Water chemistry variation in tropical high-mountain lakes on old volcanic bedrocks

Water chemistry and its ecological implications have been extensively investigated in temperate high-mountain lakes because of their role as sentinels of global change. However, few studies have considered the drivers of water chemistry in tropical mountain lakes underlain by volcanic bedrock. A survey of 165 paramo lakes in the Cajas Massif of the Southern Ecuador Andes identified 4 independent chemical variation gradients, primarily characterized by divalent cations (hardness), organic carbon, silica, and iron levels. Hardness and silica factors showed contrasting relationships with parent rock type and age, vegetation, aquatic ecosystems in the watershed, and lake and watershed size. Geochemical considerations suggest that divalent cations (and related alkalinity, conductivity, and pH) mainly respond to the cumulative partial dissolution of primary aluminosilicates distributed throughout the subsurface of watersheds, and silica and monovalent cations are associated with the congruent dissolution of large amounts of secondary aluminosilicates localized in former hydrothermal or tectonic spots. Dissolved organic carbon was much higher than in temperate high-mountain lakes, causing extra acidity in water. The smaller the lakes and their watersheds, the higher the likelihood of elevated organic carbon and metals and low hardness. The watershed wetland cover favored metal levels in the lakes but not organic carbon. Phosphorus, positively, and nitrate, negatively, weakly correlated with the metal gradient, indicating common influence by in-lake processes. Overall, the study revealed that relatively small tropical lake districts on volcanic basins can show chemical variation equivalent to that in large mountain ranges with a combination of plutonic, metamorphic, and carbonate rock areas.