Currents could disrupt ocean food chain

If increased precipitation and sea surface heating from global warming disrupts the Atlantic Conveyer current – as some scientists predict – the effect on the ocean food chain in the Atlantic and other oceans could be severe, according to a new study just published in Nature.

In a worst case scenario, global productivity of phytoplankton could decrease by as much as 20 percent and in some areas, such as the North Atlantic, the loss could hit 50 percent. The study was conducted by Andreas Schmittner, an assistant professor in the College of Oceanic and Atmospheric Sciences at Oregon State University.

In his sophisticated computer model, Schmittner does not predict that the Atlantic Conveyer current, which drags warm water from the southern tropics into the North Atlantic and warms Europe, will be disrupted. Rather, his study is one of the first to examine what would happen to the ocean food chain if such a disruption did take place.

“Phytoplankton are the basis of the entire marine food web,” Schmittner said. “They ultimately affect everything from zooplankton to the larger fish that people consume.”

The Atlantic Conveyer current has the strongest impact in the North Atlantic, but it is a global phenomenon, Schmittner said. Surface waters from the Pacific Ocean, the Indian Ocean, the Arabian Ocean and the southern Atlantic are pulled northward where they are cooled by the atmosphere in the North Atlantic. As the water cools, it sinks 2,000 to 3,000 meters and begins flowing southward. The upwelling from the mixing of waters constantly replenishes the supply of phytoplankton at the surface, forming a rich nutrient source at the bottom of the marine food web.

There is growing concern by a number of scientists, however, that higher levels of human-generated carbon dioxide could increase water and air temperatures and decrease salinity in the North Atlantic at a rate significant enough to prevent the sinking and ultimate mixing of the water. That would not only disrupt the Atlantic Conveyer current, Schmittner said, it would prevent nutrient-rich waters from triggering phytoplankton growth.

“When the Atlantic Conveyer current works, the dead plankton sink to the bottom and are replaced at the surface with nutrient-rich water that encourages further production,” Schmittner said. “When the current is disrupted, and the mixing slows, that production also is disrupted.”

The shutdown of the Atlantic Conveyer current isn’t just idle speculation. A growing body of evidence suggests that it switched on and off 20 to 25 times during the last ice age.

“During the last ice age, from about 100,000 years before present to 20,000 years B.P., thick ice sheets over Canada sporadically dropped armadas of icebergs into the North Atlantic where they melted, sufficiently freshening the water to disrupt the conveyer,” Schmittner said.

“There is some evidence backing that up,” he added. “Deep ocean sediment core samples show pebbles from land delivered by the floating icebergs.”

Schmittner said scientists also have examined ice cores from Greenland and measured isotopes that show rapid temperature changes, which coincide with changes in ocean nutrient concentrations measured in deep-sea sediment cores.

“One full oscillation of these switches took 1,500 years,” Schmittner said, “but the individual transitions happened surprisingly fast. The climate went from a cold state to a warm state in as little as 20 to 50 years. Surface temperatures in Greenland increased 20 to 30 degrees Fahrenheit and water temperatures increased 10 to 20 degrees.”

Schmittner said the impact of the current on the Pacific Ocean generally isn’t as great, even though the system is a global one. Still, he added, plankton production would also decrease in the Pacific if the current was reduced.