Comparative Limnology African Great Lakes – Victoria Tanganyika Kivu

Spatial – Temporal Variability in Surface Meteorology

Spatial Variability in Surface Meteorology

Thermal Structure in northern waters of Lake Victoria ToC Fish Thermal Structure in northern waters of Lake Victoria 1952-1954; 1960-1961; 1994-1996 Talling Oxygen, ppm Romero, MacIntyre and Kling

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

Spatial Variability in Surface Temperatures February 2000 August 2000

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.

Thermal Structure in northern waters of Lake Victoria ToC Fish Thermal Structure in northern waters of Lake Victoria 1952-1954; 1960-1961; 1994-1996 Talling What is the source of the cold water in August? Oxygen, ppm Romero, MacIntyre and Kling

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

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

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

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

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

How does climate warming affect deep, tropical lakes?

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

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.

North basin Verburg et al. 2003

Deep water temperatures increased since 1913 Verburg et al. 2003. 1913 Deep water temperatures increased since 1913 Heat gain: 0.43 J · s-1· m-2

1913 -2000

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.

What is the evidence for such change in Lake Tanganyika?

High visibility a new development in the lake

Dissolved Silica north basin

Deep water temperatures 1913 1973 1975 1913 2000 1913 1973 1975 1913

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

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?

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

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?

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

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).

Thermal, conductivity and density structure

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

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.

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.

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.

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.

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.

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).

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).

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

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. 2005 and Kling and W.C. Evans, unpublished.

Gas Extraction has begun.

Lake Kivu Region

What factors control primary productivity in this lake?