Sperm whales may put a gentle (and unwitting) brake on climate change
In a somewhat unusual research project, scientists have found that sperm whale faeces may help oceans absorb carbon dioxide from the air.
Australian researchers calculate that Southern Ocean sperm whales release about 50 tonnes of iron each year.
This stimulates the growth of tiny marine plants - phytoplankton - which absorb CO2 during photosynthesis.
They note in the Royal Society journal Proceedings B that in the end, this also provides more food for the whales.
Phytoplankton are the basis of the marine food web in this part of the world, and the growth of these tiny plants is limited by the amount of nutrients available, including iron.
Faecal attraction
Over the last decade or so, many groups of scientists have experimented with putting iron into the oceans deliberately as a "fix" for climate change.
Not all of these experiments have proved successful; the biggest, the German Lohafex expedition, put six tonnes of iron into the Southern Ocean in 2008, but saw no sustained increase in carbon uptake.
But the Australian group calculates the natural fertilisation by the 12,000 or so sperm whales estimated to inhabit the Southern Ocean result in the absorption of about 40,000 tonnes of carbon from the atmosphere every year - more than twice as much as they release by breathing.
The Lohafex expedition was the latest to probe iron fertilisation
Although 40,000 tonnes of carbon is less than one-thousandth of the annual emissions from burning fossil fuels, the researchers note that the global total could be more substantial.
There are estimated to be several hundred thousand sperm whales in the oceans, though they are notoriously difficult to count; and lack of iron limits phytoplankton growth in many regions besides the Southern Ocean.
So it could be that whale faeces are fertilising plants in several parts of the world.
Crucial to the idea is that sperm whales are not eating and defecating in the same place - if they were, they could be just abrosbing and releasing the same amounts of iron.
Instead, they eat their diet - mainly squid - in the deep ocean, and defectate in the upper waters where phytoplankton can grow, having access to sunlight.
Releasing the iron here is ultimately good for the whales as well, say the researchers - led by Trish Lavery from Flinders University in Adelaide.
Phytoplankton are eaten by tiny marine animals - zooplankton - which in turn are consumed by larger creatures that the whales might then eat.
The scientists suggest a similar mechanism could underpin the "krill paradox" - the finding that the abundance of krill in Antarctic waters apparently diminished during the era when baleen whales that eat krill were being hunted to the tune of tens of thousands per year.
