No abstract

The larger aquatic plants growing in wetlands are usually called macrophytes. These include aquatic vascular plants, aquatic mosses and some larger algae. The presence or absence of aquatic macrophytes is one of the characteristics used to define wetlands, and as such macrophytes are an indispensable component of these ecosystems. As the most important removal processes in constructed treatment wetlands are based on physical and microbial processes, the role of the macrophytes in these has been questioned. This paper summarizes how macrophytes influence the treatment processes in wetlands. The most important functions of the macrophytes in relation to the treatment of wastewater are the physical effects the presence of the plants gives rise to. The macrophytes stabilise the surface of the beds, provide good conditions for physical filtration, prevent vertical flow systems from clogging, insulate the surface against frost during winter, and provide a huge surface area for attached microbial growth. Contrary to earlier belief, the growth of macrophytes does not increase the hydraulic conductivity of the substrate in soil-based subsurface flow constructed wetlands. The metabolism of the macrophytes affects the treatment processes to different extents depending on the type of the constructed wetland. Plant uptake of nutrients is only of quantitative importance in low-loaded systems (surface flow systems). Macrophyte mediated transfer of oxygen to the rhizosphere by leakage from roots increases aerobic degradation of organic matter and nitrification. The macrophytes have additional site-specific values by providing habitat for wildlife and making wastewater treatment systems aesthetically pleasing.

Macrophytes have several intrinsic properties that makes them an indispensable component of constructed wetlands. The most important functions of the macrophytes in relation to the treatment of wastewater are the physical effects brought about by the presence of the plants. The macrophytes stabilise the surface of the beds, provide good conditions for physical filtration, prevent vertical flow systems from clogging, insulate against frost during winter, and provide a huge surface area for attached microbial growth. Contrary to earlier belief, the growth of macrophytes does not increase the hydraulic conductivity of the substrate in soil-based subsurface flow constructed wetlands. The metabolism of the macrophytes affects the treatment processes to different extents depending on the design of the constructed wetland. Plant uptake of nutrients is only of quantitative importance in low-loaded systems (surface flow systems). Macrophyte-mediated transfer of oxygen to the rhizosphere by leakage from roots increases aerobic degradation of organic matter and nitrification. The macrophytes have additional site-specific values by providing habitat for wildlife and making wastewater treatment systems aesthetically pleasing.

Internal pressurization and convective through-flow are demonstrated to be common attributes of wetland plants with cylindrical culms or linear leaves. Eight of 14 species tested produced static internal gas pressure differentials of 200-1,300 Pa relative to ambient and internal convective airflows of 0.2 to > 10 cm3 min-' culm-I, depending on species. Four species produced internal static pressure differentials of < 100 Pa. Two species did not pressurize. The driving forces are gradients in temperature and water vapor between the internal gas spaces of the plants and the ambient atmosphere (thermal transpiration and humidity-induced pressurization). A clear diel variation in pressurization and convective flow was observed; rates were highest in the afternoon and lowest at night, responding to ambient changes in light, temperature, and humidity. The resistance to airflow at the stem-rhizome junction was very high for some species, resulting in a low ability to convert internal pressurization into convective airflow through the rhizomes. Species with a high potential for internal pressurization and a low internal resistance to convective flow seem to have a competitive advantage over species that rely exclusively on diffisive gas transport, which allows them to grow in deeper waters.

Long-distance dispersal and weak breeding barriers appear to be recurring phenomena, not only in the GC, but worldwide. We present data strongly suggesting that interspecific hybridization and introgression among different Phragmites species take place and appear to have contributed significantly to the diversification processes within the genus. Hence, the application of traditional species concepts within Phragmites might be inappropriate.