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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Discussion papers
https://doi.org/10.5194/bg-2016-300
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
https://doi.org/10.5194/bg-2016-300
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Submitted as: research article 21 Jul 2016

Submitted as: research article | 21 Jul 2016

Review status
This discussion paper is a preprint. It has been under review for the journal Biogeosciences (BG). The manuscript was not accepted for further review after discussion.

Anaerobic methane oxidation in an East African great lake (Lake Kivu)

Fleur A. E. Roland1, François Darchambeau1, Cédric Morana2, Sean A. Crowe3, Bo Thamdrup4, and Alberto V. Borges1 Fleur A. E. Roland et al.
  • 1Chemical Oceanography Unit, Université de Liège, Belgium
  • 2Department of Earth and Environmental Sciences, Katholieke Universiteit Leuven (KU Leuven), Belgium
  • 3Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Canada
  • 4Institute of Biology and Nordic Center for Earth Evolution, University of Southern Denmark, Denmark

Abstract. This study investigates methane (CH4) oxidation in the water column of Lake Kivu, a deep meromictic tropical lake containing large quantities of CH4 in the anoxic deep waters. Depth profiles of dissolved gases (CH4 and nitrous oxide (N2O)) and of the different potential electron acceptors for anaerobic methane oxidation (AOM) (nitrate, sulfate, iron and manganese) were determined during six field campaigns between June 2011 and August 2014. Bacterial abundance all along the vertical profiles was also determined by flow cytometry during three field campaigns, and denitrification measurements based on stable isotopes were performed twice. Incubation experiments were performed to quantify CH4 oxidation and nitrate consumption rates, with a focus on AOM, without and with an inhibitor of sulfate-reducing bacteria activity (molybdate). Nitrate consumption rates were measured in these incubations. Substantial CH4 oxidation activity was observed in oxic and anoxic waters, and in the upper anoxic waters of Lake Kivu, CH4 is a major electron donor to sustain anaerobic metabolic processes coupled to AOM. The maximum aerobic and anaerobic CH4 oxidation rates were estimated to 27 ± 2 and 16 ± 8 µmol L−1 d−1, respectively. We observed a decrease of AOM rates when molybdate was added for half of the measurements, strongly suggesting the occurrence of AOM linked to sulfate reduction, but an increase of AOM rates was observed for the other half. Nitrate reduction rates and dissolved manganese production rates tended to be higher with the addition of molybdate, but the maximum rates of 0.6 ± 0.02 and 11 ± 2 µmol L−1 d−1, respectively, were never high enough to explain AOM rates observed at the same depths. We also put in evidence a difference in relative importance of aerobic and anaerobic CH4 oxidation between the seasons, with a higher importance of aerobic oxidation when the oxygenated layer was thicker (in dry season).

Fleur A. E. Roland et al.
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Fleur A. E. Roland et al.
Fleur A. E. Roland et al.
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Short summary
We studied methane consumption in a tropical Great Lake (Lake Kivu, East Africa). Lake Kivu has huge methane concentrations in its deep anoxic waters, but is a very poor emitter of methane to the atmosphere, which suppose a strong methane consumption in the water column. During this study, we put in evidence high aerobic and anaerobic consumption rates, whose relative importance varied with the season (higher aerobic rates in dry season, when the oxic compartment is wider).
We studied methane consumption in a tropical Great Lake (Lake Kivu, East Africa). Lake Kivu has...
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