Sources of methylmercury in freshwater suspended particulate matter: Importance of In-situ methylation and adsorption

Suspended particulate matter (SPM) plays a pivotal role in the transport and biogeochemical cycling of methylmercury (MeHg) in freshwater ecosystems. However, the trophic bioavailability and dominant sources of SPM-associated MeHg remain poorly characterized, limiting our understanding of its ecological impacts. This study conducted a comparative analysis of five representative freshwater systems in Southwest China, comprising three lakes and two rivers, to elucidate system-specific dynamics of SPM-associated MeHg. Results indicate that SPM contained substantially higher total MeHg concentrations and trophic-bioavailable fractions than surficial sediments. Remarkably, lake SPM accounted for 48.9% of the trophic-available MeHg in surficial sediment inventories, significantly higher than the contribution of river SPM (10.9%), underscoring the central role of lake systems as reservoirs of trophic-available MeHg and amplifiers of food web exposure. In lakes, MeHg in organic-rich SPM originated predominantly from in-situ methylation and aqueous-phase adsorption, with minimal contributions from sediment resuspension. Conversely, river SPM accumulated MeHg primarily through adsorption processes, reflecting distinct source mechanisms between the two systems. These findings challenge conventional paradigms that prioritize sediment resuspension as the dominant MeHg source in freshwater SPM and highlight the critical role of water-column processes, particularly in lacustrine environments, in driving SPM-associated MeHg dynamics. Altogether, these insights have profound implications for assessing trophic transfer risks and managing Hg contamination in aquatic environments.