Environmental Controls on Seasonal Carbon Dioxide Exchanges in a Salt Marsh of Virginia

Carbon dioxide exchange between a salt marsh and the atmosphere was measured to investigate the biophysical controls affecting seasonal carbon dioxide assimilation. Measurements using the eddy covariance technique were performed over a tidal salt marsh in Virginia from December 2015 through November 2016. Seasonal carbon dioxide exchange variations were driven primarily by hydrologic conditions, photosynthetically active radiation, and plant phenology. Hydrologic conditions, in the low marsh zone, modified the daytime atmospheric exchange of carbon dioxide differently depending on the season. Throughout the year, flooding created anoxic conditions that suppressed uptake of carbon dioxide by marsh plants (peak height of 0.56 m). Relatively low daily carbon dioxide assimilation (-4 µmol CO₂ m⁻² s⁻¹) occurred during the spring and winter seasons due to lower temperatures and solar irradiance. Flooding (inundation greater than 0.9 m) enhanced the daytime carbon dioxide assimilation by the ecosystem, most likely due to lateral transport of pelagic material from the ocean. As plant productivity and standing biomass increased during the summer and autumn, the daily carbon dioxide exchange increased (-9 µmol CO₂ m⁻² s⁻¹) and the carbon dioxide assimilation by oceanic material during the flooding periods was negligible by comparison. Additionally, inundation of the marsh affected soil temperature and resulted in a reduction of ecosystem respiration during all the seasons. With increasing sea level and a modified environment, it is necessary to clarify these environmental influences on carbon cycling in the salt marsh ecosystem to determine its resilience as a function of regional and global environmental changes.