Chemical speciation, source apportionment, and pollution assessment of phosphorus in sediments from typical urban lakes in Southern China

Urban lakes play a vital role in flood disaster mitigation, water supply, and climate maintenance. However, excessive phosphorus inputs resulting from urban development constitute a significant factor in the deterioration of aquatic ecosystems. In this study, samples of sediment profiles (within 40 cm) from three representative urban lakes (Xianghu Lake, Qianhu Lake, and Aixihu Lake) in Nanchang were collected to investigate the chemical speciation characteristics, analyze the sources and assess the pollution condition of phosphorus. Results indicated that sediments in Qianhu Lake exhibited the highest concentration of total phosphorus (923.06–1630.17 mg/kg), followed by Aixihu Lake (616.58–1532.88 mg/kg), while Xianghu Lake exhibited the lowest levels (538.79–1481.57 mg/kg). Calcium-bound phosphorus (Ca-P) dominated in all three lakes (accounting for over 30% of total phosphorus), followed by iron-bound phosphorus (Fe-P) and detrital phosphorus (De-P). While the organic phosphorus (Org-P) and exchangeable phosphorus (Ex-P) constituted the lowest proportions, with lower than 15% of each speciation. The distribution of phosphorus speciation in the lake sediments exhibited vertical spatial heterogeneity, reflecting the differentiation of influencing factors and sources. In all the lakes, clay dominated the sediment composition (≥ 34%), followed by coarse silt, while fine silt was the least abundant. Correlation analysis revealed that grain size and organic matter constrained the concentrations of phosphorus speciation. The Absolute Principal Component Score-Multiple Linear Regression model (APCS-MLR) revealed that phosphorus pollution in Xianghu Lake, Qianhu Lake, and Aixihu Lake was predominantly driven by agricultural non-point sources, with the contributions of more than 35%, followed by industrial emissions and domestic sewage, which corresponded with the extensive agricultural land around the lake. Nemerow integrated pollution index analysis revealed that all the lakes were heavily polluted, and ranked as follows: Qianhu Lake (F ≥ 2.26) > Aixihu Lake (F ≥ 2.04) > Xianghu Lake (F ≥ 1.92). Consequently, comprehensive management measures, including optimizing agricultural fertilization–irrigation forms and restricting pollutant discharges, are required to prevent further deterioration. This study is of practical significance for the prevention of lake eutrophication. Also, it provides a scientific basis for research on biogeochemical cycles and the protection and management of aquatic ecosystems in the urban lakes.