Warming Oceans Put Kink in Food Chain, Study Says

John Roach
for National Geographic News
January 30, 2007
The growth of tiny plants at the base of the ocean food chain is tightly
linked to changes in the climate, according to a recent study.

The finding shows that as temperatures warm, the growth of single-celled ocean plants called phytoplankton slows at Earth's mid and low latitudes. The plants' growth increases when the climate cools.

While the findings are related to short-term changes in climate, they help scientists predict how the ocean will respond to long-term climate change, according to Jorge Sarmiento, an atmospheric and ocean scientist at Princeton University in New Jersey.

"This is telling us we can expect reduced biological production [the ability to support life such as plants, fish, and wildlife] with global warming in many regions of the world," he said.

Sarmiento is a co-author of the study, which was published last month in the science journal Nature.

Michael Behrenfeld, a botanist at Oregon State University in Corvallis, was lead author of the study. He said the research demonstrates a solid link between climate change and marine life.

The growth of phytoplankton, for example, influences how much food fish have to eat, which in turn affects the marine birds that eat the fish.

As the Earth warms, Behrenfeld said, changes in the upper ocean will change not only phytoplankton but also the species that dominate different regions of the environment.

"It will change the structure of the ecosystem," he said.

Shifting Growth

Behrenfeld's team calculated the growth rates of the tiny ocean plants using satellite data collected between 1997 and 2006. The growth rates were then correlated with records of Earth's climate during that time.

The satellite imagery detected shifts in the color of the ocean from blue to green. The color shifts reflect changes in the amount of phytoplankton in the water.

The researchers found a surge in growth between 1997 and 1999, when the ocean underwent a cooling trend related to a strong shift between El Niño and La Niña events.

El Niño is a periodic warming of surface waters in the equatorial Pacific Ocean that influences global weather patterns. La Niña is a cooling of the equatorial Pacific waters.

Since 2000 the ocean surface temperatures have gradually warmed, and growth rates of the plankton have declined almost in lockstep, Behrenfeld noted.

"Projection into the future would suggest that as temperatures continue to warm, the climate will continue to suppress biology on a global basis," he said.

Behrenfeld explained that the link between climate and biology results from the way climate affects the stratification, or layering, of ocean waters.

As the ocean surface warms, it becomes lighter than the cold, dense water below it. The cold water is full of nutrients that phytoplankton need for growth. The increased layering essentially cuts off the phytoplankton from their food supply.

Critical Phytoplankton

In addition to playing a prominent role in the food chain, Behrenfeld said, the single-celled plants are responsible for about half the photosynthesis on Earth.

(Read related story: "Source of Half Earth's Oxygen Gets Little Credit" [June 7, 2004].)

Photosynthesis is the process plants use to convert sunlight, water, and carbon dioxide into carbohydrates and oxygen.

Photosynthesis by phytoplankton absorbs about 50 billion tons of carbon dioxide—a greenhouse gas tied to global warming—each year, according to Behrenfeld.

"That has a profound influence on the carbon cycle on Earth," he said, referring to the process by which carbon, a key building block of life, is recycled in the environment.

(See a National Geographic magazine feature on threats posed to Earth's carbon cycle.)

According to Sarmiento, however, most of the carbon dioxide taken up by phytoplankton is absorbed from the water, not the atmosphere.

As a result, he said, the changes in the plankton's growth due to changes in the climate may not have much of an effect on the amount of carbon dioxide in the atmosphere.

"[But] the fact [that] biology is impacted is quite remarkable in and of itself," he added.

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