Dual diagnosis of acidification dynamics in Amazonian urban lakes: multivariate analysis and calcite saturation index for status and vulnerability assessment

Abstract This study presents a dual diagnosis of acidification processes in the lakes in Lakes Água Preta (AP) and Bolonha (BL), two strategic reservoirs supplying the metropolitan area of Belém, Eastern Amazon (Brazil). We assessed the status and vulnerability of these ecosystems by integrating multivariate statistical analysis (PCA) with the Acidification Sensitivity Index (ASI). Monthly monitoring was conducted between September 2023 and August 2024, evaluating 21 parameters across the surface and bottom layers. Results indicate that both systems are chronically sensitive, with annual mean ASI values of -1.03 (AP) and -1.10 (BL), reflecting low alkaline reserves. Principal Component Analysis (PCA) revealed that Lake AP exhibits greater metabolic instability, with a score amplitude of 9.95 compared to 7.72 for BL. During the dry season, stress was predominantly biogenic; dissolved CO 2 reached 317.0 μmoles L −1 , accompanied by frequent bottom anoxia, with 62.0% of Dissolved Oxygen (DO) samples falling below the legal limit of 5.0 mg L −1 . Conversely, during the wet season, vulnerability was exacerbated by ionic dilution, as Total Alkalinity (TA) dropped to critical levels (39.0–48.0 μmoles L −1 ) and the ASI reached a minimum of -2.57. While Lake AP is primarily governed by the inorganic carbon cycle, BL is driven by cultural eutrophication. This study concludes that stand-alone pH monitoring is insufficient to capture the thermodynamic complexity of these tropical reservoirs. These findings provide novel scientific evidence for the necessity of seasonally differentiated management to ensure water security in urban-impacted Amazonian systems.