The decline - about 1% per year - could be ecologically significant as plankton sit at the base of marine food chains. Algal blooms can be imaged from space
Researchers find trouble among phytoplankton, the base of the food chain, which has implications for the marine food web and the world's carbon cycle.
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.
This is the first study to attempt a comprehensive global look at plankton changes over such a long time scale.