Coastal wetlands are important habitats that buffer terrestrial-aquatic interactions and can exert a significant influence on processes in adjacent coastal waters. One of the more certain impacts of global climate change is sea level rise, which will move the salt gradient upriver into historically freshwater wetlands. Tidal freshwater marshes (and, along the southeast Atlantic and Gulf coasts, tidal freshwater swamps) are ecologically important ecosystems located at the interface between the terrestrial and aquatic zones, where rivers enter the coastal zone. These wetlands are extensive throughout the Southeast. In South Carolina, all major rivers in the coastal zone have abundant tidal freshwater wetlands, which account for over 20% of the tidal marsh area in the state. They provide a host of ecosystem services, including acting as water quality filters (removing nutrients), sequestering C, serving as nursery habitats for juvenile fishes, and buffering storm and flood waters.

This project, which was originally funded by the University of South Carolina, is designed to complement earlier EPA-STAR funded research on the effects of salt water intrusion into tidal freshwater wetlands. The current project is using field manipulations of salinity to address similar objectives. Beginning in June 2008 and continuing through November 2011, diluted salt water was added to study plots in a tidal freshwater marsh at Brookgreen Gardens, a private sculpture garden located on the Waccamaw River in South Carolina. Porewater salinities were raised from freshwater (salinity < 0.5) to oligohaline levels (salinity ~2-5). At roughly monthly intervals for the entirety of the study, the uptake and emissions of CO₂ and CH₄ from field plots were measured so that plant photosynthesis and respiration could be modelled. The first 1.5 years of gas flux data were recently published in Estuaries and Coasts (see Publications page). Additionally, we have data on the plant community (species composition, density, biomass), rates of soil CO₂ and CH₄ production at multiple depths, and porewater chemistry (salinity, dissolved inorganic carbon, dissolved CH₄, pH). In cooperation with Dr. Rima Franklin (Virginia Commonwealth University), we will gain an understanding of how soil microbial communities and enzyme activities have responded to the long-term experimental manipulations at this site. Dr. Michael Piehler (University of North Carolina, Institute of Marine Sciences) is making some measurements of denitrification and dissimilatory nitrate reduction to ammonium (DNRA) to help us understand some potential impacts of saltwater intrusion on the nitrogen cycle. Finally, we have a coldroom full of soils from the experimental plots that have not been fully analyzed; one set of cores is being steadily processed for soil carbon and nitrogen concentrations and pools whereas other cores are being analyzed to determine belowground biomass responses to saltwater intrusion.

Visually, the largest change to the system has been the opening of the plant canopy due to the death of salt-sensitive plant species. This has led to reduced carbon inputs to the soil and reduced rates of both soil CO₂ and CH₄ production. In sharp contrast to our EPA-STAR project, CH₄ emissions decreased (as would be expected based on thermodynamic considerations). One of the motivations for my new NSF collaboration with Dr. Leigh McCallister (Virginia Commonwealth University) is to look at respiratory CO₂ and CH₄ emissions from a variety of freshwater wetland soils in order to try and identify the mechanisms that determine why some tidal freshwater wetlands show increased rates of soil carbon mineralization in response to saltwater intrusion, whereas others do not. For now, all we can say is that it appears that not all wetlands will respond similarly to salt water intrusion.