Lakes provide a wide range of ecosystem services that are increasingly at risk because of multiple anthropogenic pressures. Santa María del Oro crater lake lies in rural northwest Mexico and is of interest for global change studies and as a natural resource that sustains the economies of local communities. Temporal trends of trace element (As, Co, Cr, Cu, Ni, Pb, V, and Zn) concentrations over the past century were assessed in four ²¹⁰Pb-dated sediment cores, under the hypothesis that recent anthropogenic activities have promoted increasing trace element inputs. Data were analyzed within three periods: (1) pre-1900s, from which background levels were estimated, (2) pre-1950, and (3) post-1950. Trace element provenance was determined through a chemometric approach. Most trace element concentrations varied little in the past ~ 100 years, with no Co, V, and Zn enrichment, and minor enrichment by Cr, Cu, Ni and Pb in a few sections of the cores collected from the deepest locations, within the hypolimnion, and by Pb along the shallowest core. In contrast, As enrichment in the deep cores reached moderately severe levels during the post-1950 period. Cobalt, Pb, V, and Zn input to the lake was related to terrigenous sources. Lead enrichment was mainly anthropogenic (the shallow coring site is close to human settlements). Copper and Cr variability were mostly the result of diagenetic reactions. Arsenic enrichment, however, was attributed to biogeochemical cycling processes, likely exacerbated by drought conditions and enhanced calcite precipitation. Contradicting our hypothesis, recent anthropogenic impact on the lake´s environmental quality was small. However, a preliminary assessment, based on international guidelines, indicated that As, Cr, and Ni concentrations could be harmful to benthic biota, even at background levels. High As concentrations could affect the aquatic food web and human health, the latter through fish consumption. Given predictions for increasing droughts in the area, which induce greater evaporation and lower water levels, this issue could worsen in the future. Further studies on As contamination in the lake are needed to better understand potential hazards and enhance response capacity. Such data must be part of conservation efforts and should be integrated into management policies to ensure the lake’s sustainability.

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