Optimal submerged macrophyte coverage for improving water quality in a temperate lake in China

Abstract Submerged vegetation is essential for maintaining a clear water state in shallow lakes. To determine the optimal submerged macrophyte coverage required to improve water quality, field investigations and a simulation experiment were carried out in a sub-lake of Baiyangdian Lake, China. The effects of two seasons and five submerged macrophyte coverage gradients (0 [C-0], 25 [C-25], 50 [C-50], 75 [C-75], and 90% [C-90]) were examined in the field study, while the five submerged macrophyte coverage gradients were examined in an 80-day simulation experiment. In the field study, plant coverage, season, and their interaction all had a significant effect on most physicochemical parameters. Compared with C-0, the Secchi depth and pH of the water were higher, while the total nitrogen and total phosphorus concentrations were lower when submerged vegetation was present. Lowest values of total nitrogen, total phosphorus, and conductivity all appeared under C-50. Meanwhile, in the simulation experiment, the total nitrogen content decreased by 76.43, 90.98, 88.28 and 84.32%, and the total phosphorus content decreased by 85.15, 88.48, 87.76 and 79.00% at C-25, C-50, C-75 and C-90 compared with C-0, respectively. Moreover, plant coverage and time had a significant effect on all eight physiochemical parameters. There were no significant differences in the total nitrogen content among C-50, C-75 and C-90, but values were significantly lower than at C-0 and C-25. In contrast, the total phosphorus content was significantly higher at C-0 than all other groups, and the lowest value was observed at C-50. Overall, these findings suggest that 50% is the optimal submerged macrophyte coverage for improvements in water quality in temperate shallow lakes.