Dredging project caused short-term positive effects on lake ecosystem health: A five-year follow-up study at the integrated lake ecosystem level

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Sediment dredging is a controversial technology for lake eutrophication control. A lengthy and holistic assessment is important to understand the effects of a dredging project on a lake ecosystem. In this study, a dredging project was followed for 5 years. To understand the variations of lake ecosystems before, during and after the project, water quality, phytoplankton, zooplankton and benthic animal biomass were monitored; Four subindicators, including eco-exergy (Ex), structural eco-exergy (Exst), buffer capacity of total phosphorus for phytoplankton (β(TP)(phyto)) and trophic level index (TLI) were calculated and developed to an integrated ecosystem health indicator (EHI). The monitoring results showed that the dredging project caused many short-term positive effects such as decreased total nitrogen, total phosphorus, permanganate index and phytoplankton biomass throughout the entire lake water, increased Secchi disk depth in the whole lake and increased benthonic animal biomass in the nondredged regions. However, these positive effects disappeared overtime. Water chemistry and biomass returned to the initial state before dredging. EHI showed that the dredging project caused negative effects on the lake health in the dredged region at first. Subsequently, the health status of the entire lake, including the dredged and nondredged regions, improved until 1–2 years after the project finished. Because of the lack of other timely ecological restoration measures, the lake gradually returned to its initial health status. However, the health status in the dredged regions was only slightly better than before dredging and often worse than that of the nondredged regions. Our study suggested that dredging projects may only cause short-term positive effects on lake ecosystem health. The external interception and dredging ratio were important. A dredging project should be combined with other ecological lake restoration measures when the project has caused positive effects in a lake.