By Fiona Hodge

Global phytoplankton numbers have declined over the last hundred years, according to research published in Nature. Phytoplankton are the single celled marine plants that fuel all other sea life. Their importance also extends beyond the sea explains scientist Boris Worm, "Phytoplankton are a critical part of our planetary life support system. They produce half of the oxygen we breathe, draw down surface CO2, and ultimately support all of our fisheries”.

The new research used data collected from as far back as 1899. The earliest data comes from water clarity measurements made with the Secchi disc, which was actually designed to improve the boating prowess of the papal navy! Researchers lower the Secchi disc, a large white circle, into the sea and record the depth where it is no longer visible. From the early 1900s scientists were able to measure the amount of chlorophyll, found in all phytoplankton, in seawater samples. More recently satellites provide information on chlorophyll concentration in the sea surface.

The analysis out this week combined one hundred years of secchi and chlorophyll data, over 400,000 measurements, to investigate long term trends in phytoplankton. The researchers found a global decline in phytoplankton abundances of around 1 percent per year. At the regional scale eight out of the ten regions showed phytoplankton declines since 1950, including the South Pacific.

Envisat Medium Resolution Imaging Spectrometer satellite image shows blue-green algae blooms in the Baltic Sea.

Why the declines? It was shown that Phytoplankton losses correlated with warming sea surface temperatures. Over the last century almost all sea surfaces have warmed, most less than one degree. Warming sea surface temperatures create a layer of warm water that is more resistant to mixing with the deep, nutrient filled, dark water below. Phytoplankton need these nutrients from the deep to be available in the lit surface waters.

It’s wonderful to be able to compare our world to that of a hundred years ago, even if changes are worrying. Such comparisons require historical measurements that may have no obvious use at the time of collection. Just as we needed Father Secchi, and countless others, out there taking measurements for this study, we need Universities, DoC, and Niwa out there taking measurements today for future studies. Who knows what trends we might want to investigate fifty to a hundred years from now? Assessing the global phytoplankton population would have been unimaginable a hundred years ago.