The impact of climate change in Lake Tanganyika by using physical parameters
Student: Melania Nyimbo
Mentor: Catherine O´Reilly
Objective
The aim of the study was to investigate changing hydroclimate in Lake Tanganyika by using physical
parameters and to investigate how global warming influences the rate of upwelling in the pelagic zone of
Lake Tanganyika (Kigoma bay) during my research period. I also made a comparative study of the
limnological data for the last nine years of the Nyanza Project.
Introduction
Lake Tanganyika is a large Rift valley lake (length 65 km, mean width 50 km, mean and
maximum depth 570 m and 1470 m) near the equator (3 to 9 degrees). Along with its high water
temperature (23.25° to 27.25°C), thermal stratification is well marked and varies seasonally above an
apparently permanent anoxic hypolimnion. The lake can be classified as meromictic. The lake experiences
two main seasons: a 4 month dry season from May through August characterized by cooler dry conditions
and a fairly constant southerly wind and a wet season during the rest of the year when winds are generally
lighter and mainly northerly. In the lake environment most cooling and mixing takes place in the dry season
and maximal stratification occurs during the warmer wet season. The lake is oligotrophic and permanently
thermally stratified into:
1. Epilimnion (mixed layer Craig et al, 1974), which can approach thermal and gaseous
equilibrium with the atmosphere. This is the upper stratum of relatively uniform warm and circulating
water.
2. Metalimnion: The stratum just below the epilimnion where wind energy is not sufficient to mix
the water at these depths. This is a region of greatest change in temperature and the plane of maximum rate
of decrease of temperature with depth (Thermocline).
3. Hypolimnion: The lowest stratum in the lake, which is anoxic and generally colder.
Local climate change can be influenced by different factors including: human activities such as
cutting down trees, pollution and global warming. Climate warming through increased water density
gradient slowed vertical mixing and reduced primary productivity in Lake Tanganyika (O´Reilly et al
2003).
Study site
The water samples of pelagic zone were collected at Kigoma Bay (S04°, 51.605’and E029°,
35.324’) on the 13th, 16th, 19th, 23rd, 26th, 30th of July and 2nd of August 2007 between 9:30 and 12:00
am. Sampling was conducted twice per week, Monday and Thursday. Also I used a meteorological data
from the Kigoma airport for an investigation of wind intensity.
Materials and methods
Water samples were collected from different seven depths at 0m, 10m, 20m, 40m, 60m, 80m, and
100m using a 7.4 liter water sampler bottle and put into four liter bottles. Transparency measurements were
taken using a 20cm diameter Secchi disk (SD) at the start of the sampling period .The mean value of three
readings was recorded. CTD profiles Dissolved Oxygen (in % or mg/l), pH, Conductivity (mg/l) and
65
Temperature (°C) in the pelagic zone were conducted from the surface to 150 m depths using the R/V Echo
winch, and then downloaded to a PC. Unfiltered water was taken to the wet lab for turbidity measurements
(NTU) using a Hach Turbidity meter 2100P model, and for alkalinity measurements.
Results
Wind Velocity
The mean wind run on June 2007 was 5.1km/hr and July was 6.0km/hr. Mean air temperature was
26.1°C over the course of the year.
Transparency
Turbidity ranged between 0.10 NTU to 0.30 NTU on all sampling days, except for the first day on
13th of July at 80m, when it was 0.96 NTU. The Secchi depth was observed to be between 10.3m and
13.4m with the maximum value on July 30th. During the sampling days water sampling was observed to be
clear.
Alkalinity
Alkalinity fluctuated over the sampling period. On the 19th day of sampling at 40m and 100m the
alkalinity value was as high as 295, while on 30th from 20m-100m alkalinity value decrease to 275.
Temperature
Temperature decreased both over time and with depth, and the thermocline shallowed over the
period of study. The highest water temperature was observed on the surface with the maximum value of
26.41°C with STD (0.01) on 13th of July and minimum value of 26.10°C with STD (0.15) on July 23rd and
2nd of August, which was the last day of sampling. On the first day of sampling the epilimnion ranged
from 0m-79m, the themocline was between 79m –100m and the hypolimnion was below this, and on the
last day of sampling which was on 2nd August 2007, the epilimnion was between 0m - 50m, the
thermocline was ranged from 50m-90m, and the hypolimnion was below that.
