Variability of methane in the epilimnion of Lake Kivu

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0.001 0.01 0.1 1 10 100 1000 10000 100000 1 10 100 1000 10000 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_{, C. Morana}3_{, S. Bouillon}3_{, and F. Darchambeau}1

1_{University of Liège (BE),} 2_{University of Bordeaux 1 (FR),} 3_{Katholieke Universiteit Leuven (BE)}

* alberto.borges@ulg.ac.be

European Geosciences Union General Assembly, 22-27 April 2012

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), CH₄

concentrations 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 CH₄in 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 CH₄to 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 CH₄ in 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 (7200 µmol m-2 _d-1_{) in the}

main basin of lake Kivu is 200 times higher than the average CH₄

emission 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 )

mid rainy season late dry season early dry season late rainy season

0 10 20 30 40 50 60 70 0 5000 10000 15000 CH4 oxidation (nM d-1) d e p th ( m ) 10 100 1000 10000 100000 1000000 [CH4] (nM) CH4 oxid. O2 [CH4] 0 1 2 3 4 5 6 7 [O2] (mg L-1) Ishungu r2 = 0.82 0 2000 4000 6000 8000 10000 12000 -35 -30 -25 -20 δ13C POC (‰ ) C H 4 o x id a ti o n ( n M d -1 )

Available CH₄ oxidation rates (June 2011) were low or

undetectable at CH₄ concentrations < 3300 nM. CH₄

oxidation rates were undetectable in the anoxic layer

during this cruise. Hence, maximal CH₄ oxidation rates

were observed in lower part of the oxycline. CH₄

oxidation rates were well correlated with 13_δ_POC,

confirming the presence and probable dominance of

methanotrophs among the bacterial community