We compared the burial efficiency of organic carbon (buried OC : deposited OC) in a diverse set of 27 different sediments from 11 lakes, focusing on the potential effects of organic matter source, oxygen exposure, and protective sorption of OC onto mineral surfaces. Average OC burial efficiency was high (mean 48%), and it was particularly high in sediments receiving high input of allochthonous organic matter (mean 67%). Further, OC burial efficiency was strongly negatively related to the oxygen exposure time, again particularly so in sediments receiving high allochthonous loads. On the other hand, OC burial efficiency was not related to the mineral surface area, which was used as a proxy of the sorption capacity of the mineral phase for OC. The high OC burial efficiency in many lake sediments can thus be attributed to the frequent and significant input of allochthonous organic matter to lakes, as well as to a strong dependence of OC burial efficiency on oxygen exposure time. This study demonstrates that the carbon sink in lake sediments alters the OC export from the continents to the sea and that the fate of OC in lake sediments (burial vs. mineralization to carbon dioxide and methane) is highly sensitive to environmental conditions.

[1] Unique worldwide, Lake Kivu stores enormous amounts of CH 4 and CO 2 . A recent study reported that CH 4 concentrations in the lake have increased by up to 15% in the last 30 years and that accumulation at this rate could lead to catastrophic outgassing by ∼2100. This study investigates the present-day CH 4 formation and oxidation in Lake Kivu. Analyses of 14 C and 13 C in CH 4 and potential carbon sources revealed that below 260 m, an unusually high ∼65% of the CH 4 originates either from reduction of geogenic CO 2 with mostly geogenic H 2 or from direct inflows of geogenic CH 4 . Aerobic CH 4 oxidation, performed by close relatives of type X CH 4 -oxidizing bacteria, is the main process preventing CH 4 from escaping to the atmosphere. Anaerobic CH 4 oxidation, carried out by CH 4 -oxidizing archaea in the SO 4 2−

Lake Kivu is one of the large African Rift lakes situated between the Democratic Republic of the Congo and Rwanda. In its permanently stratified hypolimnion, unusually high methane concentrations have increased further in recent decades. Because methanogenesis is, in part, dependent on supply of organic material from the photic zone, it is necessary to quantify upward nutrient fluxes from the saline, nutrient-rich deep waters. These upward fluxes are mainly driven by advection caused by subaquatic springs. Biogenic calcite precipitation drives surface-water depletion and deep-water enrichment of Ca 2+ , Sr 2+ , and Ba 2+ . Methane is mainly oxidized aerobically at the redox interface at 60 m, with a small contribution of anaerobic methane oxidation. A subaquatic spring that sustains the major chemocline at 250 m depth was depleted of N, P, and CH 4 , and concentrations of major ions were slightly lower than in the lake water of the same depth. Enrichment of the deep waters with nutrients and CH 4 are driven by mineralization of settling organic material, whereas SiO 2 is influenced by uptake and mineralization of diatoms and inputs through subaquatic springs. Dissolved inorganic phosphorus and Si fluxes supplied by internal loading through upwelling were found to be lower than the estimations for Lakes Malawi and Tanganyika. In contrast, N flux was within the lower range for Lake Malawi, whereas it was assumed to be totally lost by denitrification in Lake Tanganyika. In Lake Kivu, nutrient uptake by primary production is three times higher than nutrient upward fluxes.The deep tropical lakes of the African Rift Valley are characterized by a specific limnology with permanent stratification (Kilham and Kilham 1990) accompanied by an export of nutrients from the mixed surface layer to the deep waters via settling of particulate organic material. The resulting large reservoirs of dissolved nutrients in the deep water sustain internal loading to the photic zone via upward fluxes, which are crucial for nutrient availability and phytoplankton growth (Hecky et al. 1996). Recent studies showed that primary production in Lake Malawi and Lake Tanganyika is driven by the import of nutrients from the deep waters via turbulent diffusion and upwelling (Bootsma and Hecky 1993;Hamblin et al. 2003). This upward transfer fuels dark microbial production at the oxycline, where large amounts of nutrients can be consumed in biogeochemical processes such as methanothrophy (Joye et al. 1999;Camacho et al. 2001;Hadas and Pinkas 2001) and coupled nitrification and denitrification (Hecky et al. 1996). These processes can be so intense that they limit the upward fluxes of nutrients otherwise available for phytoplankton growth. Quantifying upward nutrient fluxes across the redox interface to the epilimnion is therefore essential to determine nutrient cycling and primary production of these tropical lakes.Lake Kivu is one of those large East African rift lakes. It is meromictic, and the oligotrophic (Sarmento et al. 2006) epilimnion is...

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