Nitrous oxide and methane seasonal variability in the epilimnion of a large tropical meromictic lake (Lake Kivu, East-Africa)
Roland, Fleur A. E. | Darchambeau, François | Morana, Cédric | Borges, Alberto V.
We report a data-set of monthly vertical profiles obtained from January 2012 to October 2013, from the surface to 70 m depth of nitrous oxide (N₂O) and dissolved methane (CH₄) in Lake Kivu, a large and deep meromictic tropical lake (East Africa). Vertical variations of N₂O were modest, with ranges of 6–9 and 0–16 nmol L⁻¹ in surface and bottom waters, respectively, and occasionally peaks of N₂O (up to 58 nmol L⁻¹) were observed at the oxic-anoxic interface. On the contrary, steep vertical gradients of CH₄ were observed with values changing several orders of magnitude from surface (19–103 nmol L⁻¹) to 70 m (~113,000–520,000 nmol L⁻¹). Seasonal variations of CH₄ were caused by annual cycles of mixing and stratification, during the dry and rainy seasons, respectively. This mixing allowed the establishment of a thick oxic layer (maximum 65 m deep), leading to decreased CH₄ concentrations (minimum of 8 nmol L⁻¹), presumably due to bacterial CH₄ oxidation. During the stratification period, the oxic mixed layer was thinner (minimum 25 m deep), and an increase of CH₄ concentrations in surface waters was observed (maximum of 103 nmol L⁻¹), probably due to a lower integrated CH₄ oxidation on the water column. Lake Kivu seasonally alternated between a source and a sink for atmospheric N₂O, but on an annual scale was a small source of N₂O to the atmosphere (on average 0.43 µmol m⁻² day⁻¹), while it was a small source of CH₄ to the atmosphere throughout the year (on average 86 µmol m⁻² day⁻¹). Vertical and seasonal variations of N₂O are discussed in terms of nitrification and denitrification, although from the present data-set it is not possible to unambiguously identify the main drivers of N₂O production.
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