Is dredging an effective ecological restoration method to improve water quality in freshwater ecosystems?

Sediment dredging has been widely applied worldwide as a pivotal pollution-mitigation method, and facilitating the restoration of biological communities and ecosystem functions and services. However, there are still some crucial questions remain unclear, including the effectiveness of dredging in improving water quality and how deep should we dredge. To address these uncertainties, we conducted a comprehensive meta-analysis by examining 71 studies including 800 effect sizes that focus on sediment dredging practices in freshwater ecosystems across China. Our results revealed significant improvement in twelve out of the fourteen assessed water quality indices by sediment dredging except for pH and total organic carbon in sediment (TOC_sediment). Specifically, we found increasing in dissolved oxygen, TOC_sediment, and transparency by 3.30 %-24.71 %, while other indices reduced by 8.40 %-38.53 %, suggesting significant improvement of water quality by dredging. Dredging effectiveness varied by many factors including dredging depth, post-dredging time, dredging status, and climatic factors. Dredging depth negatively correlated with the effect sizes of six water quality indices such as total phosphorus (TP) and NH4+, and the recommended dredging depth is 40-60 cm to maximize water quality improvements. Unexpectedly, the effect sizes of total nitrogen (TN), TP, NH4+, chlorophyll-a, and total suspended solids decreased with longer post-dredging time, indicating stronger beneficial of water quality improvement over time. Interestingly, dredging had more profound impacts in waterbodies of higher than lower pollutant concentrations. Combined projects (dredging + other methods), post-dredging, and deeper dredging (>40 cm) consistently improved water quality more than standalone dredging, ongoing-dredging, and shallow dredging depth, respectively. We also found significant effects of climatic factors, experimental type, and dredging quantity on the effectiveness of dredging on water quality. Collectively, our study highlights dredging potential for water quality improvement, especially for reducing concentrations of TN_sediment, TP_sediment, and chlorophyll-a. Such knowledge is fundamental for decision-makers to balance between environmental benefits and cost.