Estimating Climate Change's Effects On Gulf Wetlands

View Enlarged Image Environ. Sci. Technol.

ELEVATION STATUS Much of Louisiana's coastal wetlands are under 1 meter above sea level.

Coastal wetlands store nutrients such as organic carbon and nitrogen that feed the surrounding ecosystems. As the climate changes and sea levels rise, scientists expect these coastal wetlands will slowly disappear, washing away important nutrients. Now researchers estimate how much organic carbon and nitrogen Louisiana's wetlands could lose by 2050 (Environ. Sci. Technol., DOI: 10.1021/es200909g).

These wetlands serve as significant carbon sinks: Plants pull carbon dioxide from the atmosphere, and, as the plants die and decay, the carbon dioxide is deposited as organic carbon in the soil. Allowing that carbon to wash into the sea will add carbon back into the global carbon cycle and contribute to further climate change, experts say. Meanwhile, when soils rich in organic nitrogen erode into the Gulf of Mexico, the nutrients could feed the Gulf's dead zone, a low-oxygen area where few organisms can survive.

The Intergovernmental Panel on Climate Change estimates that global sea levels could rise by more than 30 cm by 2050. Yi Jun Xu, an associate professor at Louisiana State University, and Biao Zhong, now at Nanjing Audit University, in China, wanted to calculate how much organic carbon and nitrogen would be lost in Louisiana based on models of resulting wetland loss through 2050.

The researchers first estimated the amounts of organic nitrogen and carbon stored at various depths in soils throughout Louisiana's wetlands. They assigned soil densities to different soil depths based on a previous study, and then assumed that the soil, in general, contained 58% organic carbon, also based on earlier studies. These assumptions allowed them to calculate how much organic material was at a specific depth and thus how much organic carbon was present. To estimate the amount of organic nitrogen at each soil level, they assumed that the organic carbon-to-nitrogen ratio was 15:1, based on observations from an earlier study.

Xu and Zhong next used a report that projected the extent of wetland loss by 2050 to determine what elevations of soil would wash away in the state's wetlands. Based on that model, the researchers calculated that between 1990 and 2050, Louisiana's wetlands could lose about 700,000 tons of organic carbon per year and about 47,000 tons of organic nitrogen per year. Those loses are equivalent to 17% of the organic carbon and 8% of the organic nitrogen that the Mississippi River delivers to the Gulf each year.

Denise Reed, a professor at the University of New Orleans, says estimating nutrient and carbon loss in wetlands is an important issue that few researchers have tackled.

However, she says, the study is a bit simplistic. The model assumes that all submerged wetland soils will dissolve into the Gulf. In reality, she says, the ocean's currents will redistribute some soil to other spots in the wetlands. Also, she points out that the researchers didn't consider the latest data on land elevations in the region, so their estimates of submerged soils could be inaccurate.

Still, Reed agrees with Xu and Zhong that identifying the areas with highest levels of organic matter could help Louisiana prioritize its wetland restoration and protection efforts in hopes of mitigating the environmental effects of sea level rise.

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