Deep Antarctic waters are teeming with krill 鈥� small shrimp-like creatures near the bottom of the food chain 鈥� in a discovery which has amazed scientists.
The marine invertebrates, which are a crucial food source in Antarctica鈥檚 ecosystem, were thought to inhabit surface waters only. But a submersible remotely operated vehicle (ROV) has captured surprising images of large numbers of krill living and feeding on the floor of the Southern Ocean, some 3500 metres below sea level.
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鈥淭his is first time that anyone has looked down there with a camera,鈥� says at the British Antarctic Survey (BAS), who led the team. 鈥淭here have been cameras taken down in Antarctic waters before but they have never gone this deep.鈥�
鈥淭he pictures were taken in January, which is in the second half of the Antarctic summer,鈥� Clarke says. He believes that the Antarctic krill (Euphausia superba) dive down to the Antarctic abyss to follow the phytoplankton they feed on.
鈥楲umps of gunge鈥�
鈥淭here鈥檚 a phytoplankton bloom in December and early January,鈥� he says, 鈥渁nd as it matures, those single-celled plants flocculate, forming great lumps of gunge that rain down onto the ocean floor.鈥�
Finding krill living at such depths suggests they must have an 鈥渆xtremely flexible鈥� physiology to withstand the huge water pressure on the ocean floor, notes Clarke. 鈥淚鈥檓 not sure I can think of any other organisms that would routinely travel such distances over that pressure gradient.鈥�
Magnus Johnson of the University of Hull, UK, agrees. 鈥淥n one of my voyages, we brought an anemone up from the deep,鈥� he says. 鈥淎t depth it looked fantastic but at the surface it fell apart 鈥� it just turned into snot. It鈥檚 amazing that these krill can function at 3000 metres as well as they can at 10 metres.鈥�
Reaching the ocean floor also involves passing through the chilly 鈥渨inter water鈥� layer which lies at a depth of about 100 m. The water temperature drops from about 4 掳C at surface to -2 掳C in this icy layer, says Clarke, rising to about 1or 2 掳C below 500 m.
Warming threat
But the ability to withstand that temperature drop does not mean krill could cope with the warming of the Antarctic waters, believes Katrin Schmidt, also at BAS but not a member of this team. 鈥淭he krill have had a long time to adapt to temperatures between about -2 and 4 掳C,鈥� she says. 鈥淏ut a quick rise to 6 or 10 掳C would definitely kill them.鈥�
Even if krill do cope with warmer Antarctic waters, Clarke does not think that will guarantee the survival of the larger animals that feed on them.
鈥淢ost ecologists working with food webs will tell you that it鈥檚 rarely the obvious factors that affect the webs,鈥� he says. Even if krill remain abundant, the rising temperatures will have a unique impact on each krill-eating Antarctic species. 鈥淟ife might have a surprise for us,鈥� says Clarke. 鈥淚t generally does.鈥�
Journal reference: (DOI: 10.1016/j.cub.2008.01.059)
