Some 250 billion microscopic pieces of plastic are floating in the Mediterranean Sea, creating a biological hazard that reverberates up the food chain, says research supported by green campaigners.

The estimate comes from French and Belgian marine biologists who analyzed water samples taken in July off France, northern Italy and Spain to a depth of up to 15 centimetres.

"The rough estimate is that there are roughly 250 billion pieces of micro-debris in all the Mediterranean," Francois Galgani of the French Institute for Exploration of the Sea said.

The figure derives from 4,371 minute pieces of plastic -- average weight 1.8 milligrams -- found in the samples, "which extrapolates to roughly 500 tonnes for the entire Mediterranean," Galgani said.

Ninety per cent of the samples, taken by volunteers from Expedition MED (Mediterranean in Danger) on a 17-metre yacht, had such fragments.

The sampling only covered surface waters and is a preliminary evaluation. Further samples, off Gibraltar, Morocco,

Algeria, Tunisia, Sardinia and southern Italy, will be taken in 2011 to get a wider picture.

Micro-sized plastic is an enduring hazard because it becomes mixed with plankton, which is then eaten by small fish that are then eaten by larger predators, says Expedition MED.

It says there is an accumulating pile of evidence of the damage that this does to larger forms of marine life, including seals and tortoises.

(Source: '250 billion' plastic pieces likely in Mediterranean Sea [Province])