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.
Plankton declining due to rising water temperatures
Posted on Saturday, July 31 2010 @ 20:25 CEST by Thomas De Maesschalck
BBC News reports the amount of phytoplankton seems to be declining due to rising water temperatures.