J. Limnol., 2018; 77(3): 483-504 ORIGINAL ARTICLE DOI: 10.4081/jlimnol.2018.1821 This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0). INTRODUCTION The 476 lakes currently recognized in active and quiescent volcanic systems (VHub, CVL Group page; Rouwet et al., 2014) can be subdivided, on the basis of their physicochemical features, into two main groups, as follows (Pasternack and Varekamp, 1997): i) high-activity lakes, i.e. those strongly affected by inputs of heat and hydrothermal-magmatic gases and ii) low-activity lakes, which receive CO 2 -rich fluids from geogenic source(s) at relatively low rate. Lakes pertaining to the second group typically show relatively low water temperature and salinity, neutral to slightly acidic pH and permanent (or, at least, seasonal) thermal and chemical vertical gradients (Rouwet et al., 2014). Inputs of CO 2 from external sources and stable water stratification are regarded as the main causes for the development of CO 2 -dominated dissolved gas reservoirs in the anaerobic water layers (Tassi and Rouwet, 2014, and references therein). Microbial processes, mostly occurring within the hypolimnion and in the bottom sediments, produce CH 4 that may show concentrations comparable to those of CO 2 (Tietze, 1980; Schoell et al., 1988; Caliro et al., 2008; Tassi et al., 2009; Vaselli et al., 2012; Cabassi et al., 2013, 2015). Methanogenesis, likely favoured by endogenous inputs of CO 2 , is counteracted by the activity of methanotrophs in the epilimnion (Rudd et al., 1974; Bastviken et al., 2002 Blees et al., 2015) and, at a lesser extent, in the hypolimnion (Schubert et al., 2010). Low-activity volcanic lakes can be regarded as important sources and sinks of CO 2 and CH 4 (Cole et al., 1994; Battin et al., 2009; Tranvìk et al., 2009). Nevertheless, the estimation of the contribution of these two greenhouse gases (IPCC, 2013) to the atmosphere from volcanic lakes, and more in Mechanisms regulating CO 2 and CH 4 dynamics in the Azorean volcanic lakes (São Miguel Island, Portugal) Franco TASSI, 1,2* Jacopo CABASSI, 2 Cesar ANDRADE, 3 Cristiana CALLIERI, 4 Catarina SILVA, 3,5 Fatima VIVEIROS, 3 Gianluca CORNO, 4 Orlando VASELLI, 1,2 Enrico SELMO, 6 Andrea GALLORINI, 1 Andrea RICCI, 1 Luciano GIANNINI, 1,2 Josè V. CRUZ 3 1 Department of Earth Sciences, University of Florence, Via G. La Pira 4, 50121 Florence, Italy; 2 Institute of Geosciences and Earth Resources, National Research Council of Italy, Via G. La Pira 4, 50121 Florence, Italy; 3 Instituto de Investigação em Vulcanologia e Avaliação de Riscos, Universidade dos Açores, Rua Mãe de Deus, 9500-321, Ponta Delgada, Açores, Portugal; 4 Institute of Ecosystem Study, National Research Council of Italy, Largo Tonolli 50, 28922 Verbania, Italy; 5 Centro de Informação e Vigilância Sismovulcânica dos Açores, Universidade dos Açores, Rua Mãe de Deus, 9500-321 Ponta Delgada, Açores, Portugal; 6 Department of Chemistry, Life Sciences and Environmental Sustainability, University of Parma, Parco Area delle Scienze 11/a, 43124 Parma, Italy *Corresponding author: franco.tassi@unifi.it ABSTRACT 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 CO 2 and CH 4 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 NH 4 + , NO 3 – , P and other minor species (Fe, Mn, Zn, As) controlled by microbial activity and minerogenetic processes occurring within the lake sediments. The strongly negative δ 13 C-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 CO 2 and CH 4 . The δ 13 C-CO 2 values were intermediate between those measured in the hydrothermal fluids and those typical of biogenic CO 2 . Dissolved CH 4 , 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 CH 4 production. Further investigations, focusing on the recognition of microbial populations able to produce CH 4 at different redox conditions, may be useful to corroborate these intriguing hypotheses. Key words: Volcanic lake; methane paradox; lake stratification; dissolved gas reservoir; microbiological activity; Azores. Received: April 2018. Accepted: July 2018. Non-commercial use only