Previously fingered as climate culprits because they breathe out carbon dioxide (CO2), the commonest greenhouse gas, Southern Ocean sperm whales are now seen as an unexpected ally in the fight against global warming, removing the equivalent carbon emissions from 40,000 cars each year thanks to their faeces.
But this is only a part of the picture, according to a paper, published in the British journal Proceedings of the Royal Society B.
Australian biologists estimated that the estimated 12,000 sperm whales in the Southern Ocean each defecate around 50 tonnes of iron into the sea every year after digesting the fish and squid they hunt.
Iron is a terrific food for phytoplankton -- marine plants that live near the ocean surface and which suck up CO2 from the atmosphere through photosynthesis and, as a result of faecal fertilisation, the whales remove 400,000 tonnes of carbon each year, twice as much as the 200,000 tonnes of CO2 that they contribute through respiration.
The Southern Ocean is rich in nitrogen but poor in iron, which is essential for phytoplankton.
The whales' faeces are so effective because they are emitted in liquid form and close to the surface, before the mammals dive, said the paper.
By way of comparison, 200,000 tonnes of CO2 is equal to the emissions of almost 40,000 passenger cars, according to an equation on the website of the US Environmental Protection Agency (EPA).
Industrialised whaling not only gravely threatened major carbon "sink," the scientific term for something that removes more greenhouse gases than it produces, said the paper.
Before industrial whaling, the population of this species was about 10 times bigger, which meant around two million tonnes of CO2 were removed annually, it added.
The scientists suspect that because sperm whales cluster in specific areas of the Southern Ocean there is a clear link between food availability and cetacean faeces.
This could explain the "krill paradox," they believe. Researchers have previously found that when balleen whales are killed, the amount of krill in that sea area declines, which thus affects the entire food chain.
The future of sperm whales and other species comes under scrutiny next week in Agadir, Morocco, where the International Whaling Commission (IWC) discusses a plan to relax a 24-year moratorium on commercial whaling.
KG06