CO2 and CH4 Emissions from Two Maar Crater Lakes (São Miguel, Azores)

This study was made to quantify ΦCO2 and ΦCH4 in two crater monomictic lakes on São Miguel Island (Azores, Portugal). The studied water bodies are both within maar craters, with depths equal to 22 m and 33 m, at the Congro and Santiago lakes, respectively. ΦCO2 values in both lakes are lower in summer, with a mean equal to 2.9 and 1.4 g m−2 d−1, respectively, in Congro and Santiago lakes, when the water column is stratified. ΦCH4 displayed a different seasonal trend, with higher emissions during the period when the water column is stratified (mean value equal to 0.036 and 0.020 g m−2 d−1, respectively). Bacterial families commonly associated with CO2 production, through heterotrophic respiration, fermentation, and syntrophic metabolism, were identified in both lakes, being the most abundant taxa Clostridiaceae, Enterobacteriaceae, Holophagaceae and Methylomonadaceae. The microbial community composition suggests seasonal variations, reflected by shifts in the relative abundance of specific microbial taxa. In particular, Clostridiaceae and Enterobacteriaceae exhibited higher relative abundance during winter, consistent with an increased contribution of fermentative microorganisms during this period. In contrast, Methylomonadaceae, a family commonly associated with methane oxidation and subsequent CO2 production, reached higher relative abundance during summer in both lakes, suggesting enhanced methanotrophic activity during this period. Regarding CH4 production, methanogens showed a slightly higher abundance during summer at Congro Lake, such as Methanosarcinaceae. In Santiago Lake, Syntrophaceae and unclassified Syntrophales were more abundant during summer, coinciding with higher CH4 emissions compared to winter. The present research contributes to understanding volcanic gas release from lake water bodies in active volcanic framework, helping also to constrain GHG budgets in oceanic islands.