Since 1950: Phytoplankton declines 40 percent

September 03, 2015

Halifax (Bernd F. Laeschke – 29.07.2010): Plankton, a synonym for drifting organisms that inhabit the top layers of the Earth oceans and seas, declined by about 1 percent annually over the last century, a new study published in the journal Science suggests. Researchers from the Dalhousie University in Halifax, Canada, did combine satellite-derived observations of phytoplankton with historical data. The conclusion: Algal biomass decreased around 40 percent since 1950, and the trend has accelerated in recent years.

A possible cause is global warming, researchers say. Warmer surface water makes it harder for plankton to get vital nutrients. “It's concerning because phytoplankton is the basic currency for everything going on in the ocean,” said Boris Worm, a study co-author and Dalhousie University biology professor.

While satellite data only dates back to 1978, the researchers were able to access observations from the pioneering days of oceanography.  The Vatican did ask the Italian priest Pietro Angelo Secchi around 1865 to measure the clarity of the Mediterranean Sea. He simply lowered a white disk into the water and noted the depth when the object started to move out of sight.

The tiny organisms that make up plankton provide a crucial food source for larger marine life such as fish. Phytoplankton also plays an important role in the global carbon cycle. Through photosynthesis, Phytoplankton produces about half of the oxygen in Earth's atmosphere. It absorbs carbon dioxide and transforms it into organic matter, which sinks to the ocean or seafloor when the plankton dies. The amount of plankton in the water varies widely horizontally, vertically and seasonally, based on the availability of light and nutrients.

Researchers say that lower visibility is usually caused by higher amounts of plankton in the water. Combining data from the 19th century with observations made by satellites points to a long-term decline in plankton growth. “They're creating a climate record out of something that really wasn't designed to do this, using sophisticated techniques,” said David Siegel, a marine scientist with the University of California who co-wrote a commentary on the paper. “It fills in a piece of our history, so we're able to tell the story of what has been happening in the last 100 years.”