In the Azores there is a significant number of volcanic lakes, distributed along some islands of the archipelago. The São Miguel Island has the largest number of volcanic lakes, as well as contains about 90% of the surface area and 93% of the water storage. In the present study, samples were collected along the water column of the five largest volcanic lakes on São Miguel Island. The changes on heat flux and degassing rates that may occur in these water bodies are reflected through variations on physico-chemical parameters and over the chemical composition of these lakes, thus also allowing the surveillance of active volcanic systems.
The results shown that all lakes present a monomictic character, with the water column being stratified during summer, while during winter they present mixing conditions throughout the water column.
Waters are mainly cold, low mineralized and are mainly of the Na-Cl type due to the deposition of salts of marine origin. Bicarbonate is another anion that is present in a significant content in some lakes due to the absorption of volcanic volatiles, contamination by fluids of volcanic origin, as well as from dissolution reactions of silicate minerals from volcanic rocks. The enrichment of HCO3 also results indirectly from the decomposition of organic matter mainly at depth, enhanced by the ongoing eutrophication process in some lakes (e.g. Sete Cidades Lake).
The main findings of the present research enable the definition of the lake geochemical baseline, which is of interest for the ongoing volcanic monitoring in the Azores.