How does saltwater intrusion affect tidal freshwater wetlands? A consideration of experimental approaches
In recent years, there has been an increase in the number of studies in tidal freshwater wetlands that address the biogeochemical and ecological effects of saltwater intrusion, an environmental stressor that results in elevated salinities in historically freshwater ecosystems. In this presentation, I will review the variety of approaches and methods that have been used to understand how saltwater intrusion affects carbon biogeochemistry, specifically the mineralization and preservation of organic carbon in tidal freshwater wetland soils. These approaches range from salinity manipulations in the laboratory/field to space-for-time sampling along existing salinity gradients and vary in terms of their spatial extent, temporal scale, and what components of the ecosystem are included. Each researcher presumably selects the experimental approaches that are best suited for addressing specific research questions. Understanding the strengths and limitations of other approaches becomes especially important when synthesizing results from multiple studies.
Using my own data and the literature, I will discuss how different experimental methods can affect our ability to understand biogeochemical responses to saltwater intrusion. For instance, in situ saltwater additions to a South Carolina tidal freshwater marsh reduced annual CO2 emissions to the atmosphere, yet rates of CO2 production from soils at the same site increased following laboratory salinity manipulations. Sometimes, as in this case, apparently contradictory findings are due to methodological differences (e.g., short-term vs. longer-term responses to saltwater intrusion, or comparing a soil-only system with another system that also contains plants). Other times, there are real differences between different sites/systems/studies. There is no single experimental design that is ideal for all research questions, but a more complete understanding of the strengths, limitations, and caveats of different approaches will improve our ability to place our research findings in the context of existing literature and determine how saltwater intrusion affects the mineralization and storage of organic carbon in tidal freshwater wetlands.