Constructed wetlands have been built in many rivers, lakes, and reservoirs to reduce the risk of algal blooms and eutrophications as well. Although constructed wetlands are designed by replicating the structure of natural wetland ecosystems, it is still questionable if their ecosystem functions are also secured. Therefore, this study is to evaluate the functions of constructed wetlands based on quantification methods, particularly focused on the function to sequester sediment, contaminants, and nutrients. Water qualities in ten constructed wetlands located in different climate zones have been monitored seasonally over five years. The average nutrients reduction rates were estimated by examining the difference between influent and effluent nutrient concentrations applying the mean concentration method. Additionally, seasonal and geographical variations were checked by statistical analysis.
Results/Conclusions
The average nutrient reduction rate of TN, TP, and COD are 35.9%, 1.1%, and 5.3%, respectively. TN reduction rate was relatively high with low regional variations, which means that nitrogen fixation and denitrification process in the constructed wetlands seems to be stable regardless of ecoregions. However, COD and TP reduction rate were low having high geographical variations. It is explained in some part by the effects that the plant population densities of the wetlands are very high and dominated by a few aquatic plants. Organic matters synthesized by them are recirculated into the water as they shed their shoots. Besides, phosphorus removal processes in the wetlands such as sorption and soil accretion are offset by resuspension of soil particles because the hydrological cycles of the constructed wetlands heavily rely on surface flow. Moreover, the nutrient reduction rate was typically higher in the growing seasons of wetland plants and microbiomes. Overall, constructed wetlands seem to have limited ecological functions compared to natural wetlands; however, they are useful for the water bodies where TN is a limiting factor for algal blooms.