The amount of phytoplankton - tiny marine plants - in the top layers of the oceans has declined markedly over the last century, research suggests. Writing in the journal Nature, scientists say the decline appears to be linked to rising water temperatures.
They made their finding by looking at records of the transparency of sea water, which is affected by the plants.
The decline - about 1% per year - could be ecologically significant as plankton sit at the base of marine food chains.This is the first study to attempt a comprehensive global look at plankton changes over such a long time scale.
"What we think is happening is that the oceans are becoming more stratified as the water warms," said research leader Daniel Boyce from Dalhousie University in Halifax, Nova Scotia, Canada.
"The plants need sunlight from above and nutrients from below; and as it becomes more stratified, that limits the availability of nutrients," he told BBC News. Phytoplankton are typically eaten by zooplankton - tiny marine animals - which themselves are prey for small fish and other animals.