Turbidity Currents in Reservoirs
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TANGANYIKA LAKE, MODELING THE ECO-HYDRODYNAMICS Jaya Naithani1, Pierre-Denis Plisnier2, Eric Deleersnijder1 1 G. Lemaître Centre for Earth and Climate Research (TECLIM), Institute of Mechanics, Materials and Civil Engineering (iMMC), Université catholique de Louvain, Earth and Life Institute (ELI), Louvain-la-Neuve, Belgium 2 Royal Museum for Central Africa, Tervuren, Belgium
Introduction Lake Tanganyika is one of the Great Rift Valley Lakes of East Africa (Figure 1) and is situated between 3 S and 9 S. It is 650 km long with a mean width of around 50 km and an average depth of 570 m. It is a freshwater Lake characterized by a quasi-permanent thermocline. The Lake is meromictic – complete overturning of the water never takes place – and mixing occurs only partially. The epilimnion (surface layer) undergoes seasonal temperature change annually, while the hypolimnion is anoxic with an invariant temperature. The hypolimnion is a vast reservoir of nutrients largely isolated from surface influences (Hecky and Fee, 1981; Hecky et al., 1991). The average transparency of the Lake is close to 11 m. The solar radiation around the Lake varies very little in the year because of its closer proximity to the equator. The nutrients supplied to the mixed layer, where photosynthesis can occur, are mainly internal nutrients from within the Lake, whereas riverine and atmospheric input of nutrients is considered negligible for Lake Tanganyika (Hecky and Fee, 1981; Sarvala et al., 1999a; Langenberg et al., 2003a). Nutrients from the hypolimnion are supplied to the epilimnion mainly by wind-driven upwelling of the strong southeast winds during the dry season from March/April
until August/September (Hecky et al., 1991; Plisnier et al., 1999; Langenberg et al., 2003b). The wind stress pushes the warmer surface water away from the southern end of the Lake toward the northern end, resulting in a well-known compensating upwelling in the south. This upwelling results in the seasonal enhancement of the nutrients in the euphotic layer initiating phytoplankton blooms. Apart from this major wind-induced southern upwelling, small coastal upwellings can also be seen from time to time propagating clockwise around the western boundaries of the lake, which are the internal Kelvin wave packets (Naithani and Deleersnijder, 2004). During the wet season, the primary production is less important and is primarily dependent on the nutrients regenerated within the epilimnion (Coulter and Spigel, 1991). The above mentioned thermodynamic, hydrodynamic, and ecological characteristics are incorporated into an eco-hydrodynamic model to study the primary food web of Lake Tanganyika. The hydrodynamic model comprises nonlinear, reduced-gravity equations (Naithani et al., 2002, 2003). The ecological model components include one nutrient, phytoplankton biomass, zooplankton biomass, and detritus (Naithani et al., 2007a, b, 2011).
Materials and method The model consists of a four-component ecosystem model, coupled to a hydrodynamic model. The hydrodynamic model consi
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