​The CO₂ degassing from lakes on Pico Island (Azores archipelago) were characterized in order to estimate the total diffuse CO₂ output and identify the possible sources of CO₂. Two surveys have been made in each lake (Capitão, Caiado, Rosada, Peixinho, Paúl and Seca), in the winter and summer periods. These water bodies show small surface areas and are rather shallow, with depths ranging from 1.8 to 8.6 m. Water samples are cold, both in winter and summer periods, not presenting variations along the water column, with acid to neutral pH (5.26-7.06). The electrical conductivity values point out to very diluted waters (mean range between 27 and 33.4 μS cm⁻¹), of the Na-Cl type, corresponding to meteoric waters influenced by marine salts.

To measure the CO₂ flux at the lakes surface the modified accumulation chamber method was used, and a total of 1632 measurements were accomplished (711 in winter surveys and 921 in summer). Two statistical analysis (GSA and sGs) were applied to the results of diffuse CO₂ flux measurements, showing that the CO₂ flux values measured in theses lakes are relatively low (0.60-20.47 gm⁻² d⁻¹), what seems to indicate a single source for CO₂ (biogenic source), also suggested by the water δ¹³C isotopic signature. CO₂ emissions range between 0.04 t d⁻¹ (Rosada_1) and 0.25 t d⁻¹ (Caiado_1) during the winter surveys, being in general similar to the values recorded during the summer surveys that vary between 0.03 t d⁻¹ (Peixinho_2 and Seca_1) and 0.30 t d⁻¹ (Caiado_2). Taken into account the surface area of the lakes, the highest values were estimated for both surveys made in Seca Lake (~13 t km⁻² d⁻¹). The occurrence of a dense macrophyte mass in a few of the studied lakes, such as Caiado and Seca, seems to enhance the CO₂ flux from these water bodies.