Underwater light limitation controls long-term submerged plant persistence in Lake Poyang floodplains, an important Ramsar site

Submerged macrophytes are critical to the integrity of large river-floodplain ecosystems (RFE), yet ecological constraints on their persistence remain poorly understood. Attention has mainly focused on water level fluctuations or lateral hydrological connectivity as primary drivers. In contrast, the role of underwater light, the most widely recognized driver of submerged macrophyte dynamics in shallow lakes, has often been overlooked. This study examined whether underwater light limitation regulates long-term dynamics of Vallisneria spinulosa, a dominant submerged plant in numerous floodplain sub-lakes of Lake Poyang, an important Ramsar site. We integrated field vegetation surveys, long-term Landsat imagery (1986-2021), lake optics modelling, and systematic literature surveys to explore changes in underwater light during floodings, their impacts on V. spinulosa growth and its long-term habitat distributions, and the fundamental drivers behind these changes. Underwater light availability during the flooding period determined V. spinulosa biomass densities, with a minimal light threshold of SD/Dep (Secchi depth to water depth) at 0.2. Based on the threshold, derived habitat distribution in numerous sub-lakes showed strong agreement with both field observations (89 % accuracy) and satellite-classified distributions (80 % accuracy). V. spinulosa habitat distribution exhibited high interannual variability with no consistent long-term shrinkage, and even expansion in historically poor habitat zones. Underwater light limitation, associated with sediment dynamics influenced by rainfall, flow regimes, and sand dredging, appear to be the dominant constraint. Our findings suggest that enhancing water clarity via sediment management is essential for conserving submerged vegetation in floodplain lakes under changing hydrological and climatic conditions.