Seasonal dynamics of aquatic organic carbon driven by floating-mat formation in a terrestrializing lake

Abstract

Lakes are important components of the land carbon cycle, but how floating mats during lake terrestrialization reshape aquatic organic carbon remains unclear. Here we show, using seasonal field observations, carbon isotope tracing, and structural equation modelling, that floating mats reorganize dissolved and particulate organic carbon in a floating wetland system. During the growing season, acidic and hypoxic water beneath floating mats forms a hotspot of dissolved organic carbon accumulation, with concentrations exceeding 200 mg/L, compared with values below 40 mg/L in nearby open-water zones. Ice-covered conditions weaken this local gradient and produce more uniform dissolved organic carbon distributions. Source tracing indicates a shift from floating-mat-derived organic matter near the mats to stronger nearshore contributions away from them. Structural equation modelling further links carbon variability to biological production, water chemistry, temperature, and sediment–water exchange. These findings show that floating mats act as both sources and modulators of aquatic organic carbon during lake terrestrialization. Floating peat mats regulate aquatic organic carbon during lake terrestrialization by acting as sources and modulators shaped by biological production, water chemistry, temperature, and sediment-water exchange, based on field observations, isotope tracing, and structural equation modelling.