Abstract: Nfixing cyanobacteria can create extra nitrogen for aquatic ecosystems. Previous studies reported inconsistent patterns of the contribution of biological nitrogen fixation to the nitrogen pools in aquatic ecosystems. However, there were few studies concerning the effect of Nfixation by cyanobacteria on the nitrogen removal efficiency in constructed wetlands. This study was performed at Beijing Wildlife Rescue and Rehabilitation Centre, where a constructed lake for the habitation of waterfowls and a constructed wetland for purifying water from the lake are located. The composition of phytoplankton communities, the concentrations of particulate organic nitrogen (PON) and nitrogen fixation rates (R_n) in the constructed lake and the constructed wetland were compared throughout a growing season. We counted the densities of Anabaena and Microcystis cells, and explored their relationships with PON and R_n in water. The proportions of PON from various sources, including the ambient N₂, waterfowl faeces, wetland sediments and nitrates in water, were calculated by the natural abundance of ¹⁵N with the IsoSource software. The results revealed that the constructed lake was alternately dominated by Anabaena and Microcystis throughout the growing season, and the R_n was positively correlated with PON and the cell density of Anabaena (P<0.05). This implied that the fixed nitrogen by Nfixing Anabaena might be utilized by nonNfixing Microcystis, maintaining the fixed nitrogen with PON form. The contributions of ambient N₂ to the PON in the constructed lake and wetland were 0.5%-82.0% and 50.0%-86.0% respectively during the growing season. The proportions of PON from N₂ increased to more than 80% when the R_n reached the highest in September. The results demonstrated that the nitrogen fixed by Anabaena might be utilized by nonNfixing Microcystis, which formed water blooms in summer. Therefore, the decline of the removal efficiency of PON in the constructed wetland in summer might indirectly result from the nitrogen fixation, since the proliferated algae were difficult to sediment in surface flow wetlands.