Diffuse CO2 and CH4 emissions of two volcanic lakes on Pico Island (Azores)

This study investigates the diffuse flux of carbon dioxide (ФCO2) and methane (ФCH4) from Capitão and Caiado lakes, on Pico Island (Azores archipelago, Portugal), aiming to characterize the spatial and temporal variability of greenhouse gas (GHG) emissions in volcanic lake systems. Field measurements were conducted using dynamic and static accumulation chambers for ФCO2 and ФCH4 during multiple seasonal surveys. The study highlights that ФCO2 and ФCH4 in these volcanic lakes are mainly driven by biogeochemical processes rather than direct volcanic degassing. Total ФCO2 at Capitão Lake ranged from 0.10 t d−1 (4 t km−2 d−1) in spring to 0.08 t −1 (3.64 t km−2 d−1) in summer, likely due thermal stratification limiting CO2 release in the latter period. In contrast, Caiado Lake exhibited higher ФCO2, increasing from 0.31 t −1 (5.96 t km−2 d−1) to 0.40 t −1 (7.69 t km−2 d−1), possibly influenced by methane oxidation. Methane emissions were significantly lower than CO2 but showed notable seasonal variation. The ФCH4 at Capitão Lake increased tenfold from 0.096 kg −1 (4 kg km−2 d−1) in warmer conditions to 0.96 kg −1 (40 kg km−2 d−1) in winter, likely due to water column mixing allowing methane to easily escape from oxidation. At Caiado Lake, ФCH4 increased from 1.39 kg −1 (23.96 kg km−2 d−1) to 2.67 kg −1 (46.03 kg km−2 d−1) in the same period. Carbon dioxide largely dominates GHG emissions in the studied lakes, even computing CH4 in terms of CO2-eq, being the ФCO2 2 to 3 orders of magnitude higher than ФCH4.