Phosphorus Joins Nitrogen as a Culprit in Gulf's 'Dead Zone'; but nitrogen is still as important, Rutgers oceanographer says

Phosphorus Joins Nitrogen as a Culprit in Gulf's 'Dead Zone'; but nitrogen is still as important, Rutgers oceanographer says

Nov. 15, 2006


EDITORS NOTE: Attention science and environmental writers and editors


But nitrogen is still as important, Rutgers oceanographer says


NEW BRUNSWICK/PISCATAWAY, N.J. Nitrogen is flowing down the Mississippi River and into the Gulf of Mexico faster than it can be consumed by phytoplankton, increasing the size of the dead zone off the Louisiana coast, according to researchers at Rutgers, The State University of New Jersey. As a result, phytoplankton blooms are growing, and the zone of hypoxia the dead zone of the Gulf possessing too little dissolved oxygen to sustain most life is getting bigger.

In a pristine system, the nutrients would flow down the river and into the Gulf, and there would be limited phytoplankton growth and no hypoxia, said James Ammerman, Associate Research Professor of marine science at Rutgers Institute of Marine and Coastal Sciences. The heavy use of fertilizers containing nitrogen and phosphorus in the agriculture of the Mississippi Valley has thrown the system out of balance.

The findings, based on analysis of data gathered in 2001, have been published in the journal Environmental Science and Technology. Jason B. Sylvan, a graduate student at the Institute of Marine and Coastal Sciences, is the lead author of the paper, Phosphorus Limits Phytoplankton Growth on the Louisiana Shelf During the Period of Hypoxia Formation. Ammerman is the corresponding author. They are joined by co-authors Quay Dortch, of the Louisiana Universities Marine Consortium and the National Oceanic and Atmospheric Administration; Wendy Morrison, also of the Louisiana Universities Marine Consortium; Alisa F. Maier Brown of Louisiana State University; and David M. Nelson of Oregon State University. The work was funded by the National Science Foundation and the National Oceanic and Atmospheric Administration.

This research shows that nitrogen and phosphorus nutrients, river water inputs and timing, and coastal circulation and salinity all interact to fuel biological productivity in the Gulf and create 'dead zones' there," said Philip Taylor, director of NSF's biological oceanography program. "Multiple nutrients need to considered in solving the nutrient problem in the Gulf of Mexico.

According to Ammerman, phytoplankton need both nitrogen and phosphorus to grow. Since they require a 16-to-1 ratio of nitrogen to phosphorus, phytoplankton usually run out of nitrogen first, since most coastal surface waters have ratios lower than 16-to-1.

Federal agencies and state and tribal governments have agreed to voluntary reductions on the amount of nutrients, especially nitrogen, coming down the Mississippi on the assumption that the sooner nitrogen runs out, the quicker the phytoplankton will stop growing and the sooner the dead zone can be reduced. That phosphorus ran out first in the spring and summer of 2001 might lead some people to think that such efforts had been successful, or werent really necessary in the first place. That would be a mistake, Ammerman said.

What it really means is that there is now so much nitrogen in the Gulf that, even though phytoplankton consume it faster than they consume phosphorus, they cant get rid of it fast enough, and its the phosphorus, instead of nitrogen, that runs out first and becomes the limiting nutrient, he said.

Scientists have been measuring the flow of nutrients down the river since the 1950s, and the dead zone, first noticed as a seasonally recurring feature in the late 1980s, has since doubled in size to an area larger than New Jersey. The problem may be older than that, but we dont really know, Ammerman said. There are models that suggest hypoxia wasnt a problem there before the 1970s, and there is also some data indicating occasional hypoxic events going back to the 19th century, but only in years of extremely high freshwater flow.

Contact: Ken Branson
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