Greenhouse gas exits sea

A UH study says tiny grazers keep carbon dioxide from sinking

UNIVERSITY of Hawaii researchers are questioning the ocean's role as storage for carbon dioxide to reduce the impact of greenhouse gases on global warming.

A study published today in the journal Science shows carbon dioxide does not always sink to the ocean depths where it is stored and prevented from re-entering the atmosphere as greenhouse gases.

Instead, sinking particles of carbon are often consumed by animals and bacteria in an area known as the twilight zone -- about 300 to 3,000 feet below the surface, the study says.

UH oceanographer Robert Bidigare, director of the Center for Marine Microbial Ecology and Diversity, and UH microbial oceanographer Dave Karl are among 18 co-authors of the paper.

"Unless the carbon that gets into ocean goes all the way down into the deep ocean and is stored there, the carbon can still make its way back into the atmosphere," Ken Buesseler, Woods Hole Oceanographic Institution biochemist and project leader, said in a news release. "The twilight zone is a critical link between the surface and the deep ocean."

A project called VERTIGO (VERtical Transport in the Global Ocean), involved Pacific cruises in 2004 and 2005 and long-term data from Hawaii and Japan sampling sites. More than 40 scientists from 14 institutions and seven countries participated in the National Science Foundation-funded program.

Samples collected from the water column and buoyant sediment traps lowered from a ship were sent to Bidigare's lab to determine the composition of tiny plants (phytoplankton) and microscopic animals (zooplankton) grazing at the sites.

"The work conducted at Station ALOHA during the VERTIGO program in 2004 clearly showed us that the ocean does not behave exactly the way we thought it would/should," Karl said.

Karl and other UH scientists have been monitoring waters since 1988 at Station ALOHA, a Hawaii Ocean Time-series (HOT) research site about 60 miles north of Oahu. Japan has a similar site in the northwestern Pacific.

Only 20 percent of the carbon at Station Aloha made it through the twilight zone compared with 50 percent at the site near Japan.

Big differences showed up at the sites because of nutrition and temperature, Bidigare said by telephone from Woods Hole, where he was meeting with scientists.

Algae are much larger in the northwestern Pacific because of iron blown into the ocean, "and the bigger you are, the faster you sink," he pointed out. Algae cells at the nutrient-rich Japanese site also "live in glass houses," he said, explaining that their shells are impregnated with silica, so they tend to sink faster.

Hawaiian waters have higher temperatures and carbon consumption rates, he said.

Karl said scientists are just beginning to understand processes controlling the ocean's ability to absorb and store carbon dioxide. "It is imperative to know how much of the carbon produced in the surface waters is actually lost and exactly where (and how deep) it goes.

"Only a few percent of the carbon" produced daily in Hawaiian waters is lost to deep water, he said. "Most of it (greater than 95 to 97 percent) is simply recycled back to carbon dioxide with a 'neutral impact' on climate."

At times, however, large plankton blooms occur near the surface and transport carbon particles to the deep ocean more effectively, he said. The UH Center for Microbial Oceanography: Research and Education (C-MORE) is planning a cruise this summer on the UH vessel Kilo Moana to investigate the blooms, said Karl, the center's principal investigator.