This page is dedicated to the main characteristics of the lake (i.e. the physical and chemical statistics.)
Surface area: 32,600 km²
Size of the watershed: approximately 231,000 km² (not including Lake Kivu)
Altitude of surface: 773 meters above sea level
Max. length: 650 km
Mean width: 50 km
Max width: 80 km
Max depth: 1470 m
Mean depth: 570 m
Volume: 18,880 km3
Length of shoreline: 1,838 km
Major inflows: Malagarasi River (Tanzania, Burundi), Ruzizi River (Burundi)
Outflow: Lukuga River (DRC, only outflow)
Inflow (I): between 14 and 18.2 km³ per year
Precipitation volume (P): 29 km³ per year
Outflow (O): between 2.7 and 4.2 km³ per year
Evaporation: ~ 43.5 km³ per year
Flushing Time (Volume/O): 7,000 years (for an overview of flushing time click here)
Residence time (Volume/P+I): 440 years (for an overview of residence time click here)
Surface temperature: 23.75-27.5 °C
Hypolimnion temperature: 23.25-23.5 °C
Conductivity (K₂₀) surface: 610 µS/m
offshore Uvira: 703-765 µS/cm
Bay of Bujumbura: 568-589 µS/cm
pH (surface) offshore Uvira: 9-9.1
(100m) offshore Uvira: 8.4-8.5
Transparency (Secchi disk)-offshore: 22 m
offshore outer Mwela: 5-17.5 m
inshore Nera Mpulungu: 3-12 m
offshore: 8-18 m
off Uvira: 11.3-15.3 m
Bay of Bujumbura: 1-12 m
Depth of euphotic zone (mean): 28 m
Lake Tanganyika is a meromictic type lake, in that the thermally stratified layers (epilimnion, metalimnion, and hypolimnion) do not ever mix.
Epilimnion: The lake's epilimnion undergoes seasonal temperature variation between 23.25 and 27.5 °C. It is typically 30-80 meters thick in the northern region and 150 meters thick in the southern. **(see below for explanation of the difference between north and south)
Metalimnion: The metalimnion is a region of transition between the depths of 200 and 300 meters.
Hypolimnion: Below 300 meters lies the cooler hypolimnion around 23.25 °C. This region is completely anoxic and contains hydrogen sulfide.
Though the lake's top and bottom layers do not intermix, there is a cycle of mixing between the epilimnion and metalimnion, divided into three phases:
September to November: The main heating of the lake takes place, where the average surface temperature rises from 23.75-25 °C to 27.5 °C. In this phase 11,650 cal/cm² is gained, which is almost the entire annual heat budget.
December to April: The lake is at maximum temperature stability, and the internal waves are at their peak prominence.
May to September: The dry season sets in and the epilimnion cools back to the 23.75-25 °C temperature range. Heat loss is primarily due to evaporation due to south to north blowing wind action in combination with yearly minimum temperatures.
**As south winds (south to north blowing) are the prominent wind patterns during the dry season, the metalimnion becomes tilted. The epiimnion's water dragged by the wind accumulates in the north, pushing the meta and hypolimnion towards the south, resulting in upwelling of the hypolimnion and turbulent mixing of the metalimnion with the epilimnion.
The south winds are also responsible for creating the internal seiche of Tanganyika. As the winds blow the water northward, the epilimnion is expanded and forced downward. When the winds stop in October, the density differences between the tilted epilimnion and hypolimnion work to flatten the layer as a way to minimize the energy of the system. This creates a wave action from north to south throughout the year, with a waves). The seiche continues relatively unaffected until the winds begin again in the April/May dry season.of 25 to 30 days and an amplitude of 30 to 40 meters (
The currents of Tanganyika have not been well studied, however a notable clockwise current has been reported.
Tanganyika and the other western Rift Valley lakes are of the sodium-potassium-magnesium bicarbonate type, and low chloride subtype. It has a very similar chemistry to the mineral rich inflowing Ruzizi River which, along with evaporation, is responsible for the high ionic concentrations in the lake.
The nutrient and ionic composition is represented in the chart below in microMoles (µM). I have calculated the following formula to convert from micro-Molariity to mg/L:
(A µM x [molar mass of ion/nutrient])/1000 = B mg/L
B = converted value to mg/L
Nutrient/Ion Surface 400 m
Na 2700 2900
K 820 900
Ca 270 310
Mg 1650 1740
DIC 5880 6720
Cl 750 780
SO₄ 37 -
SRSi 9.5 244
No₃ -N 0.3 0.0
NH⁴⁺ - -
PO₄ -P (SRP) .15 5.73
Total P -
Alkalinity (meq/L) 6.52 6.91
Conductivity (mS/cm) 610 -
Dissolve Oxygen (mg/L) 7-9, (9.5) 0
Physical factors such as upwelling, mixing, and diffusion along with algal photosynthesis are the primary mechanisms for the distribution of the major plant nutrients (nitrate, ammonia, phosphate, and silicon). Studies have revealed that these nutrients increase in concentration as depth increases, due to uptake by plants for photosynthesis and the meromictic condition.
At the oxic-anoxic interface, between 160 and 200 m depth, there is a very low level of dissolved fixed nitrogen, due to intense denitrification and other microbial activity.