[Influence of Potamogeton crispus on Lake Water Environment and Phytoplankton Community Structure]

The decomposition of submerged macrophytes is generally associated with dramatic changes in the water environment, such as the large release of nutrients (e.g., nitrogen and phosphorus) and organic carbon to the surrounding waters, which may result in significant changes in phytoplankton community structure. In this study, Potamogeton crispus, physicochemical variables, and phytoplankton samples were collected in 14 shallow lakes in the middle and lower Jianghuai Plain in spring (growing period of P. crispus) and summer (decomposition phase of P. crispus) of 2018. The effects of the decline of P. crispus on water quality and phytoplankton community structure were quantified. The results showed that water transparency increased significantly in spring because the growth of P. crispus inhibits sediment resuspension and macrophytes can transport the nutrients from the water column to the sediment. The values of dissolved oxygen and pH also increased significantly due the photosynthesis by macrophytes. In contrast, the decomposition of P. crispus during summer months caused a significant increase in water turbidity and organic matter. There were considerable differences in phytoplankton biomass and cyanobacterial biomass in the sites with or without P. crispus, and the corresponding ratios of cyanobacterial biomass to the total algal biomass were 18.96% and 34.05%, respectively. Higher values of cyanobacterial biomass were observed with the decomposition of P. crispus than its counterpart in summer because ① the decomposition of macrophytes provided sufficient organic matter and nutrient (nitrogen and phosphorus) resources for cyanobacterial growth; ② P. crispus decline in summer significantly increased water turbidity, which makes cyanobacteria occupy a better ecological niche and more efficiently utilize nitrogen and phosphorus.