Abstract
Hule and Río Cuarto are maar lakes located 11 and 18 km N of Poás volcano along a 27 km long fracture zone, in the Central Volcanic Range of Costa Rica. Both lakes are characterized by a stable thermic and chemical stratification and recently they were affected by fish killing events likely related to the uprising of deep anoxic waters to the surface caused by rollover phenomena. The vertical profiles of temperature, pH, redox potential, chemical and isotopic compositions of water and dissolved gases, as well as prokaryotic diversity estimated by DNA fingerprinting and massive 16S rRNA pyrosequencing along the water column of the two lakes, have highlighted that different bio-geochemical processes occur in these meromictic lakes. Although the two lakes host different bacterial and archaeal phylogenetic groups, water and gas chemistry in both lakes is controlled by the same prokaryotic functions, especially regarding the CO2-CH4 cycle. Addition of hydrothermal CO2 through the bottom of the lakes plays a fundamental priming role in developing a stable water stratification and fuelling anoxic bacterial and archaeal populations. Methanogens and methane oxidizers as well as autotrophic and heterotrophic aerobic bacteria responsible of organic carbon recycling resulted to be stratified with depth and strictly related to the chemical-physical conditions and availability of free oxygen, affecting both the CO2 and CH4 chemical concentrations and their isotopic compositions along the water column. Hule and Río Cuarto lakes were demonstrated to contain a CO2 (CH4, N2)-rich gas reservoir mainly controlled by the interactions occurring between geosphere and biosphere. Thus, we introduced the term of bio-activity volcanic lakes to distinguish these lakes, which have analogues worldwide (e.g. Kivu: D.R.C.-Rwanda; Albano, Monticchio and Averno: Italy; Pavin: France) from volcanic lakes only characterized by geogenic CO2 reservoir such as Nyos and Monoun (Cameroon).
Highlights
Volcanic lakes are peculiar natural systems on Earth, they are a common feature of volcanic systems characterized by recent activity, being present in 476 volcanic structures worldwide (VHub, Commission on Volcanic Lakes (CVL) Group page; [1])
This paper presents the geochemical and microbiological results obtained from samples collected in 2010 during the 7th Workshop of the Commission on Volcanic Lakes (CVL; Costa Rica 10–21 March 2010), which is part of the International Association of Volcanology and Chemistry of the Earth’s Interior (IAVCEI), by a group of geochemists, limnologists, biologists and volcanologists from different universities and scientific institutions
Hule and Rıo Cuarto are meromictic maar lakes mainly fed by meteoric water, and characterized by significant amounts of dissolved gases, partially consisting of CO2 having a hydrothermalmagmatic origin, in their hypolimnion
Summary
Volcanic lakes are peculiar natural systems on Earth, they are a common feature of volcanic systems characterized by recent activity, being present in 476 volcanic structures worldwide (VHub, CVL Group page; [1]). Volcanic lakes were basically classified, as follows [1,4]: i) ‘‘high-activity’’ lakes affected by the addition of significant amounts of hot and hyperacidic hydrothermal–magmatic fluids; ii) ‘‘low-activity’’ lakes, characterized by CO2-dominated fluid inputs at a relatively low rate from sub-lacustrine fluids discharges, favoring the establishment of a stable vertical stratification and possibly the accumulation of high amounts of dissolved gases in the deep water layers At these conditions, a lake overturn triggered by either i) external events, such as earthquakes, landslides or extreme weather conditions or ii) the progressive attainment of gas saturation conditions may cause the abrupt release of toxic gas clouds in the atmosphere. Low activity lakes are commonly indicated as ‘‘Nyos-type’’ lakes
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