Macrophyte-Mediated Oxygen Transfer in Wetlands: Transport Mechanisms and Rates
Hans Brix
Abstract
In constructed wetlands with subsurface water flow, wastewater is believed to be purified during contact with the surface of soil/gravel particles and with the roots of macrophytes. Sediments in wetlands are continuously water saturated and therefore generally anoxic or anaerobic. This chapter summarizes the existing knowledge on internal gas transport mechanisms in wetland plants and oxygen leakage from their root system. Most studies have been done using oxygen microelectrodes to measure radial oxygen losses from individual roots in oxygen-depleted solutions. The decreased gas permeability of the root walls minimizes radial leakage outward, allowing more oxygen to reach the apical meristem. Oxygen release from fine laterals at the base of roots can be significant, but generally, no release of oxygen from old roots and rhizomes are detected. Oxygen release rates from roots depend on the internal oxygen concentration, the oxygen demand of the surrounding medium, and the permeability of the root walls.