photo from the NOAA (public domain)

Phytoplankton – the microscopic algae that form the basis for marine food chains – have declined by 40% since 1950, at a rate of 1% per year. Phytoplankton also absorb CO2 and produce roughly half the Earth’s breathable oxygen.

According to the first large-scale plankton-measuring study of its kind, researchers have correlated the decline in phytoplankton to climate change.

From an article in The Ecologist:

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.

Weather phenomena such as El Niño also affect phytoplankton levels in the short term. The recent algal blooms in the Baltic Sea are examples of how unusually warm weather can cause algae ‘carpets’ to spread, which deprives deeper marine life – such as various kinds of phytoplankton – of crucial oxygen.

From a BBC News report:

The plants need sunlight from above and nutrients from below; and as it becomes more stratified, that limits the availability of nutrients.

–Daniel Boyce, Dalhousie University in Halifax, Nova Scotia, Canada.

Additional resources:

Nature – Phytoplankton in retreat