Physical and biogeochemical limits to internal nutrient loading of meromictic Lake Kivu

Abstract

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...

Contrasting taxonomic stratification of microbial communities in two hypersaline meromictic lakes

“…The depth profiles of ammonium concentration were comparable for both Ursu and Fara Fund lakes, and similar to the ones described in other stratified lakes (Auguet et al, 2012;La Cono et al, 2013;Yau et al, 2013). The vertical profiles of nutrients (sulfate, sulfide, nitrite, nitrate and phosphate) were found to be a result of sedimentation, biogeochemical cycling or conservative mixing (Pasche et al, 2009), and were also comparable with those found in other studied lakes (Lepère et al, 2010;La Cono et al, 2013;Marteinsson et al, 2013).…”

Microbial Ecology of Lake Kivu

“…In the intermediate 60-90 m zone, the upward (diffusing and advective) fl uxes of HS -are greater than the downward fl uxes of SO 4 2− (Pasche et al 2009 ) . The inverse pro fi les of HS -and SO 4 2− observed in this zone are mainly explained by O 2 -driven sul fi de oxidation.…”

“…In Lake Kivu, the inverse pro fi les of CH 4 and SO 4 2− in the intermediate zone from 60 to 90 m (Pasche et al 2011 ) suggest the relevance of this process. Nevertheless, the sulfate fl ux budget indicates that only 3% of the CH 4 would be oxidized with SO 4 2− (Pasche et al 2009(Pasche et al , 2011 . This estimate is a maximum because sulfate reducers can also use organic matter as an electron donor, pursuing anaerobically the heterotrophic decomposition of settling phytoplankton.…”

“…During the dry, windy season, the upper 60-65 m of the water column is well ventilated and mixed. This layer, called the mixolimnion, then contains oxygen with concentrations near the saturation, whereas deeper waters are anoxic and contain reduced forms of N (NH 4 + ), C (CH 4 ) and S (hydrogen sulfi de ion, HS -) Pasche et al 2009 ) . Most chemolithotrophic microbes typically grow at redox interfaces where anoxic water containing reduced substances come into contact with O 2 via water fl ow or molecular diffusion (Burgin et al 2011 ) .…”

Direct sensing of total alkalinity profile in a stratified lake

“…In anoxic systems, sulphate reduction in the sediments and the release of reduced substances like NH 4 + further contribute to an increase of alkalinity with depth (Pasche et al, 2009). Precise profiling of alkalinity (Stumm and Morgan, 1996) in stratified waters is therefore essential to quantify the dynamics of the carbon cycle, to constrain the pathways of organic carbon mineralisation, and to understand the burial of carbonates in the sedimentary record (Schrag et al, 2013).…”