Researchers led by Boris Worm, a marine biologist at Dalhousie University in Halifax, Canada, have published a paper in the journal Nature showing that marine phytoplankton have declined substantially in the world’s oceans over the past century.

Phytoplankton are the basis of the entire marine food chain. They produce around half of the oxygen in Earth’s atmosphere and drive the ‘biological pump’ that fixes 100 million tonnes of atmospheric carbon dioxide a day into organic material, which eventually sinks to the ocean floor.

The scientists found that the average global phytoplankton concentration in the upper ocean currently declines by around 1% per year. Since 1950 alone, algal biomass has decreased by around 40% and the rate of decline has increased in recent years.

In most regions tested, the phytoplankton decline seems to be the result of a 0.5 – 1.0 °C warming of the upper ocean over the past century. The warming leads to enhanced vertical ‘stratification’ of ocean layers, thus limiting the supply of nutrients from deeper waters to the surface.

Boris Worm says that reduced phytoplankton growth adds a new dimension – comparable to coral bleaching, overfishing and acidification – to the problems of global change in the ocean. "We don’t know what happened before 1899, and we’re not sure about what will happen in the future but we absolutely need to monitor this worrying trend and watch how it is unfolding."