Sunday, January 18, 2009

Fish to play more imp role in marine carbon cycle

http://www.thehindu.com/holnus/008200901181122.htm

New York (PTI): Fish may play a more important role in the marine carbon cycle than previously thought, a new study shows.

Researchers have found that fish excrete prodigious amounts calcium carbonate, that had been thought to come almost exclusively from marine plankton such as shelled algae, Nature reported quoting the study.

Biologists, Nature reported, knew that bony fish a group that includes most fish apart from cartilaginous ones such as sharks and rays produced calcium carbonate in their guts to rid themselves of excess calcium ingested from seawater. But this process hadn't been factored into models of ocean chemistry.

"This is the first study that has even tried to link carbonate production by fish to global carbon cycles," Rod Wilson, a fish physiologist at the British University of Exeter, is quoted as saying Wilson and his colleagues from the United Kingdom, the United States and Canada set about estimating the contribution of fish to global marine carbonate production, Nature said.

They took X-rays to observe carbonate formation in fish intestines and measured the amount excreted by the European flounder (Platichthys flesus) and the Gulf toadfish (Opsanus beta), model species studied previously in the authors' labs. Then, they used two independent computer models to calculate the total mass of fish in the world's oceans.

The models suggested that there are between 0.8 billion tonnes and 2 billion tonnes of fish biomass in the oceans. And this indicates that bony fish produce 40 million 110 million tonnes of calcium carbonate per year, the study says. The range accounts for 3 per to 15 per cent of the estimated total.

The lab results can be extrapolated to global fish populations, Wilson is quoted as saying, because the predictions are based on well-studied relationships between fish metabolism, mass, activity level and temperature.

The estimate is conservative and could be as high as 45 per cent of total calcium-carbonate production under more liberal assumptions, he says. The study appears in www.nature.com.

"They hit on an important but, before this, unrecognized source of calcium carbonate in the ocean," Victoria Fabry, an oceanographer at California State University, San Marcos is quoted as saying.

And this might elucidate why ocean surface waters are more alkaline, or less acidic, than models have predicted, the article says, adding that the carbonate coming from plankton doesn't dissolve until it sinks to depths greater than 1,000 metres. But carbonate produced by fish contains more magnesium, an impurity that causes the mineral to dissolve more readily and reduce the acidity of the water.

Fish, the article argues, may boost their carbonate production rate in response to increased carbon-dioxide levels, the researchers suggest.

Ocean scientists, it noted, have warned that plankton and corals will produce less calcium carbonate as the amount of carbon dioxide in the atmosphere rises, but Wilson is quoted as saying "what's a bit peculiar is we think fish go in the other direction."

Wilson says fish make calcium carbonate by combining calcium from seawater with carbonate ions generated from carbon dioxide in their bodies. If the amount of atmospheric carbon dioxide goes up as expected leading to a carbon dioxide increase the fish may produce more carbonate ions and thus more calcium carbonate.

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