Eutrophication of freshwater ecosystems is a major global problem, but restoration can be difficult due to ongoing problems relating to water pollution, sedimentary nutrient stores, and altered aquatic biodiversity. Mitigation of water quality stressors is often conducted alongside transplantation of submerged macrophytes and dredging, but knowledge of ecosystem response to post-dredging transplantation of submerged macrophytes is limited. 

Here, we report a long-term (2008-2018) in-situ monitoring study to evaluate the effects of two different restoration measures: dredging only (Dredged) and dredging with post-dredging transplantation of submerged macrophytes (Dredged with macrophytes) conducted in five subtropical eutrophic lakes in Lake Taihu basin, China. Water and sediment nutrients, bloom-forming algae Microcystis, and macroinvertebrate were monitored every two years for each treatment and compared with reference areas (Control) established in unrestored parts of the same lakes. 

Dredging only decreased sediment nutrients (nitrogen, phosphorus, total carbon, and water total phosphorus significantly, however, this effect diminished about five years later. Dredged with macrophytes had a stronger, longer-lasting positive effect on water quality than Dredged alone. Disturbance caused by dredging (without macrophytes transplantation) decreased the biomass of Microcystis, while transplantation of submerged macrophytes shortly after dredging did not contribute to the decrease of Microcystis biomass. The biomass of Microcystis in Dredged with macrophytes areas was always similar to Control over the period of our monitoring. 

A positive effect of submerged macrophytes transplantation post-dredging was found for macroinvertebrate abundance and diversity: Dredged with macrophytes areas had significantly higher macroinvertebrate biomass and richness than Dredged areas after 9 years’ recovery. Macroinvertebrate richness in Dredged with macrophytes areas nearly doubled compared to Control; while Dredged areas were just restored to Control levels. 

Synthesis and applications. Our study provides an in-situ long-term field monitoring with new findings about the benefits and caution of submerged macrophytes transplantation post-dredging, and the effect of partial restoration, which could inform eutrophic waterbody restoration schemes.