June 10, 1998 An interdisciplinary team of scientists has found a surprisingly high rate of carbon and nutrient turnover by microbes in one of Georgia's coastal salt marshes, a highly productive ecosystem.
The team of researchers, all associated with the Georgia Institute of Technology, is conducting a long-term study at Sapelo Island, Ga., to examine the marsh's biogeochemical processes — that is, the exchange of biogeochemical elements such as carbon, phosphorus, nutrients and metals between living and non-living components of the environment. They want to know how these processes relate to the productivity, faunal activity and hydrology of the marsh system. An understanding of these relationships is crucial to predicting the effects of global warming on the coastal environment.
They are presenting their findings to date at the joint meeting of the Ecological Society of America and the American Society of Limnology and Oceanography to be held June 7 -12 in St. Louis, Mo. This presentation is scheduled for 11:30 a.m. June 11 in the conference facilities at the Adam's Mark Hotel.
"We observed some of the highest rates of organic matter decomposition ever measured in marine systems," said Dr. Joel Kostka, a Georgia Tech adjunct assistant professor and a researcher at the Skidaway Institute of Oceanography, a research unit of the University System of Georgia. One reason for the higher than expected results may have been the length of time the study was conducted; very few studies have looked at decomposition rates by microorganisms over a two-year period, as this ongoing study has done, Kostka added.
Researchers believe microorganisms in salt marsh sediments play a significant role in the cycling of materials in the ecosystem. By examining microorganisms, such as bacteria that occur in salt marsh sediments, the scientists hope to determine what drives microbial activity. By looking at the marsh environment across several seasons, they are learning how the nutrients flow through the system.
Numerous variables affect microbial activity in the sediments, said Dr. Philippe Van Cappellen, an associate professor in Georgia Tech's School of Earth and Atmospheric Sciences. He and Tech graduate student Alakendra Roychoudhury are working with Kostka on this study. Those variables include temperature, inundation by the tides, plant composition in the area, the hydrology of the area, the input of organic material, runoff from the adjoining land area and the mixing of the sediment.
Along with the microorganisms, larger forms of animal life also affect nutrient cycling in the marsh. Large populations of fiddler crabs that inhabit the mud flats in the salt marsh can greatly enhance nutrient cycles in the area, Kostka said. Their burrows, which can extend 20 centimeters into the soil, allow saltwater to infiltrate the mud, which in turn introduces oxygen and other nutrients into the sediments.
"The injection of oxygen changes microbial activity," Kostka said, "and that affects chemical and nutrient release."
Sapelo's salt marshes provide an ideal model system in which to study biogeochemical cycling in the environment, Van Cappellen said. Stretching along the eastern coast of North America from Nova Scotia to Florida, salt marshes are not only extensive, he said, but also extremely productive.
Here, great expanses of Spartina, or cord grass, form the basis of the food chain. Salt marshes play a critical role as nursery grounds for fishes, shellfish and other marine organisms by providing a place where the young stages of animals can grow rapidly.
The 10-acre Sapelo study site also provided another advantage to the research team; years of research have been conducted at the site and, hence, a great deal is already known about the dynamics of the ecosystem. The study represents a major departure from the typical approach to studying salt marshes, which is traditionally ecology based, according to the researchers.
With the introduction of biogeochemists, oceanographers and microbiologists, the researchers were able to gather more quantitative data that will help them better understand the factors controlling the cycling of these vital materials in salt marsh sediments.
"Clearly, not enough interdisciplinary studies have been done," Kostka said, referring to the value of the study and its many contributors. The research, which was funded by Georgia Tech's School of Earth and Atmospheric Sciences, the Georgia Sea Grant College program, the Office of Naval Research, the Skidaway Institute of Oceanography and the National Science Foundation through an initiative aimed at encouraging interdisciplinary studies, takes a different look at the dynamics of an important natural system.
The utility of the data being collected can be expanded well beyond the particular habitat at Sapelo where the study is under way. The researchers point out that the knowledge gained about the salt marsh can be extended to other natural systems. Such information will be useful as scientists grapple with the problems associated with global warming, Van Cappellen said.
"If sea levels rise with global warming, we need to understand the stability of these environments and determine if we will lose them," he said. With an increased knowledge of the biogeochemical cycles at work, humans can better predict the future of these dynamic and important areas, he added.
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