| Abstract | Chemical and isotopic vertical profiles from the volcanic lakes of Sete Cidades, Santiago, Fogo, Congro and Furnas (Island of Sao 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 δ13C-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. |
| Text | 398717 2018 10.4081/jlimnol.2018.1821 Scopus 2 s2.0 85058179803 Volcanic lake; methane paradox; lake stratification; dissolved gas reservoir; microbiological activity; Azores Mechanisms regulating CO2 and CH4 dynamics in the Azorean volcanic lakes São Miguel Island, Portugal Tassi F.; Cabassi J.; Andrade C.; Callieri C.; Silva C.; Viveiros F.; Corno G.; Vaselli O.; Selmo E.; Gallorini A.; Ricci A.; Giannini L.; Cruz J.V. Department of Earth Sciences, University of Florence, Via G. La Pira 4, Florence, 50121, , Italy; Institute of Geosciences and Earth Resources, National Research Council of Italy, Via G. La Pira 4, Florence, 50121, , , Italy; Institute of Geosciences and Earth Resources, National Research Council of Italy, Via G. La Pira 4, Florence, 50121, , , Italy; Instituto de Investigação em Vulcanologia e Avaliação de Riscos, Universidade dos Açores, Rua Mãe de Deus, Ponta Delgada, Açores, 9500 321, , Portugal; Institute of Ecosystem Study, National Research Council of Italy, Largo Tonolli 50, Verbania, 28922, , Italy; Centro de Informação e Vigilancia Sismovulcanica dos Açores, Universidade dos Açores, Rua Mãe de Deus, Ponta Delgada, Açores, 9500 321, , Portugal; Department of Chemistry, Life Sciences and Environmental Sustainability, University of Parma, Parco Area delle Scienze 11/a, Parma, 43124, , Italy Chemical and isotopic vertical profiles from the volcanic lakes of Sete Cidades, Santiago, Fogo, Congro and Furnas Island of Sao 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 δ13C 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. 77 Published version http //www.scopus.com/inward/record.url eid=2 s2.0 85058179803 partnerID=q2rCbXpz Articolo 2018_J_Limnol_77_3_483_504.pdf Articolo in rivista Istituto per lo Studio degli Ecosistemi. 1129 5767 Journal of limnology Testo stamp. Journal of limnology Testo stamp. J. limnol. Testo stamp. Journal of limnology. Testo stamp. orlandovaselli VASELLI ORLANDO jacopo.cabassi CABASSI JACOPO cristiana.callieri CALLIERI CRISTIANA gianluca.corno CORNO GIANLUCA |
