Seasonal shifts in mercury speciation and relative methylation potential in cold-arid seasonally ice-covered lakes.

Approximately 50% of global lakes experience seasonal ice cover for an average of 217.5 days per year, a period that has traditionally been regarded as an ecological "dormant phase". During this period, lake ice regulates light availability, thermal structure, gas exchange, redox conditions, and hydrodynamic processes, all of which can influence mercury (Hg) speciation, transport, and transformation. However, most Hg biogeochemistry studies have focused on the open-water period, leaving Hg cycling during the ice-covered period insufficiently understood. To address this knowledge gap, six representative cold-arid lakes in northern China were investigated across ice-covered and ice-free periods. We quantified the seasonal distribution, ecological risk, and environmental responses of total mercury (THg), methylmercury (MeHg), and Hg methylation potential, expressed as MeHg% (MeHg/THg)×100%, in water and sediments. The results showed that THg and MeHg concentrations in both water and sediments were generally higher during the ice-free period, which may be related to enhanced external inputs from agricultural non-point sources, atmospheric deposition, hydrological recharge, and in-situ production. In contrast, MeHg% was significantly elevated during the ice-covered period, indicating a higher relative methylated fraction of the Hg pool rather than higher absolute MeHg concentrations or direct evidence of enhanced net MeHg production; this pattern may partly reflect lower THg concentrations under ice-covered conditions. Water depth was significantly associated with sediment MeHg%, and additional partial correlation and multiple regression analyses showed that this relationship remained significant after controlling for measured sediment oxidation-reduction potential, organic matter, and season. These results suggest that deeper-water environments may favor Hg methylation potential through coupled effects involving light limitation, reduced sediment-water oxygen exchange, and more reducing sediment conditions. In the water column, pH, salinity, and total dissolved solids were significantly associated with Hg species and MeHg%, suggesting that salinization and alkalization may modify Hg methylation risk in cold-arid lakes. However, their effects are likely condition-dependent and should not be interpreted as purely positive drivers. Overall, this study demonstrates that the ice-covered period represents an important but previously underrepresented stage of Hg methylation potential in cold-arid lakes. Risk assessments based only on ice-free monitoring may underestimate seasonal Hg exposure risks. These findings support the incorporation of ice-covered periods into routine Hg speciation monitoring and provide a basis for improving Hg risk assessment in seasonally frozen inland waters under ongoing climate and environmental change.