Decomposition of emergent macrophytes is now recognized as an internal nutrient source for shallow lakes. Temperate lakes always experience seasonal ice cover in winter, but the influences of emergent macrophytes decomposition on water quality have rarely been examined under ice. Here, we conducted an incubation experiment to investigate winter decomposition of two common emergent macrophytes species (Typha orientalis and Phragmites australis) and its influences on water quality in the Hengshui Lake, North China. Mesocosms simulating a lake ice regime were incubated in the field for 120 days in winter and were treated with and without plant material addition. Water quality was monitored through dissolved oxygen (DO), dissolved organic carbon (DOC), total nitrogen (TN), total phosphorus (TP), ammonium nitrogen (NH₄-N), and nitrate nitrogen (NO₃-N). We found that both species were significantly decomposed in winter and that the majority of mass loss occurred in the first 10 days of decomposition when the water surface of mesocosms were already frozen. The concentrations of DO rapidly dropped to values close to zero after plant material submergence. At the end of incubation, the concentrations of DOC, TN, and NO₃-N in the mesocosms with plant material addition were significantly higher than initial concentrations. In contrast, the concentrations of DOC, TN, TP, NO₃-N, and NH₄-N in the mesocosms without plant material addition were equal to or less than initial concentrations. Our research suggests that winter decomposition of emergent macrophytes produces negative influences on water quality under ice that lasts for the whole winter.

A comprehensive analysis of the spatial and temporal variations of heavy metals in wetland sediment can delineate the changes in possible contamination sources, providing valuable conservation strategies for further wetland management. Using the pollution index, enrichment factors, and potential ecological risk index, the spatial and temporal variations in heavy metals (Cd, Hg, As, Pb, Cr, Cu, and Zn) were evaluated in Hengshui Lake in north China in 2005 and 2020. The results demonstrated that the concentrations and assessment index for most heavy metals all decreased, with that of As decreasing the most (−54.3%), which mainly benefited from the implementation of a series of ecological conservation and restoration projects. Although the assessment indexes for most heavy metals indicated non-pollution status, Hg and Cd exhibited medium enrichment and moderate potential ecological risk. Especially for Cd, the related indexes increased by 860.0%, mainly influenced by anthropogenic activities. Furthermore, the high pollution was mainly distributed nearby the regions of dense enterprises and wastewater overflow zone (i.e., Wangkou sluice, the Jizhou Small Lake and its causeway). This was primarily attributed to the discharge of industrial wastewater and Cd-polluted ecological diversion water. These findings demonstrated the necessity of the continued and targeted implementation of wetland conservation and restoration projects and identified possible contamination sources and important pollution regions that could provide insights into contamination control options and targeted management strategies for Hengshui Lake.