Scuba Diving Fiji: Found: The clearest ocean waters on Earth

Monday, 9 July 2007

Found: The clearest ocean waters on Earth

As clear as the clearest lakes on the planet, salty as ocean waters, and roughly the size of the Mediterranean – this, say researchers, is the clearest and most lifeless patch of ocean in the world. And it is in the middle of the Pacific.

"Satellite images that track the amount of chlorophyll in ocean waters suggested that this was one of the most life-poor systems on Earth," explains Patrick Raimbault of the University of the Mediterranean, in Marseille, France (see image, right).

In October 2004, Raimbault and colleagues set out to study the remarkable patch of ocean water on a three month cruise – called BIOSOPE – that left from Tahiti in French Polynesia, passed by Easter Island and ended on the Chilean coast. Along the way, they sampled the water's chemistry, physics and biology.

Marc Tedetti, also from the University of the Mediterranean, was on the expedition to investigate the water's clarity. He was struck by the colour of the water, which he describes as closer to violet than to blue.

Beautiful but barren

Tedetti returned having found "unequivocally" the clearest ocean waters on the planet. "Some bodies of freshwater are equally clear, but only the purest freshwater," Tedetti told New Scientist. "For instance, researchers have found equivalent measurements in Lake Vanda in Antarctica, which is under ice, and is really extremely pure."

At the clearest point of the south-east Pacific, near to Easter Island, Tedetti found that UV rays could penetrate more than 100 metres below the surface.

This correlates with Raimbault's chlorophyll measurements, which suggest the patch contains roughly 10 times less chlorophyll that is found in most ocean waters. Raimbault says the patch of ocean is the least productive marine region known to man.

In a sense, the patch is isolated from the global river and ocean circulation, which explains its lack of life. Being far away from the coast it does not benefit from continental run-off, and the thermohaline circulation – the "global conveyer belt" – which ferries ocean waters around the world, also mostly runs along the continental shelves.

To compound things, this area of ocean does not benefit from seasonal variations which tend to bring nutrients up from the seabed.

Carbon rich

Elsewhere, winter temperatures cool surface waters, making them denser and causing them to sink and push deeper, nutrient-rich waters up to the surface. But the surface waters of the southeast Pacific are warm year-round, which means they tend to perpetually "float" on top of the deeper, colder waters.

In spite of this, the expedition found the clear water is able to support a food chain, which Raimbault suspects relies heavily on the organisms' ability to recycle nutrients. "As there is no supply, there cannot be any loss either," he says.

Raimbault made another surprising discovery: the patch of the ocean that is poorest in life appears to be extremely rich in dissolved organic carbon.

He is currently teasing apart data in an attempt to explain the apparent contradiction, but believes it may be that the limited availability of nutrients such as nitrogen and phosphorus means the bacteria that would normally degrade the dissolved organic matter are not able to complete the task.

Journal reference: Geophysical Research Letters (DOI: 10.1029/2007GL029823)

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http://environment.newscientist.com/article/dn12162-clear-waters-run-deep-in-the-pacific.html