Phytoplankton worry

Green wet stuff continues to make the news. A new study in Nature indicates a large decline in the amount of phytoplankton in the oceans over the last century:

Phytoplankton activity fluctuates widely according to season and location, making long-term monitoring of trends difficult. An earlier study², based on satellite observations of ocean colour, suggested a link between climate variability and ocean productivity, but this was limited to observations from 1997 to 2006. Boris Worm, a marine biologist at Dalhousie University in Halifax, Canada, and his team have now combined satellite-derived observations of phytoplankton with historical shipboard measurements stretching back to the pioneering days of oceanography.

The research reveals an unsettling centennial downwards trend, superimposed on shorter-term variability. 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 decreased by around 40%, probably in response to ocean warming — and the decline has gathered pace in recent years.

"Clearly, 40% is a huge number," says Paul Falkowski, an oceanographer at Rutgers University in New Brunswick, New Jersey. "This implies that the entire ocean system is out of steady state, slowing down.

Something to worry about? Well, yes:

"This is severely disquieting," adds Victor Smetacek, a marine biologist at the Alfred Wegener Institute of Polar and Marine Research in Bremerhaven, Germany. "One must really digest the very magnitude of this decline and its possible implications."

The culprit is believed to be ocean warming:

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.

But ocean warming does not explain reduced productivity in regions, including the Arctic Ocean, where algal growth is mainly constrained by sunlight. So scientists must try to find out what other drivers, such as changes in wind and ocean circulation, might force the decline, says Falkowski.

No one is pointing the finger at ocean acidification yet, and (from memory) experiments with bubbling CO2 through phytoplankton have had mixed results. But there was this story recently that increased acidification may affect the availability of iron, which phytoplankton need to grow well. (There is more detail on that study at my earlier post.) So, I wonder if acidification over the last century is part of the explanation.