Variability of methane in the epilimnion of Lake Kivu

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Kabuno

Main basin

Lake surface area (km2₎

C H4 ( n M )

Variability of methane in the epilimnion of Lake Kivu

Borges A.V.1_{, G. Abril}2_{, B. Delille}1_{, S. Bouillon}3_{, J.-P. Descy}4_{& F. Darchambeau}1

1_{Université de Liège (BE),} 2_{Université de Bordeaux 1 (FR),} 3_{Katholieke Universiteit Leuven,} 4_{Université de Namur (BE) * alberto.borges@ulg.ac.be}

3

rd

_{International Symposium on Organic Matter Dynamics, 11-14 July 2011}

We report a data-set of methane concentrations (CH₄) in the surface waters of Lake Kivu obtained during four cruises covering the two main seasons (rainy and dry). Spatial gradients of CH₄ concentrations were modest in the surface waters of the main basin. In Kabuno Bay (a small sub-basin), CH₄concentrations in surface waters were significantly higher than in the main basin. The likely higher contribution of deepwater springs in Kabuno Bay than in the main basin increases the upward flux of solutes and might explain the higher CH₄ concentrations we observed in Kabuno Bay than in the main basin. Seasonal variations of CH₄ in the main basin were strongly driven by deepening of the mixolimnion and mixing of surface waters with deeper waters rich in CH₄. On an annual basis, both Kabuno Bay and the main basin of Lake Kivu were over-saturated in CH₄ with respect to atmospheric equilibrium (7330% and 2510%, respectively), and emitted CH₄ to the atmosphere (39 mmol m-2 _yr-1 _{and 13 mmol m}-2 _yr-1_{, respectively). A global cross-system} comparison of CH₄ in surface waters of lakes shows that both Kabuno Bay and the main basin are at the lower end of values in lakes globally, despite the huge amounts of CH₄in the deeper layers of the lake (concentrations 106 _{higher than in surface). This is related to the strongly meromictic} nature of the lake that promotes an intense removal of CH₄ by bacterial oxidation. Indeed, the average CH₄ oxidation rate (2628 mmol m-2_yr-1_{) in the main basin of lake Kivu is 200 times higher} than the average CH₄emission to the atmosphere in the main basin.

March 2007 30 30 33 34 34 38 44 46 ₄₈ 48 48 53 53 75 167 Bukavu Kabuno September 2007 54 55 58 62 62 68 70 70 72 79 82 89 103 145 197 Bukavu Kabuno June 2008 120 46 77 59 98 77 133 67 67 56 44 46 62 50 30 Bukavu Kabuno April 2009 51 66 83 72 32 26 33 42 303 18 35 37 36 41 35 Bukavu Kabuno 20 30 40 50 60 10 30 50 70 90 110 130 March 2007 Sept. 2007 June 2008 April 2009

Mixed layer depth (m)

s u rf a c e C H4 ( n M )