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Fish may help fight ocean acidification

Fish are a major source of calcium carbonate production in marine ecosystems, a finding that has implications for ocean acidification, report scientists writing in the journal Science.

The authors — consisting of an international team of researchers — estimate that by continuously ‘drinking’ calcium-rich seawater and excreting calcium carbonate as a waste produce, fish may produce 3-15 percent of oceans’ calcium carbonate. Importantly, the carbonate excreted by fish is in a form that rapidly dissolves in seawater, helping balance ocean chemistry by making waters more alkaline. The impact is greatest in depth of less than 1,000 meters (3,300 feet).

Until now, most scientists believed that the oceans’ calcium carbonate came from plankton. The new study suggests that carbonates produced by plankton are less soluble than carbonates produced by fish, meaning that have a slower impact on ocean acidity.

Marine scientists are dually concerned about the impact of rising atmospheric carbon dioxide levels on oceans, which become more acidic as they absorb CO2, and depletion of the world’s fisheries as a result of unsustainable fishing practices. The new research indicates that fish and ocean chemistry are intrinsically linked — healthy fish populations will help slow ocean acidification.

“The combination of future increases in sea temperature and rising CO2 will cause fish to produce even more calcium carbonate, which is in sharp contrast to the response by most other calcium carbonate producing organisms,” explained the University of Miami’s Rosenstiel School of Marine and Atmospheric Science, which supported in research, in a statement. “Fish’s metabolic rates are known to increase in warmer waters, and this study explains how this will also accelerate the rate of carbonate excretion. In addition, our existing knowledge of fish biology shows that blood CO2 levels rise as CO2 increases in seawater and that this in turn will further stimulate fish calcium carbonate production.”

Dr. Martin Grosell of the Rosenstiel School says that while the finding seems promising, more research is needed to better understand the impact of fish on the ocean carbon cycle.

“Depletion of fish stocks due to overfishing will obviously influence global calcium carbonate production attributable to fish, but the prediction of the impact of overexploitation is complex. Smaller fish which often result from exploitation produce more calcium carbonate for the same unit of biomass than bigger fish, a simple consequence of higher mass-specific metabolic rates in the smaller animals. In addition, the chemical nature of the calcium carbonate produced by fish, which determines solubility, almost certainly will depend on temperature, fish species, ambient pH and CO2 levels among other factors. The influence of such factors on this newly recognized and significant contribution to oceanic carbon cycling offers an exciting challenge for further study” said Grosell, associate professor of Marine Biology and Fisheries at the Rosenstiel School.

Ocean acidification is important because it reduces the availability of free carbonate ions in sea water, making more difficult for marine organisms to extract calcium carbonate to build the aragonite and calcite shells and skeletons they need to survive. Some of the most affected creatures are tiny polyps that build coral reefs. As the calcification rates slow, so does the growth of coral reefs, leaving them vulnerable to weathering from wave action and other stress. Other marine life is affected too, including microorganisms that form the base of the oceanic food chain.

Wilson R.W. et al. “Contribution of Fish to the Marine Inorganic Carbon Cycle,” Science. 16 January 2009

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