1. Lake Tanganyika Ecosystem
Jaya Naithani1, Eric Deleersnijder1,2
and
Pierre-Denis Plisnier3
1 Institute of Astronomy and Geophysics, UCL, Belgium
2 Centre for Systems Engineering and Applied Mechanics, UCL, Belgium
3Royal Museum for Central Africa, Leuvensesteenweg, 13, B-3080 Tervuren, Belgium

2. East African Rift Lakes
ZAMBIE
RDC
TANZANIE
B URUNDI
100 km N

3. General characteristics of Lake Tanganyika
SE winds
Mpulungu
Bujumbura
Kigoma
Mpulungu
(c)
(a)
(b) i ii
iii

4. Observations (April 93-March 94)

5. Model Layout Hydrodynamic component Ecological component
Non-linear reduced gravity equations
Entrainment included
Ecological component
One nutrient
Phytoplankton biomass
Zooplankton biomass
Detritus pool

6. Sketch of the reduced-gravity model
Z=0
upwelling
Epilimnion
H=h+ h
Depth s -
Z=-h +
Hypolimnion b
downwelling

7. Hydrodynamic sub-model continuity equation
 is the downward displacement of the thermocline (m),
H =h+, is the upper layer depth (m),
h is the equilibrium depth of the thermocline (m),
we is the entrainment velocity (m/s),
x and y are the horizontal axis and t is the time.

8. Hydrodynamic sub-model (contd.) continuity equation (contd.)
x and y are the horizontal components of specific wind stress in x and y direction (m/s)2,
g is the gravitational acceleration (m/s2),
=(b - s)/ b is the relative density difference between bottom and the surface layers,
wd is the detrainment term (m/s),defined so that the annual mean of the epilimnion volume remains constant.
rtt is the relaxation time scale of the thermocline (s).

9. Hydrodynamic sub-model (contd.) momentum equations
u and v are the currents in the x and y directions,
f is the Coriolis factor,
As is the horizontal eddy viscosity in the s(=x,y) direction,
Ay= Ax(y/x)2
is the entrainment or detrainment term,
, are momentum fluxes from epilimnion to the hypolimnion.

10. Flowchart of the ecological model
z=0
surface
Phytoplankton
Copepods
copepod
grazing
soluble
excretion
fecal pellets
egestion
predation
mortality
uptake
respiratory
release
mortality
dissolved
organic
phosphorus
pelagic
detritus
epilimnion
Phosphate
pelagic
regeneration
sinking
z=H
Benthic detritus
benthic
regeneration
hypolimnion

11. Ecological sub-model (Phytoplankton)
PROD
RESP
MORTa
GRAZ
SINK
Phyto is the phytoplankton biomass in µgC/L,
Ks is the horizontal eddy diffusivity in the s(=x,y) direction,
he is the flux of Phytoplankton from hypolimnion to the epilimnion,
PROD is the gross primary production calculated using the nutrient and light limitations, rp is the maximum daily production per day.
F(I) and F(P) are the functions of light and nutrient limitation.
RESP is the respiration calculated using the percent of PROD.
MORT is the mortality is assumed to be proportional to phytoplankton standing stock with mortality rate mz per day.
GRAZ is the copepod grazing assumed to be proportional to copepod biomass (Zoo) with rate rz per day.
SINK represents the sinking phytoplankton per day.

12. Phytoplankton (contd.)
Phos is the phosphate in µgP/L.
Kphos is the half-saturation constant.
ke=(0.07 Phyto/ pa ma Phyto/100)+0.077, is the light extinction coefficient (m-1),
Ik is the light saturation constant (Em-2s-1),
Io is the photosynthetically active radiation (Em-2s-1).

13. Ecological sub-model (Zooplankton)
EXE
FEC
MORTz
PRED
Zoo is the zooplankton biomass in µgC/L,
he is the flux of zooplankton from hypolimnion to the epilimnion,
EXE, FEC and MORT are the excretion, fecel pellets egestion and mortality rates of zooplankton parameterized as fixed proportion of zooplantkon grazing (GRAZ) (GRAZ=copepods growth+EXE+FEC+MORTz).
PRED is the predation of zooplanktons by zooplanktivorous fish (FISH), which is considered equal to zooplankton biomass.

14. Ecological sub-model (Phosphate)
UPTAKE
REM
REMsa
Phos is the phosphate concentration in µgP/L,
he is the flux of phosphate from hypolimnion to the epilimnion,
REM is fractions of dead phytoplankton, fecal pellets and the dead zooplankton that are instantaneously remineralization within the water column by the microbial food web.
Soluble Excretion by zooplankton and respiratory release by phytoplankton directly replenishes the phosphate pool.
CPa and CPz are the carbon to phosphate ratio of phytoplankton and zooplankton, respectively.
REMsa is the remineralized percent of the sinking phytoplankton.

15. Ecological sub-model (Detritus pool)
Detr is the detritus concentration in µgC/L,
he is the flux of detritus from hypolimnion to the epilimnion,
rd is the detritus decomposition rate in the water column.
wd is the detritus sinking rate in m/sec.

16. April 1993 to March 1994

17. Interannual Upwelling Intensity
Data:
FAO-LTR
FAO-LTFMP
CLIMLAKE

18. Predictions off Kigoma and off Mpulungu 1970-2004

19. Lake averaged Predictions
290 gCm-2yr-1
Hecky and Fee (1981)
360 gCm-2yr-1
predictions
gCm-2yr-1
Sarvala et al., (1999)
gCm-2yr-1
predictions

20. Fish Target Species Clupeids = Sardines Perch Stolothrissa miodon
Limnothrissa
tanganicae
Lates
stappersi
Perch

21. Predictions and Fish Statistics
Bujumbura
B URUNDI
Kigoma
TANZANIA
RDC N
100 km
Mpulungu
ZAMBIA

22. Conclusions
The model satisfactorily predicts the dynamics of primary productivity in Lake Tanganyika ( gC/m2/year). The agreement between the observations and prediction is encouraging.
Primary production, which depends strongly upon light in the water column and entrainment of nutrients, is bottom-up controlled.
At least phytoplankton chlorophyll concentrations seem to have remained largely similar from the 1970’s to the present except for some year to year fluctuations due to exceptional winds.
Relationship between the mixing depth changes (due to warming) and lake productivity is not simple or straightforward.
Next I will explore the impact of increased population around the lake.

23. Why study the past climate and link it with Lake production!
To make future scenarios
And plan for the sustainable management of fisheries.