Objective  The submerged plant Ceratophyllum demersum is one of the commonly used plants in water ecological restoration projects. Therefore, determining the planting density of C. demersum is of great significance to promote the restoration of eutrophic water and accelerate the development of underwater forests.

  Method  In this study, we simulated the different densities of C. demersum, and studied the characteristics of water quality changes under different planting densities and analyzed the purification efficiency of C. demersum on total nitrogen, total phosphorus and organic matter in eutrophic water under six planting densities (36, 50, 75, 100, 110, 120 plant/m²). Finally, combined with cost analysis, the optimal planting density was selected and the thinning and harvesting time of C. demersum was determined.

  Result  The plant corresponding to the density distribution of 100 plant/m² had the highest survival rate (95.65%), with the total nitrogen removal rate of 62.86% and the total phosphorus removal rate of 74.32%. This density had the best effect on the absorption of pollutants such as nitrogen and phosphorus, and was the best planting density for aquatic ecological restoration projects. In addition, within 15 days after planting, the removal efficiency of nitrogen, phosphorus, chemical oxygen demand (COD) and other pollutants was better along with the increase of planting density in eutrophic water, and the dissolved oxygen concentration was higher. After 50 days of planting, the amount of pollutants in each configuration increased. At this time, a part of nitrogen, phosphorus and organic matter in C. demersum were released into the water with the decline of the plants, and the dissolved oxygen content decreased more rapidly.

  Conclusion  Therefore, it is necessary to consider the best purification time and timely harvest in the utilization of C. demersum to achieve water restoration.