Suspended phosphorus sustains algal blooms in a dissolved phosphorus-depleted lake.

The expansion of algal bloom in surface waters is a global problem in the freshwater ecosystem. Differential reactivity of organic phosphorus (Po) compounds from organic debris, suspended particulate matter (SPM), and sediment towards hydrolysis can dictate the extent of supply often limited inorganic P (Pi) for algal growth, thereby controlling the extent of bloom. Here, we combined solution P-31 nuclear magnetic resonance (31P NMR), sequential extraction, enzymatic hydrolysis, and 16S rRNA measurements to characterize speciation and biogeochemical cycling of P in Lake Erhai, China. Lower ratios of diester-P/monoester-P in SPM in January (mean 0.09) and July (0.14) than that in April (0.29) reflected the higher degree of diester-P remineralization in cold and warm months. Both H2O-Pi and Po were significantly higher in SPM (mean 1580 mg ·kg-1 and 1618 mg ·kg-1) than those in sediment (mean 8 mg ·kg-1 and 387 mg ·kg-1). In addition, results from enzymatic hydrolysis experiments demonstrated that 61% Po in SPM and 58% in sediment in the H2O, NaHCO3, and NaOH extracts could be hydrolyzed. These results suggested that H2O-Pi and Po from SPM were the primarily bioavailable P sources for algae. Changes of Pi contents (particularly H2O-Pi) in algae and alkaline phosphatase activity (APA) during the observation periods were likely to be controlled by the strategies of P uptake and utilization of algae. P remobilization/remineralization from SPM likely resulted from algae and bacteria (e.g., Pseudomonas). Collectively, these results provide important insights that SPM P could sustain the algal blooms even if the dissolved P was depleted in the water column.

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.