Effect of algal blooms outbreak and decline on phosphorus migration in Lake Taihu, China

Algal blooms (ABs) can affect the migration of phosphorus (P) among sediments, interstitial water and overlying water. It is important to analyze the characteristics of P and their interactions in the three media during ABs. A 5-month field study (June to October in 2016) was conducted in Zhushan Bay of Lake Taihu. P fractions, P adsorption characteristics and P diffusion fluxes at the sediment-water interface (SWI) were investigated. During the outbreak period of ABs from June to August, labile P concentrations in the sediment measured by diffusive gradients in thin films (DGT-labile P) and its diffusion fluxes across the SWI increased significantly. The equilibrium P concentration (EPC0) of the sediment was higher than the PO43--P concentration in the overlying water. During the period of decline of ABs from September to October, the concentrations and diffusion fluxes of DGT-labile P sharply decreased. However, the sediment total P (TP), overlying water TP, total dissolved P (TDP) and PO43--P concentrations increased. These results show that the ability of sediment solids to supplement interstitial water labile P was significantly enhanced by the outbreak of ABs. Labile P was then intensively released into the overlying water by interstitial water. Degraded algae became a crucial P source during the period of decline of ABs. P from the degraded algae was re-released to the sediment and overlying water. The observed DGT-labile P and DGT-labile Fe coupling in June, September and October confirmed the Fe redox-driven P release mechanism in sediment during these periods. The decoupling of DGT-labile P and DGT-labile Fe was observed in July and August and was probably caused by algal decomposition, labile organic P degradation and/or sulfate reduction in sediment stimulated by the ABs outbreak.