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A trio of oceanographers and mathematicians at Dalhousie University, in Canada, has found evidence that the populations of two major types of phytoplankton in the North Atlantic Ocean have been decreasing by approximately 2% per year for the past 60 years.

In their study, in the open-access journal PLOS One, Crispin Mutshinda, Zoe Finkel and Andrew Irwin analyzed 60 years of data to compare the levels of diatom and dinoflagellate blooms over many decades as they respond to global warming.

Every spring, large blooms of diatoms take place in the North Atlantic Ocean. Likewise, every year in late summer to early fall, large blooms of dinoflagellates take place. Both are major types of phytoplankton and thus serve as a major part of the base of the oceanic food web in the North Atlantic Ocean. Because of the critical role they play in the ecosystem, the researchers wondered how well they are faring in light of global warming.

To find out, the team pulled publicly available data from the Continuous Plankton Recorder, a database of information regarding plankton in the world's oceans. Their effort involved looking at the total amount of both types of phytoplankton measured in the North Atlantic Ocean every year for the past six decades.

They found that the levels of the two types of phytoplankton, taken as a whole, have dropped by approximately 2% every year over the 60 years they studied—except in the eastern and western shelf regions.

With this regional variability, the researchers also found changes in the percentages of the two types of phytoplankton relative to each other, and found that diatoms have increased in percentage relative to dinoflagellates by approximately 1% to 2% each year over the years 1960 to 2017, which was a surprise. Most in the field had been predicting the reverse.

These findings may go beyond the oceanic food web—phytoplankton also serve as carbon sinks, pulling massive amounts of CO₂ from the air and carrying it to the bottom of the ocean. Less phytoplankton could mean more carbon dioxide in the atmosphere, speeding up global warming.

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