Data From Antarctic Cruise Could Confirm That Rates Of Ocean Nitrogen Uptake Are Seriously Underestimated

January 28, 1997

ATHENS, Ga. -- The Southern Ocean -- that vast expanse of water that surrounds the frozen continent of Antarctica -- has been reluctant to give up it wealth of scientific secrets. But new information gathered from a voyage to the bottom of the world could have a major impact on how scientists view the role oceans play in the global carbon and nitrogen cycle.

Dr. Deborah Bronk, a biological oceanographer from the University of Georgia, undertook a six-week voyage to the Southern Ocean and to Antarctica's Ross Sea to test whether current methods for measuring nitrogen uptake in the oceans seriously underestimate new and regenerated phytoplankton production. Bronk's previous work in the Chesapeake Bay, the Caribbean Seas and the Eastern Pacific strongly suggest this to be the case.

The finding requires an overhaul in the way scientists measure the uptake rates of nitrogen.

"Very little is know about nitrogen cycling," said Bronk, "but my previous studies strongly suggest that the normally used numbers for nitrogen uptake in this area are just wrong. They're much too low."

The importance of the study lies in understanding such phenomena as global warming and even the health of the oceans. This study will provide baseline data on how the oceans recycle one of the Earth's most crucial elements.

Bronk and her colleagues in the study, Gwen Puckett, a technician in her laboratory, and Dr. Bill Cochlan of the University of Southern California, were only two of 32 scientists who were part of the Joint Ocean Global Flux Study, sponsored by the National Science Foundation. The goal of the ongoing research is to study the role of carbon in the oceans and also the role of nitrogen in how carbon is used. The study has been in progress for a decade.

The team left Littleton, New Zealand on Sept. 23, and within a week of leaving port ran into a vast storm with winds hitting as high as 80 knots and waves 50 feet high. The storm raged for several days, and the ship, an icebreaker named the Nathaniel B. Palmer, rolled as much as 30 degrees in the high seas. A portable laboratory brought by a team of trace-metal analysts suffered serious damage in the sea swells. Bronk said the scientists not-so-jokingly said they felt like "tennis shoes inside a clothes dryer."

The voyage to the research site in the Ross Sea off the coast of Antarctica took a full two weeks, but the team was lucky in that the spring activity of phytoplankton -- floating aquatic plants -- was just beginning.

"These phytoplankton are very important because they must be highly productive during the spring and summer for the food chain to go from the microscopic all the way up to whales," said Bronk. "As the water begins to warm, we can study how the phytoplankton grow and thrive."

Using the word "warm" is relative when speaking of Antarctica, because the water temperature, even during the continent's spring, is actually below freezing, though it remains liquid because of its salt content. Anyone falling overboard in such conditions would lose muscle control almost immediately and consciousness in no more than 20 seconds -- an obvious death sentence.

In oceanic (and coastal and estuarine) environments, Bronk notes, an average of 25 to 41 percent of the dissolved inorganic nitrogen taken up by phytoplankton is released as dissolved organic nitrogen or DON. A failure to account for DON production in current methods of calculating nitrogen uptake has led scientists to underestimate new and regenerated production by an average of some 30 percent and by as much as 74 percent in some studies. (Production is the formation of phytoplankton biomass, and "new production" is production fueled by nitrogen from outside the system. "Regenerated production" is fueled by nitrogen released within the system.)

Bronk said her findings during the voyage indicate that newly produced nitrogen may, in fact, not wind up in particles that sink to the ocean floor. Instead, it may remain suspended as dissolved organic nitrogen where it can be recycled.

"Though DON is the largest pool of dissolved nitrogen in the ocean, we don't know much about it," said Bronk. "We know it includes amino acids and combined amino acids, but they represent less than 10 percent of the DON pool."

As recently as 15 years ago, scientists could not identify significant numbers of bacteria in the oceans. They now know that marine bacteria are the likely consumers of DON that phytoplankton release. While the conditions were difficult at best, Bronk said she found convincing evidence that current methods of estimating phytoplankton production are wrong.

After six weeks at sea, the scientists were taken by helicopter to McMurdo Station on the continent of Antarctica, where team members were flown back to New Zealand aboard a military transport -- a long, arduous flight in the unheated cabin of a plane meant to carry equipment. (Oddly, one of the most difficult adjustments in New Zealand came with the return of night. While in Antarctica, the scientists had spent the full six weeks in daylight.)

University of Georgia undergraduate marine science major Jay Wheeler will go on the next voyage of the Joint Ocean Global Flux Study in April. Bronk and graduate student Jill Shukert will return to Antarctica on the next voyage after that in October.

University of Georgia

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