Spatial-temporal evolution of ecological risks in a typical urban lake in Southwestern China

During urbanization, excessive development and utilization pose varying risks and challenges to the ecosystems of many urban lakes. Sediments are invaluable archives of past ecosystem changes. By analyzing variations in 210Pb and other geochemical indicators in sediment cores, it is possible to assess how susceptible lakes are to the impacts of climate change and human-induced pollution, which is crucial for urban planning and the management of lake environments. This study investigated the status of heavy metal pollution in Lake Longquan, Chengdu City, and reconstructed the historical evolution of its ecological environment using 210Pb dating. Three main conclusions are drawn. (1) Currently, Cd, Cu, Pb, Zn, Cr, and organic matter in the surface sediments of Lake Longquan are highly enriched in the southeastern, central, and northeastern regions. Cd levels even reach 1.54 mg/kg in the southeastern part of the lake near the town. The high overlap of heavy metal distribution with historical aquaculture areas and some tourist areas indicates that aquaculture and tourism are important sources of heavy metals that increase their bioavailability. (2) Lake Longquan is currently at a high potential ecological risk level, and the high potential risk level of Cd may cause ecological safety problems. The biological toxicity of As should be considered, too. (3) The risk evolution in Lake Longquan can be divided into six stages: (i) Before 1977, the ancient river. With the destruction of forest vegetation and the intensification of rock weathering, ecological risks increased. (ii) 1977-1989, construction and development. Due to human factors and the joint influence of heavy precipitation events, ecological risks increased; and (iii) 1989-2010, development prioritized protection. Due to the ineffective implementation of protection policies, aquaculture increases risks. (iv) 2010-2017, development. As a provincial resort and tourism area, the significant increase in human activities has led to constantly rising sedimentation rates and heavy metal concentrations, resulting in an ecological risk peak. (v) 2017-2020, protection. The efficient implementation of lake protection and restoration policies proved to be effective, and the ecological risk started to decrease. (vi) After 2020, protective development. The government's plan to reduce fertilizer usage and increase forest cover will help control ecological risks. It is necessary to continue controlling tourism and aquaculture activities, and transitional areas should be established around lakes for city residents to engage in lakeside sightseeing and recreational sports. This study provides valuable references for controlling and preventing heavy metal pollution in urban lakes.