Chemical and isotopic vertical profiles from the volcanic lakes of Sete Cidades, Santiago, Fogo, Congro and Furnas (Island of São Miguel, Azores Archipelago, Portugal) were studied to investigate the biogeochemical processes acting at different depths, with a focus on the CO2 and CH4 dynamics. These lakes are fed by meteoric water affected by seawater spray and interacting with volcanic rocks at a relatively low extent. In addition to volcanogenic gas inputs, the biogeochemical processes are influenced by microbial activities since the lakes offer specialized ecological niches for oxic and anoxic metabolism. The lakes were sampled in two extreme conditions of (partial) mixing (winter) and stratification (summer), respectively. The seasonal thermal stratification favored the development of anaerobic hypolimnia, showing relatively high concentrations of NH4, NO3, P and other minor species (Fe, Mn, Zn, As) controlled by microbial activity and minerogenetic processes occurring within the lake sediments. The strongly negative δ13C-TDIC values measured in almost all the studied lakes suggest dominant contribution of organic carbon. Dissolved gases were mostly consisting of atmospheric compounds with significant concentrations of CO2 and CH4. The δC-CO2 values were intermediate between those measured in the hydrothermal fluids and those typical of biogenic CO2. Dissolved CH4, which was the most abundant extra-atmospheric gas in the anoxic waters, was measured at significant concentrations even in the aerobic layers, especially in the winter season. This unexpected feature may tentatively be explained by admitting i) convective mixing of shallow and deep waters, and/or ii) aerobic CH4 production. Further investigations, focusing on the recognition of microbial populations able to produce CH4 at different redox conditions, may be useful to corroborate these intriguing hypotheses.

中文翻译：

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调节亚速尔火山湖中 CO2 和 CH4 动态的机制（葡萄牙圣米格尔岛）

研究了 Sete Cidades、圣地亚哥、Fogo、Congro 和 Furnas（葡萄牙亚速尔群岛圣米格尔岛）火山湖的化学和同位素垂直剖面，以研究作用于不同深度的生物地球化学过程，重点是 CO2 和CH4 动态。这些湖泊由受海水喷溅影响的大气水补充，并与火山岩的相互作用程度相对较低。除了火山气体输入外，生物地球化学过程还受到微生物活动的影响，因为湖泊为好氧和缺氧代谢提供了专门的生态位。分别在（部分）混合（冬季）和分层（夏季）两种极端条件下对湖泊进行采样。季节性热分层有利于厌氧低体温的发展，显示出相对高浓度的 NH4、NO3、P 和其他次要物种（Fe、Mn、Zn、As）受微生物活动和湖泊沉积物中发生的成矿过程控制。在几乎所有研究的湖泊中测得的强负 δ13C-TDIC 值表明有机碳的主要贡献。溶解气体主要由含有大量 CO2 和 CH4 的大气化合物组成。δ13​​C-CO2 值介于热液流体中测量的值和典型的生物 CO2 值之间。溶解的 CH4 是缺氧水域中最丰富的大气层外气体，即使在好氧层中也能测量到显着浓度，尤其是在冬季。可以通过承认 i) 浅水和深水的对流混合，和/或 ii) 有氧 CH4 生产来暂时解释这一意外特征。进一步的研究，重点是识别能够在不同氧化还原条件下产生 CH4 的微生物种群，可能有助于证实这些有趣的假设。