A century-long decline in tiny algae called phytoplankton could disrupt the global ocean food chain, including the human consumption of fish, according to a study.

The microscopic organisms – which prop up the pyramid of marine animal life from shrimps to killer whales – have been disappearing globally at a rate of one percent per year, researchers reported.

Since 1950, phytoplankon mass has dropped by about 40 per cent, most likely due to the accelerating impact of global warming, they reported.

“Phytoplankton is the fuel on which marine ecosystems run,” said lead author Daniel Boyce, a professor at Dalhousie University in the Canadian province of Nova Scotia.

“A decline affects everything up the food chain, including humans.”

The pace of the decline – heaviest in polar and tropical regions – matched the rate at which surface ocean temperatures have increased as a result of climate change, the study said. Like all plants, phytoplankton need sunlight and nutrients to grow.

But warmer oceans become more stratified, creating a “dead zone” at the surface in which fewer nutrients are delivered from deeper layers.

The findings are worrying, the researchers said.

“Phytoplankton are a critical part of our planetary support system – they produce half the oxygen we breathe, draw down surface carbon dioxide, and ultimately support all fisheries,” said co-author Boris Worm.

Dr Boyce and colleagues combined historical and high-tech data to measure the marine algae’s progressive ebb. Satellites provided the most accurate gauge, but usable images from space of earth’s ocean biosphere have only been available since the late 1990s – too recent to show long-term trends.

To reach back further in time, Boyce and colleagues combed through logs compiled since the late 19th century using a 20-centimetre white disk lowered into sea water until an observer lost sight of it.

The degree to which light penetrates the ocean’s top layer, it turns out, is a good measure of the concentration of the chlorophyll found in all phytoplankton.

The study, published in Nature, “does not portend well for pelagic, or open water, ecosystems in a world that is likely to be warmer”, David Siegel, a researcher at the University of California at Santa Barbara, and Bryan Franz, an ocean biologist at Nasa Goddard Space Flight Centre, said in a commentary.

In a separate study, also in Nature, a team of researchers led by Derek Tittensor of Dalhousie found a close tie between sea temperatures and the concentration of biodiversity in the world’s oceans.