Phytoplankton worldwide have been shrinking for 100 years, study shows

The world’s phytoplankton appear to have been disappearing at a rate of about 1% a year for the last century, researchers announced Wednesday, a disturbing long-term trend for the microscopic algae that form the basis of the marine food chain and produce much of the world’s oxygen.

In reporting their findings in the journal Nature, the Canadian team said that, since 1950, phytoplankton biomass has shrunk about 40%. Scientists had known the population was shrinking, but the long-term nature of that reduction came as a surprise.

“A global decline of this magnitude? It’s quite shocking,” said Dalhousie University marine scientist Daniel Boyce, lead author on the study.

The study combines historical records of ocean clarity with modern satellite data, the latter of which have been available only since the 1970s. Together, the modern and historical information provide an accurate long-term view of the state of phytoplankton, something scientists haven’t had before now.


The historical data were based on measurements of ocean clarity called Secchi depth determinations. Developed by an Italian priest around 1865, the measurements were obtained by lowering what looked like a white dinner plate into the ocean until observers lost sight of it. Water murkiness increases or decreases depending on the amount of phytoplankton or, more specifically, the plant’s green chlorophyll. The fewer phytoplankton, the clearer the ocean.

Thus, the scientists were able to convert historical data into specific measures of phytoplankton populations, using it with the modern information to create a virtually seamless timeline of the algae over the last century.

“They’re creating a climate record out of something that really wasn’t designed to do this, using sophisticated techniques,” said David Siegel, a UC Santa Barbara marine scientist who co-wrote a commentary on the paper. “It fills in a piece of our history, so we’re able to tell the story of what has been happening in the last 100 years.”

The scientists noted that the global decline, which was observed in eight of 10 ocean basins, corresponded with a rise in ocean temperatures.

The scientists suspect that warming near the surface of the ocean makes each ocean layer more distinct, preventing the bottom layer, which is rich in nutrients, from mixing effectively with the upper layers and thus fertilizing the phytoplankton.

Boyce said he hoped the research would encourage more study of the phytoplankton decline. Looking into the past could help scientists determine how to reverse the drop, he added.

“This is the tip of the iceberg, in some respects,” Boyce said. “Phytoplankton are key to the whole ecosystem. In terms of climate changes, the effect on fisheries, we don’t know exactly what these effects will be.”