The tiny algae known as phytoplankton - which form the basis of the entire marine food chain - have been disappearing globally over the last hundred years.
Using historical and recent oceanographic data, a team from Canada's Dalhousie University found a decline of about one percent of the global average - every year.
The decline is particularly noticeable in the Northern Hemisphere and after 1950, and translates into a massive 40 percent drop since 1950.
The scientists found that long-term phytoplankton declines were negatively correlated with rising sea surface temperatures and changing oceanographic conditions.
Says lead author Daniel Boyce, "Phytoplankton is the fuel on which marine ecosystems run. A decline of phytoplankton affects everything up the food chain, including humans."
Thhe three-year analysis aimed to resolve whether the ocean is becoming more - or less - 'green' with algae. Previous analyses were limited to more recent satellite data, and gave variable results.
To look further back, the authors analysed historical measurements of ocean transparency going back to the late 1800s, and combined them with additional samples of phytoplankton pigment (chlorophyll) from ocean-going research vessels.
The end result was a database of just under half a million observations, which allowed the scientists to estimate phytoplankton trends over the entire globe going back to the year 1899.
They found that most phytoplankton declines occurred in polar and tropical regions and in the open oceans.
Rising sea surface temperatures were negatively correlated with phytoplankton growth over most of the globe, especially close to the equator. Warm oceans are strongly stratified, which limits the amount of nutrients that are delivered from deeper waters to the surface ocean where the phytoplankton are found.
Rising temperatures may contribute to making the tropical oceans even more stratified, leading to increasing nutrient limitation and phytoplankton declines.
"Phytoplankton are a critical part of our planetary life support system. They produce half of the oxygen we breathe, draw down surface CO2, and ultimately support all of our fisheries," says co-author Boris Worm.
"An ocean with less phytoplankton will function differently, and this has to be accounted for in our management efforts."