The seasonal performance of four differently configured constructed wetland systems was compared for the treatment efficiency of nonpoint source pollution. Dead plants in the constructed wetlands increased the nitrogen removal rate during winter by providing organic carbon, which is essential for the denitrification process. However, when the wetlands released phosphorus from the dead plants, the removal rate of phosphorus decreased. After seven growing seasons, plant coverage was nearly 100%, and the dissolved oxygen (DO) concentration was lowered to 1.3–5.4 mg/L. Open-water sections were then constructed inside the wetlands, which resulted in enhanced DO concentrations as well as improved treatment efficiency of nutrients and biochemical oxygen demand (BOD). Overall, performance of the constructed wetland was improved BOD, total nitrogen, and total phosphorus with the establishment of open water sections in the constructed wetland system.

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