Since 1950, phytoplankon mass has dropped by about 40 percent, 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 -- heavist 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.
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 longterm trends.
To reach back further in time, Boyce and colleagues combed through logs compiled since the late 19th century using a 20-centimetre (eight-inch) 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
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.
Across more than 11,000 species ranging from zooplankton to whales, the only environmental factor linked to all species groups was temperature.
"This relationship suggests that ocean warming, such as that due to climate change, may rearrange the distribution of ocean life," Tittensor said in a statement.