Conductivity
The conductivity of the lake was generally very high and was found to decrease with the sampling
days and depth. The highest conductivity recorded was 685.83S/cm (0.87 1 ), which was observed on
30th of July, and the lowest value of conductivity was 684.74S/cm on the 26th of July. The changes of
conductivity were relevant as they could be used as indicators of metalimnion movement.
pH
pH was found to be low in deeper waters and high in surface waters that were observed to be
increase with the sampling days. The highest pH was 9.20 on 2nd of August which was the last day of
sampling. The lowest value was 9.12 on 13th of July (Fig.1c).
Dissolved Oxygen
Generally the amount of dissolved oxygen was increased over time and at shallower depths. The
highest value of DO was found to be 6.45 (mg/L) with STD (0.22) on 26th of July and lowest value was
5.86 (mg/L) with STD (0.1). On the first day of sampling (13th of July) the epilimnion ranged from 0m70m, and the oxycline was between 70m-91m. On the last day of sampling (August 2nd) the epilimnion was
0m-59m, and the oxycline ranged from 59m-80m (Fig.1f ).
66
Discussion
It would appear that the productivity of Lake Tanganyika is highly dependent on the
hydrodynamic state of the lake and on climatic conditions particularly the wind and the heat budget
(Coulter et al 1974). Through my research period, temperature of the lake was observed to decrease with
depth and the sampling days. The thermocline was increasing as on the 13th of July, which was the first
day of sampling was ranged from 79m-107m, and on the last day on 2nd of August was ranged from 60m100m (Fig 1a). Dissolved oxygen was observed over the sampling period and decreased with depth, the
oxycline increased from 70m-90m to 60m-80m at the end of the sampling period (Fig.1f ). Alkalinity and
turbidity were both higher on the first day of sampling (Fig.1d and 1e).
By comparison with the last nine year, in order to get upwelling the wind run in June must be
greater than 5km/hr and July greater than 5.5km/hr. In the year 2003, the wind run in June averaged
5.1km/hr, July 5.6km/hr, in 2004 was 5.7km/hr in June and July, in 2007 was observed to be 5.1km/hr in
June and 6.0km/hr in July that made provided support for upwelling (Fig.1a, 2a-2d).
Temperature was observed to increase with years as in 1999 temperature was 25.5°C and 2007
temperature was 26.1°C, with the correlation value (r2=0.1072). In the epilimnion temperature was
observed to be higher and increase with years (P<0.001), which shows that there is a very significant
change in temperature with years. The thermocline was observed to fluctuate over time due to upwelling
during the sampling period in some years and not in others. But over time the thermocline generally
deepened. In 1999 thermocline was between 0m-47m and in 2007 was ranged from 0m-75m. Dissolved
oxygen was observed to decrease compared to other years due to upwelling fluctuation (Fig.1f). The winds
speed was observed to decrease with increasing air temperature (r2 = 0.1408).
Conclusion
Changing climatic conditions, mainly increasing heat and reduced wind speed, seems to be altering the
trends of upwelling in Lake Tanganyika. It seems that global warming has impacted Lake Tanganyika as
winds speed decreased, and air temperature increased, causing changes in the epilimnion.
References
Bygott, D. 1992. Gombe Stream National Park. Tanzania National Parks/African Wildlife
Kivyro, A., 2005. A comparison of the physical-chemical limnology in the pelagic waters of Lake
Tanganyika at Kigoma Bay. The Nyanza Project Annual Report.
Plisnier. P.-D. Limnology Notes and Field Manual
Plisnier, P.-D., V. Langenberg, L. Mwape, D. Chitamwebwa, and E. Coenen. 1999. Limnological annual
cycle inferred from physical-chemical fluctuations at three stations of Lake Tanganyika. Hydrobiologia
O’Reilly, C. M., S. R. Alin, P.-D. Plisnier, A.S. Cohen and B.A. Mckee, 2003. Climate change decreases
ecosystem productivity of Lake Tanganyika, Africa. Nature 424.
Wetzel, 1983. Limnology: Lake and River Ecosystem
Coulter, G.W.1999. Lake Tanganyika and its Life. Oxford University Press, London.
Acknowledgements
I would like to thank the following: Dr. Hudson Nkotagu from the University of Dar es salaam for giving
me opportunity to explore my interest, Andy Cohen and E.Michel for their coordination of 2007 Nyanza
Project and my mentor Catherine O’Reilly, who provided her endless support throughout the entire project.
Special thanks to Mr. Willy Mbemba for his tireless assistance during sampling, laboratory analysis and
computer work. I thank the manager of the Tanzania Metrological agency, TAFIRI staff for their
coordination during the entire project, and my fellow students for their help during my research period.
Finally I would like to thanks the National Science Foundation (ATM 0223920 and DBI-0608774) for
funding this research.
