Influence of prolonged submergence or exposure to air on photosynthesis and carbon dioxide exchange in a microtidal Spartina alterniflora salt marsh

Salt marshes are globally-distributed environments that straddle the land to sea boundary. They provide myriad ecosystem services that support coastal ecosystems and human communities. It is well documented that salt marsh plants grow best at an optimum elevation relative to sea level and modulated by local tidal patterns, with different plant species occupying different elevation niches within the marsh. For example, Spartina alterniflora, the dominant marsh grass along the Gulf and Atlantic coasts of the United States, typically grows best in a window centered around mean higher high water that results in a regular pattern of inundation and exposure that the plant is adapted to. However, in microtidal environments that characterize the Gulf coast, regional wind patterns can supersede tides and alter patterns of inundation and exposure, sometimes resulting in large areas of marsh that are inundated or exposed for a week or more at a time. Here, we report on a mesocosm-based experiment to determine the influence of these longer-term inundation/exposure events on plant metabolism and salt marsh greenhouse gas fluxes. We show that longer periods of exposure (> 5 days) increased plant photosynthetic capacity but did not alter respiration. These longer periods of inundation and exposure did not have a detectable impact on soil or plant-mediated methane fluxes. Since these prolonged periods of inundation and exposure typical of microtidal environments can result in changes of carbon assimilation, it is important to account for these differences when calculating annual salt marsh carbon budgets in the Gulf of Mexico.