Response of Coastal Vegetation to Flooding and Salinity: A Case Study in the Rapidly Subsiding Mississippi River Deltaic Plain, USA

Abstract

In rapidly subsiding coastal environments such as the Mississippi River deltaic plain, maintenance of the marsh surface in the intertidal zone is important for the survival of marsh vegetation. Coastal wetlands vertically accrete in response to increases in water level through accumulation of organic matter and mineral sediment. In brackish and salt marshes, plant productivity is related to the mineral content of the marsh soil profile. Areas which are not receiving adequate sediment supplies are experiencing marsh deterioration resulting from plant stress caused by flooding and saltwater intrusion. These factors adversely affect normal physiological functions leading to stressed plants characterised by increased anaerobic root respiration, lower carbon fixation and lower productivity. Increases in the intensity of soil reduction restricts root elongation, which reduces the effective rooting zone of wetland vegetation, thereby limiting its access to nutrient sources. Results suggest that coastal marsh vegetation requires periods of drainage or less reducing soil conditions during the growing season for root development in otherwise anaerobic soil zones. Once roots are established, anaerobic root metabolism and oxygen transport mechanisms allow plants to survive adverse soil redox conditions. Increased soil sulphide levels and salinity result in stomatal closure and reduction of photosynthetic rates in brackish and freshwater vegetation. The addition of sediment to a deteriorating marsh reduces plant stress and increases productivity.