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.