ick Bogren
Many scientists believe rising levels of carbon dioxide in the atmosphere are contributing to climate change. The increase may be related to the frequency that water moves from the ground into rivers and then into the air before returning to land as rainfall.
This water movement is called the hydrologic cycle, and an intensifying hydrologic cycle could increase global carbon cycling, said Yi-jun Xu, a hydrology researcher in the LSU AgCenter.
Xu is finishing up a short-term study of the carbon cycle on the Mississippi River.
Initially working with an earlier grant from the National Fish and Wildlife Foundation to study pollution and water quality, Xu began sampling carbon concentrations in the Mississippi River when the record-high flood began in February 2019.
With the sampling already in place, Xu applied to the National Science Foundation for a Rapid grant to study dissolved carbon in the river during the extraordinary long flood.
Xu set up locations in Baton Rouge and Belle Chasse on the Mississippi River and in Morgan City at the mouth of the Atchafalaya River, which carries approximately 25% of the waters from the Mississippi from a point just south of the Louisiana-Mississippi state line.
Xu and his team took water samples every three to five days during the entire major flood period from Feb. 27 to Aug. 5 and every two weeks after that.
“The river transported large amounts of carbon and nitrates,” Xu said. “We found extremely high mass loading of dissolved inorganic carbon and dissolved organic carbon.”
The total dissolved carbon could be detrimental to the ecosystem and water quality. “This is unprecedented. We found very high levels of dissolved carbon,” he said.
The broader implication is more carbon dioxide is moving into the atmosphere, and the change in carbon dioxide will intensify and accelerate with the increasing biogeochemical cycle, Xu said.
In streams and rivers, organic carbon comes from decomposition of terrestrial sources, such as trees and plants, while inorganic carbon can come from very different sources, including the solution of atmospheric carbon dioxide, inorganic carbon bound in sediments, groundwater, and chemical and biological processes.
Water contains gasses, including oxygen and carbon dioxide, Xu said. When dissolved carbon dioxide is in an imbalance with carbon dioxide in the atmosphere, the exchange between water and the atmosphere, called outgassing, increases.
“Normal atmospheric concentration of carbon dioxide is 400 to 410 parts per million,” Xu said. “During the flood stage, the river concentration of dissolved carbon dioxide was between 3,000 and 6,000 parts per million.”
Demineralized inorganic carbon dioxide affects the biogeochemical cycle. “As climate continues to change, the hydrologic cycle intensifies,” he said. “As the cycle becomes intensified, more carbon dioxide is discharged into the atmosphere. More carbon dioxide will lead to warmer temperatures.”
The past 100 years have seen a 25% increase in flow in the Mississippi River, mostly as a result of changes in land use and river engineering in the watershed, allowing more water to drain off hard surfaces rather than being absorbed into the soils, Xu said.
Scientists estimate the volume of water flowing in the Mississippi River will increase by 11% to 65% by 2100. “Precipitation is increasing the Mississippi River Basin,” he said.
The broader implication is that more carbon dioxide will move into the atmosphere through the increasing the biogeochemical cycle. “Terrestrial carbon is moving into the atmospheric ‘store,’” Xu said.
He stressed that the National Science Foundation is interested in the scientific observations from his research and the long-term ramifications.
“Findings gained from this research will help coastal resources managers and policy makers make better-informed decisions,” Xu said. “The research aims at an unprecedented flood event, and rapid measurements are vital for estimating the carbon movement during such extreme events in the Mississippi-Atchafalaya River System.”
The findings can be helpful in investigating carbon dynamics in other river systems worldwide, he said. One aspect of this is whether these changes will intensify and speed up the carbon cycle between the land and the oceans.