Human activities aggravate silicon limitation risks in floodplain lakes: Evidence from the sub-lakes of Poyang Lake

The biogeochemical coupling of silicon (Si) and carbon (C) in floodplain lakes has been increasingly disrupted by human activities, yet the mechanisms and trajectories of this decoupling remain poorly constrained. Here we present well-dated sediment records of biogenic silica (BSi) and associated geochemical proxies spanning the past century from three sub-lakes of Poyang Lake to fill this knowledge gap. Despite divergent BSi-content trajectories shaped by the contrasting hydrological settings among these sub-lakes, BSi/TOC mass ratios consistently reveal a systematic transition from coupled Si-C cycling toward decoupling. This transition intensified following dam construction in the 1950s-1960s and the acceleration of anthropogenic nutrient loading from the 1980s onward. Random forest modeling further reveals that following damming and eutrophication, catchment erosion and nutrient enrichment ranked as the strongest positive and negative predictors of BSi/TOC variation, respectively, with these associations strengthening as hydrological connectivity to the main lake declined. Furthermore, the decreased BSi/TOC ratios during the decoupling phase indicate possible silicon limitation and are consistent with increased inputs of non-siliceous algae. Because dam-driven silica retention and nutrient enrichment co-occur in many regulated rivers and lakes elsewhere, the risk of Si-C decoupling merits evaluation well beyond Poyang Lake, with implications for freshwater carbon sink assessments.