Comparing the effect of tailwater replenishment at different time intervals on eutrophic surface freshwater: a mesocosm simulation study

Tailwater is becoming the main water source supplied to surface freshwater worldwide. However, few studies have compared the effect of tailwater replenishment at different time intervals on eutrophic surface freshwater. In this study, we investigated the changes of water quality, sediment, and submerged macrophytes in eutrophic water in an outdoor mesocosm in response to different tailwater replenishment time intervals [every 7 days (TW7), 14 days (TW14) and 28 days (TW28)]. An 84-day simulation experiment demonstrated that there were only significant differences in the total nitrogen (TN) concentration of the overlying water, with the lowest mean value occurring in TW28. Nevertheless, the sediment TN was lowest in TW7 with a denitrification rate of 102.9 μmol/m2/h. Tailwater replenishment also increased the nitrogen content and total biomass of Vallisneria spiralis, and TW7 had the highest total biomass of 20.19 g. Additionally, tailwater replenishment also affected plant enzyme activity, causing an increase in superoxide dismutase, peroxidase, and catalase, coupled with a decrease in malondialdehyde concentration in leaves. Overall, TW28 can be adopted as a tailwater replenishment strategy to ensure water quality, whereas TW7 can be applied without a strict water quality requirement for TN.