Multi-scale response of mercury and methylmercury to submerged plant decline in a plateau wetland.

Submerged plant decline (SPD) can profoundly-but still incompletely understood-alter mercury (Hg) cycling, especially its methylation, in grass-dominated wetlands. To close this knowledge gap, we tracked the short-term and inter-annual responses of Hg and methylmercury (MeHg) to an unexpected SPD event in a plateau wetland. Immediately after SPD (2022-2023), dissolved MeHg (DMeHg) in pore water peaked in August, whereas dissolved Hg (DHg) in interface water peaked in May, revealing a temporal decoupling pattern. Spatially, seasonally inundated sediment had significantly higher DHg and DMeHg concentrations than permanently inundated sediment, an unexpected pattern that contrary to total Hg. Moreover, in May, DHg level in pore water fell below that in interface water, posing a reverse result previously recorded. The SPD-induced accumulation of algal particles, plant residues and minerals shifted water DOM toward higher protein-like fractions and aromaticity. These changes, combined with hydrological fluctuation and anaerobic condition, lead to the multi-scale distributions of Hg and MeHg. Two synchronous changing patterns were also formed: protein-like fractions and DMeHg peaked in pore water during algal blooms, while DHg and aromaticity increased in interface water at early rainy stage. The PLS-PM model further reveals that, among various environmental factors, DOM properties have the most positive impacts on MeHg production, and the contributions of DOM compositions, such as protein-like fractions, are particularly pronounced. From 2014 to 2023 following SPD event, sediment MeHg and its fraction in Hg fell to 78.95 % and 38.64 % of its 2014 level, while DHg and DMeHg declined by 44.52 % and 47.06 %, respectively. Concomitant enrichment of alkoxy carbon and sulfate at the sediment-water interface likely contributes to this long-term evolution. The SPD event is unexpected, but it reshapes the distribution patterns of Hg and MeHg, underscoring the need to incorporate this event into future Hg-risk assessments of plateau wetlands.