Fuel from within: Can suspended phosphorus maintain algal blooms in Lake Dianchi

Extensive algal bloom in the surface water is a pressing issue in Lake Dianchi that causes lake restoration to be difficult owing to complex and variable phosphorus (P) sources in the water column. P released from algae, suspended particles (SS), and sediment can provide sustainable P sources for algal blooms. However, little is known regarding the dynamic of P speciation in these substances from different sources. In this study, solution ³¹P nuclear magnetic resonance (³¹P NMR) and chemical sequential extraction were employed to identify P speciation in algae, SS, and sediment during different periods. Results showed that dissolved inorganic P (Pᵢ) directly accumulated in algae in the form of orthophosphate (ortho-P) and pyrophosphate (pyro-P). Algae preferentially utilized Pᵢ, followed by organic P (Pₒ) in the water column when the Pᵢ was insufficient during growth and reproduction. The ³¹P NMR spectra demonstrated that ortho-P, orthophosphate monoesters (mono-P), orthophosphate diesters (diester-P), and pyro-P dominated the P compounds across the samples tested. Increasing remineralization of SS mono-P driven by intense alkaline phosphatase activities was caused by increasing P needs of algae and pressure of P supply in the water column. The higher ratios of diester-P to mono-P in sediment (mean 0.55) than those in algae (mean 0.07) and SS (mean 0.11 in surface water, 0.14 in bottom water) suggested that the degradation and regeneration occurred within these P compounds during or after sedimentation. Pᵢ content in algae during growth and reproduction was controlled by its P absorption and utilization strategies. Results of this study provide insights into the dynamic cycling of P in algae, SS, and sediment, explaining the reason for algal blooms in the surface water with low concentrations of dissolved P.