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Sperm whales more than offset their carbon

Sperm whales in the Southern Ocean are responsible for soaking up more carbon dioxide than they give out, according to new Australian research.

Marine mammal biologist, Trish Lavery, from Flinders University in Adelaide, and colleagues, report their findings today in the Proceedings of the Royal Society B.

"They've well and truly bypassed being carbon neutral. They've actually gone one step further," she says.

Lavery has been researching whales as part of her PhD on the ecology of marine mammals under the supervision of Associate Professor Jim Mitchell of Flinders University's MSL Lab.

She says previous research has suggested that whales in the Southern Ocean are contributing to carbon build-up in the atmosphere.

"You didn't have to read too closely between the lines to get the idea that if we didn't have all these pesky whales swimming around in the Southern Ocean, the atmosphere would have a lot less carbon," says Lavery.

But she says whales also help in the sequestering of carbon dioxide into the deep ocean - through their faeces.

Whale faeces is a liquid mix of nutrients that floats in the surface of the ocean (as seen in the second image).

"It's like a big plume of diarrhoea," says Lavery.

Iron in whale faeces boosts phytoplankton growth, which absorbs CO2 through photosynthesis.

Once phytoplankton die, they sink to the deep ocean removing carbon from atmospheric circulation for a period of time.

Lavery says whale faeces stays in the surface layers of the ocean and is therefore an important source of iron for phytoplankton growth, which depends on sunlight.

The faeces of most other marine animals such as sharks and fish comes in pellet form and rapidly sinks without releasing its nutrients.

Calculations

Lavery sat down with a calculator to estimate how much carbon was removed from circulation as a result of phytoplankton growth due to iron from whale faeces.

She took data from studies of marine mammal diets to work out how much iron the whales were consuming - and excreting.

Lavery then had to make an estimation of how much iron would stay in the surface waters.

To do this she subtracted diet components, like squid beaks, which are not digested and sink quickly after being excreted.

Lavery used the results from 'iron fertilisation' experiments in the Southern Ocean to work out how much carbon would be removed by the iron.

And using estimates of the number of sperm whales in the region, she was able to work out the total amount of carbon they were respiring and sequestering.

Lavery found that the Southern Ocean's 12,000 whales were breathing out 200,000 tonnes of carbon per year, but were removing 400,000 tonnes of carbon per year.

"They're taking out double what they're putting in," says Lavery. "They act as carbon sinks."

She says there used to be 120,000 sperm whales in the Southern Ocean.

"If we hadn't removed them ... we'd have an extra 2 million tonnes of carbon being removed out of our atmosphere every year," says Lavery.

But she says this is less than 1% of what human acitvity is adding to the atmosphere every year so whales alone can't save us from climate change.

Lavery is now looking at the contribution of other marine species to carbon sequestration.