Greenhouse Gas Flux in Coastal Salt Marshes: Field Measurements along Estuarine Gradients in Northeastern USA

Tidal Marshes are wetland ecosystems at the marine-terrestrial interface which serve as strong sinks for atmospheric carbon dioxide and large reservoirs of soil organic carbon (SOC). However, tidal marsh soils also produce and emit the potent greenhouse gas methane (CH₄). Previous work has demonstrated that CH₄ flux is inversely related to salinity, and that methane flux is negligible compared to carbon dioxide (CO₂) uptake in marshes with salinities of >18 parts per thousand (ppt). However, in lower salinity tidal marshes, CH₄ flux is highly variable, and can spike sharply following the depletion of sulfate supply. In order to better understand drivers of methane flux across a range of salinities, we established three transects along estuarine gradients in Rhode Island and Connecticut, USA. At landward and seaward sites along each transect, we measured methane flux, salinity, and conducted various porewater and soil chemical analyses. We found that methane flux was significantly higher and more variable in marshes where salinity is < 18 ppt. The highest magnitude methane fluxes occurred when sulfate was nearly depleted in marsh porewater, indicating that sulfate abundance dampens methane production, but demonstrating the need for further investigation into processes governing sulfate depletion and replenishment in salt marshes, and the degree to which salinity is a reliable proxy for sulfate concentration. Additionally, the lack of spatial data products which delineate tidal marshes according to salinity complicates efforts to estimate methane budgets in tidal estuaries. Our results indicate that spatial differences in salinity should inform wetland mapping in order to facilitate estimations of greenhouse gas budgets, but more high-resolution monitoring of salinity is needed to accurately delineate map units.