Double-diffusive staircases with a total of 230-350 mixed layers and sharp interfaces were observed in nine microstructure temperature profiles measured during February 2004 in Lake Kivu. The presence of these staircases at depths . 120 m indicates that diapycnal turbulent mixing is weak and vertical diffusive transport is dominated by double diffusion. Contrary to previously investigated natural or laboratory double-diffusive systems, the dissolved gases CO 2 and CH 4 contribute significantly to the density stratification, thereby influencing the formation and the structure of the staircases. The density ratio (i.e., the ratio of the stabilizing effect of dissolved substances to the destabilizing effect of temperature) ranges between 2.0 and 4.5 in large sections of the deep waters, implying a high susceptibility to the formation of staircases. The mixed layers (average thickness 0.48 m) are shown to be in a state of active convection. The average thickness of the interfaces (0.18 m) is surprisingly constant and independent of the large-scale stratification. The vertical heat fluxes correlate well with the temperature steps across the interfaces. Lake Kivu receives inflows from subaquatic springs at several depths that maintain the large-scale structure of the density stratification and disturb the staircases. In comparison to earlier observations from 1972, the double-diffusive heat fluxes appear to have been reduced, leading to a heat accumulation in the deep waters. Conversely, the strengthening of the main chemocline indicates an increased discharge of the subaquatic springs that could be responsible for recent changes in the nutrient cycling and methane production in the lake.

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...