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

Published in The Hindu on January 8, 2009

Corals
Increased sea surface temperature and acidity of sea water are the most likely causes.  The observed changes in coral growth are unprecedented within the past 400 years. — Photo: Wikimedia Commons 

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 journal Science notes unprecedented effects of increased CO2 on the Great Barrier Reef. Scientists found the rate at which corals were able to build skeletons dropped by 14 per cent during the period 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.

Unprecedented

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 (CaCO3). 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. The year 1998 saw a large scale destruction of coral reefs all over the world.

Effect of salinity

Since the oceans act as sinks for carbon dioxide, increased uptake of CO2 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.

Long term effects

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.”

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