Fish Stocks To Fall As Oceans Warm

December 7, 2006

Release from: Leigh Dayton
The Australian

PRODUCTIVITY of the world's oceans could plummet if climate change continues to heat the planet's seas.

The bleak prognosis comes from a US study of 10 years of satellite data which shows - for the first time - that growth of tiny marine plants called phytoplankton is tightly linked to global warming.

"This clearly showed that overall ocean productivity decreases when the climate warms," said team leader Michael Behrenfeld a botanist and expert on remote sensing with Oregon State University in Corvallis.

Dr Behrenfeld published the findings over night in the journal Nature, along with colleagues at NASA's Goddard Space Flight Centre in Greenbelt, Maryland, and at the Universities of Maine and California, and Princeton and Rutgers Universities.

The team used images collected by NASA from the SeaWiFS satellite, launched in 1997.

According to the researchers, the lock-stem link between climate and productivity is "stratification" (layering) of the oceans. When the sea surface warms it becomes "lighter" than the cold, dense water below which is rich in nutrients.

The result is that the waters don't mix and surface-dwelling phytoplankton are separated from the nutrients they need to convert sunlight into energy.

"There is significant regional variability, with some areas showing enhanced production and some area losses," Dr Behrenfeld said.

"But on a global basis there is an inverse relationship: increased temperatures cause decreased marine phytoplankton production," he added.

That's critical because ocean-dwelling phytoplankton form the base of the marine food chain, supporting fish stocks which feed upon them.

Dr Behrenfeld's group identified two climate-driven changes in ocean production. During the first event, between 1997 and 1999, ocean productivity increased as the seas recovered from one of the strongest El Nino events on record. During El Nino sea surface temperatures in the eastern Pacific Ocean rise.

The second event was a long-term warming trend that began in 2000 and continues today. During this period, biomass dropped by over 30 per cent in some regions.

Also writing in Nature, Scott Doney _ a marine chemist at the Woods Hole Oceanographic Institution in Massachusetts _ agreed that satellites with sensors sensitive to ocean colour can detect important "climate-driven trends".