The stage-specific regulation mechanism of mercury methylation in shallow lakes water driven by the DOM structural reconstruction driven by the algal bloom life cycle

Algal blooms alter both the quantity and structure of dissolved organic matter (DOM), thereby influencing the transport and transformation of mercury (Hg) and methylmercury (MeHg) in water. This study focused on Wuliangsuhai, a shallow lake in an arid and cold region, and combined multi-stage in situ observations (2024–2025) with laboratory microcosm experiments to investigate the effects of DOM on water Hg and MeHg throughout the full life cycle of algal blooms. The results showed that Hg and MeHg concentrations in water exhibited a continuous increasing trend from the early release stage to the decomposition stage of algal blooms. In situ monitoring revealed that MeHg concentrations in the water during the decomposition stage were 2.90 times higher than those during the non-bloom period, while in simulation experiments, MeHg concentrations in the water under high algal-derived input conditions were 1.27–4.48 times higher than those under low-input treatments. During the early release and bloom stages, dissolved organic carbon (DOC) increased by 1.40 times compared to the non-bloom period, while Ultraviolet–visible (SUVA 254 ) and spectral slope ratio (S R ) decreased by 1.25 and 1.58 times, respectively. Upon entering the decomposition stage, SUVA 254 and humification index increased again, and Hg in water was further transformed into MeHg, potentially associated with enhanced microbial activity and the formation of reducing microenvironments during algal decomposition. Laboratory experiments further demonstrated a pronounced stage-dependent amplification effect of Hg and MeHg in water during the decomposition phase. This study reveals that MeHg risk persists throughout the entire algal bloom process and is significantly amplified during the decomposition stage, providing a scientific basis for developing a lake Hg risk assessment framework based on algal bloom succession.