Alarming 40% decline in phytoplankton - base of marine ecosystem

The July 28 issue of NATURE has a disturbing new study with strong evidence that phytoplankton have declined 40 percent in the last 50 years. That is based on a study of 450,000 ocean samples taken over the last century. Phytoplankton are critical because they:

  • are the base of the entire ocean food chain, analogous to grasses and grains supporting land animals
  • produce half of our life-supporting oxygen
  • sequester 100 million tons of carbon dioxide daily, offsetting a substantial amount of the greenhouse gas produced from the burning of fossil fuels.

Dr. Boris Worm, a noted marine biologist at Dalhousie University in Halifax, Nova Scotia, headed the study team. They spent three years unearthing, filtering and analyzing available data on ocean transparency and chlorophyll concentration — common 'proxies' for phytoplankton abundance. After removing data on shallow coastal waters and any obviously erroneous — that is, biologically impossible — observations, the data set still included some 450,000 globally distributed measurements collected between 1899 and 2008.

The findings add to concerns that climate change is dangerously altering marine ecosystems. "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."

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

Since 1899, ocean transparency has been measured using a simple device called a 'Secchi disk' after the Italian astronomer who invented it in 1865. The disk is lowered into the sea and a depth measurement is taken at the point where observers lose sight of it. Using optical equations, the researchers compared Secchi depth measurements of ocean transparency to measurements of chlorophyll concentrations at research sites and within phytoplankton, and to satellite observations of ocean color.

The combined data suggest that phytoplankton biomass has decreased in eight of the ten ocean regions measured, with the largest rates of decline in the South and Equatorial Atlantic, the Arctic and the Southern Ocean. Only in the Indian Ocean has phytoplankton biomass increased — slightly in the north and more markedly in the south — since 1899.

"The study adds to a growing body of global ocean research, all evidencing a fundamentally common result: the net effect of a warming ocean surface is a reduction in phytoplankton surface chlorophyll concentration," says Michael Behrenfeld, a marine ecologist at Oregon State University in Corvallis.


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