comparative limnology
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Comparative Limnology
African Great Lakes –
VictoriaTanganyika
Kivu
17 18 19 20 21 22 23Longitude (m in o f 33°)
15
16
17
18
19
20
21
Latit
ude
(min
of O
°N)
32° 33° 34°
0°
1°
2°
100 km
Entebbe
Jinja
K isum u
M usom a
M w anza
Bukoba
KageraR iver
N ileR iver
N zoiaR iver
60 m
10 km
0°
0°30'
33° 33° 30 '
NileR iver
BUG 0 1 2kmN
BuvumaIsland
0 km
8 kmBuvuma Channel Hannington
Bay
10 km
LingiraIsland
Napoleon G ulf
Buga ia Island
Thruston Bay
Hann ington B ay
Buvum a Is land
1500
900 (Apr-Aug)
900 (Sep-Apr)
5 m s
B
A
CJinja
N 1500
1500
1500
1500
1500
Fig. 1 - Rom ero, M acIntyre and K ling (Phys. L im n. V ic.)
MET
MET
900(Apr.-M ay)
900(Jun.-M ar.)
AB
C
D
E
F
G
H
S
T
R
Q
M
N
P
O
Apr. 13, 1957 (A -B )
Jun . 23, 195 7 (C-D )
Au g. 24-25, 1957 (M -N-O -P )
Sep . 5-7 , 1957 (Q -R -S -T)
N ov. 9-10 , 1957 (E -F)
Feb. 17 -18, 1958 (G -H )
M ay 20-24, 1995Apr. 5-9 , 1996
Lake Transect Legend
-1
2250
1750
1250
750
mm
yr-1
R ain Evap
2250
1750
1250
750
mm
yr-1
Rain Evap2250
1750
1250
750
mm
yr-1
R ain Evap
2250
1750
1250
750
mm
yr-1
Rain Evap
2250
1750
1250
750
mm
yr-1
R ain Evap
2250
1750
1250
750
mm
yr-1
Rain Evap
1°
31°La
titud
eLongitude
YalaR iver
M araR iver
900 (N ov.-M ay)
900(Jun.-O ct.)
Spatial – Temporal Variability in Surface Meteorology
T @
090
0 (°
C)
19
22
25
28
31
RH
@ 1
500
(%)
40
60
80
100
Pre
ssur
e @
900
(mb)
886
887
888
889
890
Clo
ud @
900
(fra
c)
0.2
0.4
0.6
0.8
1.0
Rai
n (m
m m
onth
-1)
0100200300400500
Sho
rtwav
e ( W
m-2
)
300330360390420450480510540
KisumuEntebbeBukobaMwanzaJinjaMusoma
WS
@ 0
900
(m s
-1)
0
2
4
6
8
WS
@ 1
500
(m s
-1)
0
2
4
6
8
WD
@ 0
900
(°)
0
90
180
270
360
WD
@ 1
500
(°)
0
90
180
270
360
T @
150
0 (°
C)
19
22
25
28
31
RH
@ 0
900
(%)
40
60
80
100
A
B
C
D
E
F
G
J
K
H
I
L
J F M A M J J A S O N D
Net
Lon
gwav
e 09
00 (W
m-2
)
-100
-75
-50
-25
0
Late
nt H
eat F
lux
0900
(W m
-2)
-400
-300
-200
-100
0
J F M A M J J A S O N D
Late
nt H
eat F
lux
1500
(W m
-2)
-400
-300
-200
-100
0
MJ F M A M J J A S O N DN O
Spatial Variability in Surface Meteorology
Fig. 6 - R om ero, M acIntyre and K ling (Phys. L im n. Vic.)
0
20
40
60
0
20
40
60
0
20
40
60
0
20
40
60J J A S O N D J F M A M J J A S O N D J F M A M J
1994 1995 1996
1960 1961
1952 1953 1954
Dep
th (m
)
M onth o f 1994-1996
A
B
C
D
23
23.25
23.5
23.75
24
24.25
24.5
24.75
25
25.25
25.5
25.75
26
26.25
26.5
26.75
27
Thermal Structure in northern waters of Lake Victoria
1952-1954; 1960-1961; 1994-1996
Fish
Talling
Romero, MacIntyre and Kling
ToC
Oxygen, ppm
Lake Temperatures have Increased!
Increase occurred at end of long, dry season.
February 2000
Surface
August 2000
Spatial Variability in Surface Temperatures
0
20
40
60
B
North SouthIn /O ffshoreTransects M w anza
Speke G ulfB
North SouthIn /O ffshoreTransects M w anza
Speke G ulf
Dep
th (m
)
N orth-South D istance (km )
M ay 20-24 1995 Apr 5-9 1996
0
20
40
60
0 50 100 150 200 250 3000 50 100 150 200 250 300
A B
E F
F ig. 9 - R om ero, M acIntyre and K ling (Phys. L im n. V ic.)
