Ocean acidification cuts shell weight

Ocean acidification has reduced the average shell weight of a species of foraminifera single-celled, calcite-secreting plankton in the Southern Ocean by one-third compared to pre-industrial specimens, say scientists in Australia. Their study provides the first evidence of the phenomenon from the field.

“It is clear that ocean acidiciation is happening, but up until now the potential impact on ocean chemistry and marine life has been based on projections and models,” said Will Howard of the Antarctic Climate and Ecosystems Cooperative Research Centre, Australia. “We’ve had inferences and theory, and there has been an enormous amount of discussion around the the world on the highly likely potential for ocean acidification to have this impact, but there has been little direct data from the field.”

As carbon dioxide from the atmosphere dissolves in the ocean it lowers the pH and alters the carbon chemistry of the water, making carbonate ions less available to organisms for processes such as shell building. Previous laboratory studies have indicated that this can affect calcifying organisms such as some coccolithophorids, corals and pteropods, as well as foraminifera.

Howard studied the Globigerina bulloides species of foraminifera together with colleagues at the Antarctic Climate and Ecosystems Cooperative Research Centre, the University of Tasmania, CSIRO Marine and Atmospheric Research, and the Australian Department of the Environment, Water, Heritage and the Arts. They compared current specimens with those from pre-industrial times taken from sediment cores. Today’s specimens exhibited a 30 35% decrease in shell weight.

According to Howard this decrease could have an impact both on ecosystems and the take-up of carbon by the oceans. Planktonic foraminifera make up between 25 and 50% of the total open-ocean marine carbonate flux, which influence the transport of organic carbon to the sea floor.

By using sediment cores the researchers also analysed how foraminifera shell weight had varied with carbon-dioxide concentrations in the past. Shell weights were highest during the last glacial maximum, around 18,000 24,000 years ago.

The Southern Ocean currently contains around 40% of the manmade carbon emissions that have been absorbed by the oceans. Speaking at last week’s Climate Change Congress in Copenhagen, Howard said that the Southern Ocean is one of the places in which the carbonate-chemistry shift is relatively large and will be one of the first places to go undersaturated with respect to a carbonate.

“The Southern Ocean is giving us a strong indication of an acidification process that will spread throughout the global ocean,” he said. “It will take centuries to millennia to reverse due to the long-term nature of the buffering effect. We need sustained long-term monitoring.”

The researchers reported their work in Nature Geoscience.

Liz Kalaugher, environmentalresearchweb, 18 March 2009. Article.

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