A major study published today in the journal Nature suggests that the marine food chain could be in danger of collapse due to declining levels of phytoplankton.
Phytoplankton are the bottom rung of the food chain on which all sea life depends.
Phytoplankon levels have dropped of by about 40 percent since 1950, and the Candadian scientists who authored the report link the decline to warming oceans.
"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 is heaviest in polar and tropical regions, and was in line with the speed at which surface ocean temperatures had changed there.
Phytoplankton get their energy from the sun, and need sunlight and nutrients to grow. They are the marine equivalent of grazing pasture.
With warmer oceans becoming more stratified, a "dead zone" can develop at the surface. Less vertical movement of water in the oceans means that 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.
