FAYETTEVILLE, Ark. - The Gulf of Mexico experiences a periodic "dead zone" caused by an algal bloom. Sea grasses around the world have begun to disappear from saltwater ecosystems. Scientists know little about what makes an aquatic ecosystem work, let alone what makes it fall apart. Now a University of Arkansas researcher and her colleague have found that the basic processes in life under water change on a seasonal and even hourly basis.

Susan Ziegler, University of Arkansas assistant professor of biological sciences, and Marilyn Fogel of the Carnegie Institution of Washington, D.C. will present their work on the isotopic composition and cycling of elements in a freshwater wetland at 11:45 a.m. Monday, April 2, at the 221^st national meeting of the American Chemical Society in San Diego.

Ziegler and Fogel used carbon and nitrogen isotope ratios to follow the course of organic matter in the wetlands. The microscopic plants and bacteria that live in the water incorporate different sources of these chemicals for biological processes from dissolved organic matter that can come from animals, plants or man-made sources such as fertilizer or effluent. Microorganisms and plants form some amino acids in a similar fashion, and others differently.

"One of the big questions in this field is what is the source of dissolved organic matter?" said Ziegler. "Looking at the bigger picture, we want to find out what land use change is doing to change the dynamics of carbon and nitrogen cycling."

Ziegler cites dramatic changes in an isotope of the amino acid alanine that she studied in a freshwater marsh in the Chesapeake Bay watershed as an example. The stable carbon isotopic composition of alanine serves as an indication of the metabolic activity of phytoplankton.

The researchers took samples throughout the day to create scientific snapshots of the stream in small intervals. They found an enriched carbon 13 signal from phytoplankton at the end of the day. Plants usually use the lighter carbon dioxide first, before tapping into the heavier isotope. This indicates a large draw down of carbon dioxide in the system during the day and an overall increase in primary production.

They also found seasonal variations in isotopes, indicating that plants and microbes contribute differently to the ecosystem depending upon the time of year. This may offer some explanation as to why algal blooms, such as the one in the Gulf of Mexico, occur seasonally.

"Using these techniques, you can get an idea of what processes are important at what times," Ziegler said.

Ziegler plans to use the isotope techniques in the Arkansas delta region of the Mississippi to explore the biochemical cycles in the water and how they might be altered by human actions. The Mississippi River drainage affects a large area starting in Minnesota and ending in the Gulf of Mexico, and changes in land use throughout the Midwest may have contributed changes to the entire ecosystem. Ziegler plans to start by looking at a small part of the Mississippi Delta, examining the effects of fertilizers and effluent on the aquatic ecosystem.

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