A Landscape Limnology Framework for Lake Typology: Refining Geochemical Baselines and Heavy Metal Assessment in the Middle and Lower Yangtze River Plain

Sedimentary heavy metal background levels vary among lake types in heterogeneous floodplain lake systems worldwide. Uniform regional values may therefore bias contamination assessment. We developed a source–process–sink framework linking lake typology to type-specific geochemical baselines for 91 lakes in the Middle and Lower Yangtze River Plain. Ward hierarchical clustering of 24 indicators spanning freshwater, terrestrial and human landscapes plus two spatial metrics identified four groups: shallow river network (Group I, n = 28), small peri-urban lakes (Group II, n = 20), hill–plain transition lakes (Group III, n = 23) and large river-connected lakes (Group IV, n = 20). Pre-industrial layers of four dated representative sediment cores yielded type-specific Cr, Cu, Pb and Zn baselines. Surface heavy metal enrichment and contamination were assessed using the single-factor pollution index (Pi) and geo-accumulation index (Igeo), respectively. Baselines differed markedly with lake groups, with maximum-to-minimum ratios of 2.18, 3.82, 2.39 and 3.64 for Cr, Cu, Pb and Zn, respectively. Greater catchment development triggered the strongest multi-metal enrichment in Group I: Pi values for all four metals exceeded 1 in 92.9% of lakes. Igeo values for Pb exceeded 0 in 85.0% and 78.3% of Group II and III lakes, respectively. Group IV showed the weakest overall enrichment under lower catchment development. Although Pi values for Pb exceeded 1 in 85.0% of its lakes, no lake reached moderate or higher contamination for any metal. Coupling lake typology with type-specific baselines supports enrichment assessment and differentiated management at the lake group scale in China’s Middle and Lower Yangtze River Plain.