The Azores archipelago comprises nine Islands of volcanic nature, spread along a complex geodynamic seƫng, which is reflected by intense seismic and volcanic acƟvity. Since the seƩlement in the late 15
th century, a total of 28 eruptions took place in the archipelago, both inland and offshore. Nowadays, secondary manifestations of volcanism can be found in most of the islands, comprising thermal and cold CO
2-rich springs, steaming grounds, fumaroles and diffuse degassing areas.
A total of 88 lakes are spread in the archipelago, except for Santa Maria, São Jorge, and Faial islands, lying mainly inside volcanic craters and subsidence calderas, and presenƟng a monomictic behaviour in the case of the deepest lakes. These lakes present a total water storage of about 9x107 m3 , 93% corresponding to water bodies in São Miguel Island. Besides those surface lakes, two other water bodies are located inside volcanic caves in Terceira and Graciosa islands.
Considering a representative set of 45 lakes, water temperature is cold (mean = 16.2ºC), has acidic to neutral pH (mean = 7.40) and corresponds to weakly mineralized waters (mean EC = 107 µS cm-1). The prevailing water type is Na-Cl, nevertheless lakes with Na-HCO3 and Mg-HCO3 type waters also occur. Besides the influence of water-rock interaction, the input of fluids of volcanic origin, and the deposiƟon of salts of marine origin through atmospheric processes, eutrophication may also impact water quality due to the overuse of ferƟlizers and livestock manure in pasture lands along the watersheds.
An extensive survey made in the last decade showed that lakes in the archipelago emit approximately 171x103 t yr-1 of CO2 to the atmosphere. About 42% of the total CO2 emission from the Azores lakes is of volcanic origin (~72 × 103 t yr-1), mainly from Lakes Furnas and Santiago, both in São Miguel Island, and from Furna do Enxofre (Graciosa). The higher output, equal to ~185x103 t km-2 yr-1, is also observed at Furna do Enxofre.