Effects of Lake Geo-Engineering on Plankton in a Typical Shallow Urban Lake: Evidence Based on 10-year Data

Lake geo-engineering such as submerged macrophyte restoration is generally applied to re-establish the biological components of aquatic systems and recover the ecological functions of eutrophic shallow lakes. Research on the long-term spatiotemporal variations in plankton and their correlations with environmental variables after submerged macrophyte restoration is limited. Plankton in the West Lake, Hangzhou, Zhejiang Province, China, was monitored on a monthly basis, and the main factors and relative contributions of environmental variables to phytoplankton were determined using quantitative analyses after lake geo-engineering. Dominant phytoplankton phyla exhibited a gradual decline along with submerged macrophyte restoration, and the composition changed to a more evenly distributed status on a long-term scale. Zooplankton populations sustainably increased during the initial period of submerged macrophyte restoration, and then gradually decreased until the composition sustained a stable status. Owing to lake geo-engineering, significant spatiotemporal variations in phytoplankton were attributed to the large submerged macrophyte distribution, water diversion project, and seasonal change. Spearman correlation analysis results illustrated that submerged macrophyte characteristics, especially Vallisneria natans, which was the species with the highest distribution in the West Lake, strongly affected the dominant phytoplankton. Comparison of ecological indices including abiotic and biotic factors in the monitoring survey revealed that submerged macrophytes were responsible for re-establishing a preferable aquatic system. The core effect of submerged macrophytes in the aquatic system could be explained by the interactions between environmental variables, whereby submerged macrophytes affected phytoplankton growth by releasing allelochemicals and altering water nutrient levels, sediment, and zooplankton communities. Therefore, our findings offer new evidence on the vital ecological functions of submerged macrophytes in a typical subtropical shallow urban lake under increasing anthropogenic pressure.