Phytoplankton are microscopic algae which float and drift near the top of the ocean. They carry out photosynthesis, converting sunlight into organic matter which then serves as the first step in the entire marine food chain.
But their numbers are declining by an average of 1 per cent a year, translating to a 40 per cent decline since 1950, according to researchers from Dalhousie University in Nova Scotia, Canada.
The authors suggest rising sea surface temperatures linked to global warming are the reason for the decline. As ocean temperatures rise they become more stable and less nutrients are brought up towards the surface where they are needed by phytoplankton, together with sunlight, to grow.
Scientists already know that large-scale climate fluctuations, such as the El-Niño Southern Oscillation (ENSO), affect phytoplankton on a year-to-year basis, by changing short-term oceanographic conditions.
Basis of marine food chain
The researchers in this study say the decline in phytoplankton was likely to have wider implications for the entire ocean ecosystem.
'They produce all of the organic matter of the oceans that feeds everything else from the small shrimp to the large whale - everything in the oceans depends on the activity of these plankton.
'It is as if you reduced the amount of crop plants available to humans.'
'They are also a critical part of our planetary life support system,' added the authors. 'They produce half of the oxygen we breathe, draw down surface CO2, and ultimately support all of our fisheries.'
Full study: Global phytoplankton decline over the past century
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