCO{-3}). There are a
few things that may affect the calcification process. Though the
scientists note that the precise "causes of decline" in calcification
are not known, their study suggests that increased temperature stress
and increased acidity of sea water are the most likely causes. Coral
reefs are extremely sensitive to sea surface temperature. Any changes
beyond 1 degree C for extended periods of time affect the corals.
Increase in sea surface temperature affects and destroys the symbiotic
zooxanthellae algae that live on the corals. Any damage to the algae
leads to a loss of the symbionts and a rapid whitening of the coral
host (thus the term "bleaching"). Mass coral bleaching was not
documented in the scientific literature before 1979. 1998 saw a large
scale destruction of coral reefs all over the world.Since the oceans
act as sinks for carbon dioxide, increased uptake of CO{-2} by ocean
water will make them acidic. Supersaturation of tropical sea water
with calcium carbonate is crucial for reef calcification process.
Hence acidic water will compromise supersaturation. The pH of the
ocean has decreased by 0.1 unit (become acidic) since the beginning of
the industrial revolution. And this has affected the calcification
process.The researchers studied the Porites corals using X-rays and a
technique called gamma densitometry to measure annual growth and
skeletal density. Studying the skeletal density allowed them to
calculate the amount of calcification annually. They found that the
calcification rate rose 5.4 per cent between 1900 and 1970. It dropped
by 14.2 per cent between 1990 and 2005. The drop was mainly due to a
growth slowdown from 1.43 cm a year to 1.24 cm.How the sea surface
temperature and lower pH would affect the reefs and marine organisms
in the long run cannot be accurately predicted since living organisms
and ocean are dynamic."We may not see drastic changes in a short
period. And how the increased temperature, acidity and reduced
skeletal strength due to calcite erosion would affect marine life are
not known," said Dr. M. Wafar, Senior Scientist at the National
Institute of Oceanography (NIO), Goa. "So this only calls for a more
cautious approach."

The Hindu, 8th January 2009