Dynamics of sediment phosphorus fluxes drive Microcystis blooms in shallow lakes

Phosphorus (P) is a key limiting element driving Microcystis blooms in shallow lakes. Although sedimentary P dynamics, water-column P supply modes, and algal P utilization strategies are recognized as critical factors promoting development of blooms, their connections and coupling mechanisms remain insufficiently elucidated. In this study, we combined field investigations in Wuhan's shallow lakes with controlled simulations. Using equilibrium phosphorus concentration (EPC₀) as an indicator of sedimentary P release characteristics, we found that EPC₀ values correlated positively with soluble reactive phosphorus (SRP) concentrations in the water column (P<0.01), negatively with density proportions of Microcystis (P<0.01), and positively with relative abundance of Chlorophyta and Dolichospermum (P<0.05), indicating sediment-mediated SRP supply shaped phytoplankton structure. Notably, CaCO₃-bound P (CaCO₃∼P), rather than iron-bound P, emerged as a key factor contributing to sedimentary P release and phytoplankton proliferation, reminding us to focus more on CaCO₃∼P in sediment management. Simulation experiments further demonstrated that Microcystis achieved higher growth rates under the condition of slight and continuous SRP supply compared to P-replete environments, attributable to its efficient P uptake capacity. In contrast, Chlorophyta and Dolichospermum thrived under high P availability, exhibiting superior growth and P utilization. In brief, our findings demonstrated that low-EPC₀ sediments created a slight and continuous P supply mode favoring Microcystis while providing sufficient P loads, ultimately facilitating blooms. Hence, maintaining EPC₀ within an optimal range is critical for balancing phytoplankton biodiversity and ecosystem stability. This study provides novel insights for sediment management strategies targeting eutrophication mitigation and cyanobacterial blooms control in shallow lakes.