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This study examines the spatiotemporal variations in diatom assemblages and selected metal concentrations (Pb, Cr, Cd, Al, and Zn) in bed sediments of lakes Telaga Pengilon and Telaga Warna in Dieng, Indonesia to document natural and/or anthropogenic changes in the local aquatic and terrestrial environment. The analyses focused on sediments collected from a 150-cm core taken from a sill between the two lakes, which exhibit significant differences in water chemistry. The core was subdivided into 14 stratigraphic intervals allowing for an analysis of the vertical (and temporal) variations in diatom composition and selected metal concentrations. A total of 103 taxa from 25 genera were identified in the core. Diatom assemblages were dominated by Eunotia (56%), Pinnularia (17.2%), and Frustulia (4.6%). The most abundant species was Eunotia, a diatom that can tolerate a wide range of pH conditions. Given that pH within waters of the two lakes differs significantly, the abundance of Eunotia suggests that pH in the area between Telaga Pengilon and Telaga Warna varied through time, potentially ranging from about 2.5 to 8. Changes in pH were likely related to alterations in hydrological conditions. Metal concentrations varied with depth/time of deposition within the core. Peak metal concentrations appear to be related to the influx of debris from a volcanic eruption. Based on the principle component analysis (PCA), the input of volcanic materials also influenced diatom assemblages and resulted in a distinct layer of broken diatom frustules. Relatively low metal concentrations in surface sediments suggest that the erosion of hillslope soils in response to agricultural activity did not significantly impact the lakes. Full article