"A disruption of the Atlantic meridional overturning (AMO) circulation leads to a collapse of the North Atlantic plankton stocks to less than half their initial biomass," said Andreas Schmittner of Oregon State University.

Writing in the science journal Nature, Schmittner said the steep drop in the plankton population was due to it becoming separated from deep water nutrient layers as the ocean current failed.

To date much work has been done on the potential disruption of the Atlantic Conveyor as the climate warms by an estimated two degrees centigrade this century due to man-made greenhouse gases such as carbon dioxide.

However, relatively little research has been published on the possible effect on the seaborne food chain which provides sustenance for millions of people.

"A massive decline of plankton stocks could have catastrophic effects on fisheries and human food supply in the affected regions," Schmittner wrote.

"Hence, emission pathways that lead to fast and large increases of future CO2 including the risk of a collapse or substantial reduction of the AMO should be avoided through early measures for emission reductions," he added.

He said there was evidence that the current had switched on and off during the ice ages, and his modelling work indicated that ocean productivity could drop by 20 percent as plankton vanished.

"These model results ... suggest that global ocean productivity is sensitive to changes in the Atlantic meridional overturning circulation," he said.

It is not confined to the northern Atlantic but has implications across the Indian, Pacific, Arabian and southern Atlantic Oceans, he added.

Although the effect was most noticeable in the north Atlantic where even a partial weakening in the life-giving current caused a substantial drop in productivity, it also registered globally.