1. Comparative Limnology
African Great Lakes –
Victoria
Tanganyika
Kivu

2. Spatial – Temporal Variability in Surface Meteorology

3. Spatial Variability in Surface Meteorology

4. Thermal Structure in northern waters of Lake Victoria
ToC
Fish
Thermal Structure in northern waters of Lake Victoria
; ;
Talling
Oxygen, ppm
Romero, MacIntyre and Kling

5. Lake Temperatures have Increased!
Increase occurred at end of long, dry season.

6. Spatial Variability in Surface Temperatures
February 2000
August 2000

7. Upwelling Early in the Long, Dry Season
Strong wind events induce upwelling at the end of the wet season, but mixing during the long, dry season leads to isothermal waters.

8. Thermal Structure in northern waters of Lake Victoria
ToC
Fish
Thermal Structure in northern waters of Lake Victoria
; ;
Talling
What is the source of the cold water in August?
Oxygen, ppm
Romero, MacIntyre and Kling

9. Causes of Increased Temperatures
Land use changes
Higher attenuation coefficient in northern, inshore waters – warmer temperatures
Inshore waters flow offshore - transported southward by cyclonic flow – warms the southern waters
Climate induced changes
Reduced latent heat fluxes in the south
Possible warming of Kagera River

10. Year to Year Variability in near bottom Oxygen Concentrations
February 2000
February 2001
Year to Year Variability in near bottom Oxygen Concentrations
Greg Silsbe

11. Max Chlorophyll (mg/m3) – Feb 2000
Spatial Patterns: Maximum Chlorophyll
Max Chlorophyll (mg/m3) – Feb 2000
Feb 2000
Aug 2000

12. Max Chlorophyll (mg/m3) – Aug 2000
Spatial Patterns: Maximum Chlorophyll
Max Chlorophyll (mg/m3) – Aug 2000
Feb 2000
Aug 2000

13. What factors cause the spatial-temporal variability in chlorophyll distributions?

14. How does climate warming affect deep, tropical lakes?

15. Climate warming effect on stratification in Lake Tanganyika:
Loss of productivity
Verburg et al. Science 2003

16. Lake Tanganyika
World’s longest lake.
2nd oldest – Baikal is older.
2 – 20 million years
Graben lake.
Oligotrophic – chlorophyll concentrations 1-5 ug/l
214 species of native fish, 176 of which are endemic
Upwelling during the long, dry season provides nutrients that support the productivity of the lake.

17. North basin Verburg et al. 2003

18. Deep water temperatures increased since 1913
Verburg et al
1913
Deep water temperatures increased since 1913
Heat gain:
0.43 J · s-1· m-2

21. Why would warming of a lake or ocean cause a decrease in growth of phytoplankton?
How does the stratification of the lake or ocean change?
Stratification becomes more stable!
Greater density difference between upper and lower water column.
More work will be required to mix nutrients at depth into the well lit regions where phytoplankton grow.

22. What is the evidence for such change in Lake Tanganyika?

23. High visibility a new development in the lake

24. Dissolved Silica north basin

25. Deep water temperatures 1913 1973 1975 1913 2000 1913 1973 1975 1913

26. Global Warming Consequences vary in different locations.
In Lake Tanganyika, the evidence indicates:
Increased Heat Gain and Stratification
Vertical mixing is reduced
Primary Production is reduced
Consequences for Fish Production are unknown

27. Thermocline during upwelling
South
North
Thermocline during upwelling
What other process during the long, dry season could contribute to nutrient flux?
How could climate change moderate this process?

28. Lake Tanganyika (South Basin) - Long Dry Season
Upwelling co-occurs with non-linear waves
Horizontal Transport is predicted and may supply nutrients to the North Basin
Mixing
Non-linear
waves
2nd vertical mode internal wave
Wedderburn no.
0.5
0.1

29. While Verburg et al. (2003) show that the waters of Lake Tanganyika are warmer now than 100 years ago, and provide evidence for a reduced depth of vertical mixing,
What mechanistic hypotheses can we develop to explain the changes?

30. Lake Kivu – a small (70 km long) but deep (500 m) African Great Lake

31. Schematic cross section through Lake Kivu and Lake Tanganyika showing the mode of influx and infiltration of fresh and saline water into Lake Kivu (after Tietze, 2005).

32. Thermal, conductivity and density structure

33. Stratification in Lake Kivu
Buoyancy Frequency –
A measure of the stability of density structure in lakes and oceans
N2 = g / ρ (d ρ /dz)
g = gravity 9.8 m s-2
ρ = density
d ρ /dz is density gradient, that is, the change in density with depth
Units of N are s-1 or, by recalling that there are 2 pi radians in a cycle, we convert to cycles per hour (cph).
Stratification in Lake Kivu

34. Density Structure and Internal Waves
Internal waves occur in stratified waters.
They look like waves we see on the sea surface.
They are initiated when a disturbance causes a pycnocline to tilt.
For Lake Kivu, the disturbance could be wind, an earthquake, or a volcano.
We do not know how high the amplitude of the waves will be.

35. Density Structure and Internal Waves
Internal waves occur in stratified waters.
They look like waves we see on the sea surface.
They are initiated when a disturbance causes a pycnocline to tilt.
For Lake Kivu, the disturbance could be wind, an earthquake, or a volcano.
We do not know how high the amplitude of the waves will be.

36. Density Structure and Internal Waves
Internal waves occur in stratified waters.
They look like waves we see on the sea surface.
They are initiated when a disturbance causes a pycnocline to tilt.
For Lake Kivu, the disturbance could be wind, an earthquake, or a volcano.
We do not know how high the amplitude of the waves will be.

37. Density Structure and Internal Waves
Internal waves occur in stratified waters.
They look like waves we see on the sea surface.
They are initiated when a disturbance causes a pycnocline to tilt.
For Lake Kivu, the disturbance could be wind, an earthquake, or a volcano.
We do not know how high the amplitude of the waves will be.

38. Density Structure and Internal Waves
Internal waves occur in stratified waters.
They look like waves we see on the sea surface.
They are initiated when a disturbance causes a pycnocline to tilt.
For Lake Kivu, the disturbance could be wind, an earthquake, or a volcano.
We do not know how high the amplitude of the waves will be.

39. Estimated gas pressures based on 2003/2004 measurements and perceived saturation risks (after Schmid et al. 2004).
Measured methane concentration from 1974/75 and perceived saturation risk (after Tietze 2005).

40. Earthquakes in the Kivu region.
Bottom topography of the northern part of Lake Kivu showing numerous clearly visible volcanic cones (after Schmid et al. 2004).

41. Lake Nyos in 1985 before and in 1986 after the catastrophic eruption (photos by G. Kling)
Operation of the ‘soda straw’ syphon.

42. DANGER WAS IMMINENT but FORESTALLED!
Pipe Inlet
Illustration of changes in gas content and layering in Lake Monoun, Cameroon as water is removed from 73 m depth during controlled degassing. A similar effect of lowering the stratification layers while maintaining their integrity is predicted for Lake Kivu (data from Kling et al and Kling and W.C. Evans, unpublished.

44. Gas Extraction has begun.

45. Lake Kivu Region

49. What factors control primary productivity in this lake?