20
40
60
0
4
8
12
16
chla (ug/l)
G
24.0
24.5
25.0
25.5
26.0
26.5
T (°C)
H
0123456789 D
O (m
g/L)
Secchi2m
0
20
40
60 C D109.0
111.0
113.0
115.0 SC
(uS/cm
)
0
Upwelling Early in the Long, Dry Season
0
20
40
60
B
North SouthIn/O ffshoreTransects M wanza
Speke G ulfB
North SouthIn/O ffshoreTransects M wanza
Speke G ulf
De
pth
(m
)
N orth-South D istance (km )
M ay 20-24 1995 Apr 5-9 1996
0
20
40
60
0 50 100 150 200 250 3000 50 100 150 200 250 300
A B
E F
Fig. 9 - R om ero, M acIntyre and K ling (Phys. L im n. V ic.)
20
40
60
0
4
8
12
16
ch
la (u
g/l)
G
24.0
24.5
25.0
25.5
26.0
26.5
T (°C
)
H
0123456789
DO
(mg
/L)
Secchi2m
0
20
40
60 C D109.0
111.0
113.0
115.0 SC
(uS
/cm
)
0
Strong wind events induce upwelling at the end of the wet season, but mixing during the long, dry season leads to isothermal waters.
Fig. 6 - R om ero, M acIntyre and K ling (Phys. L im n. Vic.)
0
20
40
60
0
20
40
60
0
20
40
60
0
20
40
60J J A S O N D J F M A M J J A S O N D J F M A M J
1994 1995 1996
1960 1961
1952 1953 1954
Dep
th (m
)
M onth o f 1994-1996
A
B
C
D
23
23.25
23.5
23.75
24
24.25
24.5
24.75
25
25.25
25.5
25.75
26
26.25
26.5
26.75
27
Thermal Structure in northern waters of Lake Victoria
1952-1954; 1960-1961; 1994-1996
Fish
Talling
Romero, MacIntyre and Kling
ToC
Oxygen, ppm
What is the source of the cold water in August?
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
February 2000
Year to Year Variability in near bottom Oxygen Concentrations
February 2001
Greg Silsbe
Spatial Patterns: Maximum Chlorophyll
9 . 22 1 . 03 1 . 46 . 27 . 0
3 . 93 . 6
4 . 51 3 . 5
4 . 8
7 . 11 8 . 0
4 . 3
3 . 1 1 4 . 8
2 . 12 . 1
9 . 8
3 . 99 . 5
2 2 . 16 . 8
9 . 0
3 1 . 5
8 . 94 . 2
1 1 . 41 4 . 9 1 0 . 2 4 . 5
1 4 . 9
1 5 . 51 4 . 5
1 3 . 31 5 . 3
7 . 5
2 4 . 1
1 0 . 49 . 9 1 7 . 1
8 . 3
2 0 . 5
1 2 . 15 . 2
3 . 63 . 5
2 . 6 6 . 09 . 9
28.4
16.1
5.5
8.3
35.6
3.6
50.1
23.8
15.6
6.4
27.9
5.0
4.7
10.529.2
28.1
34.7
14.5
41.8
36.1
7.3
12.4
30.3
30.8
15.3
3.5
28.0
33.4
20.8
34.8
27.6
22.2
45.7
25.3
10.4
34.5
12.7
33.1
63.7
7.6
56.1
42.2
38.5
71.0
4.6
72.6
18.348.046.7
25.23.7
Feb 2000
Aug 2000
Max Chlorophyll (mg/m3) – Feb 2000
9 . 22 1 . 03 1 . 46 . 27 . 0
3 . 93 . 6
4 . 51 3 . 5
4 . 8
7 . 11 8 . 0
4 . 3
3 . 1 1 4 . 8
2 . 12 . 1
9 . 8
3 . 99 . 5
2 2 . 16 . 8
9 . 0
3 1 . 5
8 . 94 . 2
1 1 . 41 4 . 9 1 0 . 2 4 . 5
1 4 . 9
1 5 . 51 4 . 5
1 3 . 31 5 . 3
7 . 5
2 4 . 1
1 0 . 49 . 9 1 7 . 1
8 . 3
2 0 . 5
1 2 . 15 . 2
3 . 63 . 5
2 . 6 6 . 09 . 9
Spatial Patterns: Maximum Chlorophyll
9 . 22 1 . 03 1 . 46 . 27 . 0
3 . 93 . 6
4 . 51 3 . 5
4 . 8
7 . 11 8 . 0
4 . 3
3 . 1 1 4 . 8
2 . 12 . 1
9 . 8
3 . 99 . 5
2 2 . 16 . 8
9 . 0
3 1 . 5
8 . 94 . 2
1 1 . 41 4 . 9 1 0 . 2 4 . 5
1 4 . 9
1 5 . 51 4 . 5
1 3 . 31 5 . 3
7 . 5
2 4 . 1
1 0 . 49 . 9 1 7 . 1
8 . 3
2 0 . 5
1 2 . 15 . 2
3 . 63 . 5
2 . 6 6 . 09 . 9
28.4
16.1
5.5
8.3
35.6
3.6
50.1
23.8
15.6
6.4
27.9
5.0
4.7
10.529.2
28.1
34.7
14.5
41.8
36.1
7.3
12.4
30.3
30.8
15.3
3.5
28.0
33.4
20.8
34.8
27.6
22.2
45.7
25.3
10.4
34.5
12.7
33.1
63.7
7.6
56.1
42.2
38.5
71.0
4.6
72.6
18.348.046.7
25.23.7
Feb 2000
Aug 2000
Max Chlorophyll (mg/m3) – Aug 2000
