Ecological and biogeochemical impacts of tidal impoundment on coastal salt marshes: establishing baselines for assessing post-restoration ecosystem responses

Structures restricting tidal flow to coastal marshes are common and often have negative consequences for ecosystem functioning. Restoration of tidal connectivity between impounded wetlands and saltwater bodies has the potential to restore reduced or lost ecosystem functions. Here we investigated ecological and biogeochemical processes in an impounded coastal wetland and a nearby brackish marsh with natural hydrology in Gulf Shores, AL. Surface water levels, plant communities, soil porewater chemistry, nitrogen cycling, and greenhouse gas emissions were measured seasonally over two years in each wetland. Freshwater retention was evident by higher surface water levels and lower salinities in the impounded marsh. The lower salinities and sulfate levels in the impounded site resulted in lower porewater sulfides but much higher dissolved methane and methane gas fluxes to the atmosphere. Taller plants that are typical of fresh-to-brackish wetlands were observed in the impounded marsh, which may explain the more oxidizing soil conditions and near-complete denitrification inhibition. The impounded wetland had lower denitrification rates likely reflecting competition for nitrate and dissolved organic carbon with plants and DNRA microbes, respectively, and inhibition from more oxidizing conditions. In the nearby brackish marsh, the soil carbon accumulation rates were 43% higher compared to the impounded wetland. Together, our data demonstrates that tidal impoundment reduces wetland ecosystem functioning by decreasing N removal efficiency by 21% and C accumulation rates by 30%.

Presentation Preference: Oral

Primary Presenter: Shao-Min Chen, University of Georgia (shaomin.chen@uga.edu)

Authors:

Shao-Min Chen, University of Georgia  (shaomin.chen@uga.edu)

Meagan Eagle, United States Geological Survey  (meagle@usgs.gov)

Taylor Ledford, Smithsonian Environmental Research Center  (ledfordt@si.edu)

Behzad Mortazavi, Syracuse University  (behzadm@syr.edu)

Simone Falvey Gibson, University of Georgia  (simone.gibson@uga.edu)

Alexander Thomas, University of Louisiana at Lafayette  (alexander.thomas3@uga.edu)

James Nelson, University of Louisiana at Lafayette  (jimmy.nelson@uga.edu)

Samuel Dong, University of Georgia  (samuel.dong@uga.edu)

Reese Lofgren, University of Georgia  (reese.lofgren@uga.edu)

Amanda Spivak, University of Georgia  (aspivak@uga.edu)