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Conductivity Vs Depth per day
Temperature Vs Depth per day
0
0
20
20
24.0
24.5
25.0
25.5
26.0
40
674
676
678
40
680
682
60
80
80
th( )
Depth(m)
D
60
100
100
Fig 1a
120
684
686
120
Fig.1b
140
140
13-Jul
17-Jul
21-Jul
25-Jul
29-Jul
Fri 13
02-Aug
Tue 17
Sat 21
Wed 25
Sun 29
Thu 02
Time in days
Time in days
pH Vs Depth per day
Alkalinity Vs Depth per day
0
0
20
20
40
40
8.8
8.9
9.0
9.1
9.2
60
275
280
285
290
295
D th ( )
D
80
60
th( )
100
Fig.1d
80
120
Fig 1c
100
13-Jul
17-Jul
21-Jul
25-Jul
29-Jul
02-Aug
Time in days
140
Fri 13
Tue 17
Sat 21
Wed 25
Sun 29
Thu 02
X Data
Turbidity Vs Depth per day
DO(mg/L) Vs Depth per day
0
0
20
20
0.1
0.2
0.3
40
0.4
0.5
1
2
0.6
40
0.7
0.8
60
3
4
0.9
5
6
D th( )
Depth(m)
60
80
100
Fig 1f
80
120
Fig 1e
140
100
13-Jul
Fri 13
17-Jul
21-Jul
25-Jul
29-Jul
Tue17
Sat21
Wed25
02-Aug
Time in days
Time in days
68
Sun29
Thu02
o
2003 Temperature (
o
C)
o
o
Temperature
C)
Temperature 2004
( C)
in 2004( summer
Temperature ( C) in 2003 summer
0
0
2 4 .4
2 4 .6
2 4 .8
20
2 5 .0
20
2 4 .6
2 5 .2
2 4 .8
2 5 .4
2 5 .0
2 5 .6
2 5 .8
2 5 .2
2 5 .4
2 6 .0
40
2 5 .6
2 5 .8
40
2 6 .0
Depth (m)
60
60
80
80
100
7 /1 2
100
7 /1 8
7 /2 0
7 /2 2
7 /2 4
7 /2 6
7 /2 8
7 /3 0
8 /1
7 /1 4
7 /1 6
7 /1 8
7 /2 0
7 /2 2
8 /3
7 /2 4
7 /2 6
7 /2 8
7 /3 0
8 /1
8 /3
8 /5
Sampling Days
Sam pling Days
Fig. 2b
Figure 2a
o
2005 Temperature (
C)
Temperature (
o
Temperature ( C) in 2005 summer
0
C)
0
2 4 .2
20
o
Temperature ( C) in 2006 summer
o
2 4 .5
2 4 .4
20
2 5 .0
2 4 .6
2 5 .5
2 4 .8
2 6 .0
2 5 .0
2 5 .2
2 5 .4
40
40
2 5 .6
2 5 .8
2 6 .0
Depth (m)
60
Fig 2c
80
60
80
Fig.2d
100
100
7 /2 4
7 /2 6
7 /2 8
7 /3 0
8 /1
8 /3
8 /5
7 /1 3
7 /1 5
7 /1 7
7 /1 9
7 /2 1
Sampling Days
7 /2 3
7 /2 5
7 /2 7
7 /2 9
7 /3 1
Sampling Days
Temperature Vs Depth per Year
Air temp from 1998-2007
0
31.5
31
30.5
30
Te 29.5
m
29
p 28.5
28
27.5
27
26.5
R2 = 0.1072
24.5
25.0
25.5
20
26.0
40
198519871989199119931995199719992001200320052007
D
ep
th
(m
)
60
Fig.2f
80
Years
Fig. 2e
May
Jun
Jul
Fig. 2e
Linear (Jul)
100
1999
2000
2001
2002
2003
2004
2005
2006
2007
Time in years
Monthly Wind speads variation
6.5
Variation of Temp from 1999-2007
26.5
2
R = 0.1408
Wi 6
nd
sp 5.5
ea
5
d
(K
4.5
m/
h)
4
R2 = 0.9194
Te 26
m 25.5
pe
rat 25
ur 24.5
e
24
(o
C)
23.5
3.5
19
98
19
99
20
00
20
01
Fig 2g
May
20
02
20
03
20
04
20
05
20
06
R2 = 0.0582
*** P<0.001
R2 = 0.2006
23
20
07
J 1999
J 2000
J 2001
J 2002
J 2003
J 2004
J 2005
J 2006
J 2007
Time in Year
Jun
Jul
Time in years
Fig.2h
Linear (Jul)
Av Epi
69
Av Hypo
Av Term