28.4
16.1
5.5
8.3
35.6
3.6
50.1
23.8
15.6
6.4
27.9
5.0
4.7
10.529.2
28.1
34.7
14.5
41.8
36.1
7.3
12.4
30.3
30.8
15.3
3.5
28.0
33.4
20.8
34.8
27.6
22.2
45.7
25.3
10.4
34.5
12.7
33.1
63.7
7.6
56.1
42.2
38.5
71.0
4.6
72.6
18.348.046.7
25.23.7
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.
23.023.223.423.623.824.024.2
1900 1920 1940 1960 1980 2000Year
Tem
pera
ture
(°C
)
110 m150 m200 mdeep minima
Mean Global Temperature-0.6
-0.4
-0.2
0
0.2
0.4
0.6
1900
1906
1912
1918
1924
1930
1936
1942
1948
1954
1960
1966
1972
1978
1984
1990
1996
Dev
iatio
n fr
om m
ean
(° C
)
Climatic Research Unit (Jones et al. 2001)
North basin Verburg et al. 2003
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
0
5
10
15
20
25
30
35
40
45
1900 1950 2000
Dep
th o
f 1 %
of l
ight
at s
urfa
ce
Irradiance (% PAR)
0 50 100
Depth (m
)
0
10
20
30
40
1913
1975
1996
Dissolved Silica north basin
0
20
40
60
80
100
120
140
0 20 40 60 80 100Si (uM)
Dep
th (m
)
Jul/Sep 1938Apr/Nov 1947Feb-73Apr 1975Oct-75Jul-Sep 2000Mar-Apr 2001
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
Upwelling
Thermocline during upwelling
South North
U
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
0.5 0.1Wedderburn no.
Upwelling co-occurs with non-linear waves Horizontal Transport is predicted and may supply nutrients to the North Basin
2nd vertical mode internal wave
Non-linearwaves
Mixing
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?
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).
depth
[m]
1.00510.0
0
4850.9950.0
density without pressure effect [g/cm³] -- in-situ density [g/cm³]electrical conductivity [mS/cm]
0.021.0
1000.031.0
pressure [dbar]temperature [°C]
L A K E K I V U– main basin –
© Dr. Klaus TIETZE
HL1
GL3HL4
GL4
HL5
GL5
HL6GL6HL7
GL1
HL2
GL2
HL3
HL1
GL3HL4
GL4
HL5
GL5
HL6GL6HL7
GL1
HL2
GL2
HL3
Thermal, conductivity and density structure
0 1 2x 10
-4
-400
-300
-200
-100
0
Stability
Dep
th (m
)
0 5 10 15 20 25 30
-400
-300
-200
-100
0
Buoyancy frequency (cph)
Stratification in Lake KivuBuoyancy 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 = densityd 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).
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.
Measured methane concentration from 1974/75 and perceived saturation risk (after Tietze 2005).
Estimated gas pressures based on 2003/2004 measurements and perceived saturation risks (after Schmid et al. 2004).
Bottom topography of the northern part of Lake Kivu showing numerous clearly visible volcanic cones (after Schmid et al. 2004).
Earthquakes in the Kivu region.
Lake Nyos in 1985 before and in 1986 after the catastrophic eruption (photos by G. Kling)
Operation of the ‘soda straw’ syphon.
Lake Monoun - Total Gas Pressure (bar)
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8
Dep
th (
m)
20032006saturation line
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.
DANGER WAS IMMINENT but FORESTALLED!
Gas Extraction has begun.
Lake Kivu Region
What factors control primary productivity in this lake?
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