Spatial distribution pattern of heavy metals in sediments of seasonal ice-covered lakes

About 40 % of lakes in the world have seasonal freezing characteristics, and their heavy metal accumulation status and behavior patterns during freezing are different from those of unfrozen lakes. Although the mechanism of heavy metal enrichment in lake environments has been widely explored as a toxic and highly accumulating pollutant, there is a lack of in-depth research on the possible occurrence and behavior patterns of heavy metals in these lakes during freezing, which may differ from those in non-freezing lakes. This study applied various evaluation methods such as sediment quality benchmarking method, potential ecological risk index, and geographic detector to comprehensively evaluate the occurrence characteristics, toxicity, and ecological detector of heavy metals in sediments of three major lakes (Hulun Lake, Wuliangsuhai and Daihai) in northern China during freezing and ice free periods. The results showed that, heavy metals were transported along ice-water-sediment in freshwater lakes, due to the freeze-concentration effect formed by the icing process, which made the heavy metal content in freshwater lakes during the glacial period, significantly higher than during ice-free period. However, high salt lakes are controlled by high salt water intrusion, which affects the migration process of heavy metals in sediments through competitive adsorption, ion exchange and solubility, resulting in significantly lower contents in brackish lakes, during the glacial period than during ice-free period, suggesting that icing and salinity affect the mobility of heavy metals. The biotoxicity effect and ecological risk showed that, the heavy metal composite contamination of lake is controlled by the degree of lake shrinkage, water exchange capacity and water exchange cycle, in addition to human activities. The results have significance in revealing the cycling and fate of heavy metals in global lake systems.