The trend noted in the study becomes most pronounced near the poles and in the tropics since 1950, the researchers say.
Of the factors the team considered to explain the decline, the most influential appeared to be rising sea-surface temperatures – a trend many other scientists have traced to global warming.
The size of the change "is staggering," says Daniel Boyce, the lead author of the Nature paper describing the results and how the team arrived at them.
Mr. Boyce, who is working toward a doctorate at Dalhousie, says he began the study three years ago as part of a larger effort to tease out the factors accounting for a significant decline globally in fish at the top of the food chain. Overfishing is one obvious suspect. But changes at the bottom of the food chain also could be playing a role.
Satellite-based estimates of trends in phytoplankton abundance date only to 1979. So Boyce and two Dalhousie colleagues explored the possibility of pushing the record back deeper in time.