A preliminary investigation of pelagic primary production in the
Kigoma area, Lake Tanganyika
Students: P. Isumbisho Mwapu & J. Raini
Mentor: Dr. Pierre-Denis Plisnier
Introduction
Lake Tanganyika in East Africa is known for its productive pelagic fishery, which is reported to yield
higher catches per unit area than most great lakes of the world (Coulter, 1991). These important fish yields
can only be supported by favorable primary production, which depend on hydrodynamic mixing events. It’s
a meromictic lake that undergoes periodical superficial mixing. Strong winds occur during the dry season
(May-September) causing partial upwelling of nutrients into the epilimnion (Plisnier et al., 1996, 1999)
which become available for the food web of the Lake. This study concerns an investigation on pelagic
primary production in the Kigoma area, Lake Tanganyika.
Objectives
This study has two objectives:
- Comparative analysis of Chl a as a measure of algal biomass in relation to some limnological
parameters,
- Evaluate the phytoplankton assemblage, abundance and distribution in the Kigoma bay.
Materials and Methods
The sampling were done at a pelagic site (S 4o 52’ 19’’; E 29o 35’ 99’’) of the Kigoma bay. A boat
“Titanic” was used for moving on the Lake. Water samples for analysis was collected at different depths (0,
10, 20, 30, 40, 60, 80 and 100 m ) using a 7.4-l sample bottle. In situ measurements were made for
temperature and dissolved oxygen (DO) using an oxymeter (YSI model 58); conductivity (µS/cm) using a
Cole/Parmer conductivity meter (model 2100A); pH using an Orion pH meter (model 210A); turbidity
(NTU) using a Hach turbidimeter (model 2100A). All meters were calibrated before sampling. Samples
collected from the field were stored in a cool box for the lab analysis. Alkalinity was determined through a
titration procedure using an automatic titrator with continuous stirring by a magnetic stirrer. Total
phosphorus, soluble reactive phosphorus, nitrogen and silica were analysed, in the lab using a HACH
DR/2010 Spectrophotometer. Chlorophyll a (µg/l) was calculated after reading the absorbance at 665 nm
and 750 nm using a Spectronics 21D Spectrophotometer. This was done after leaving the samples in a
refrigerator for 23 hours in an extraction process using 94.1% methanol. Algal biomass was estimated by
using Chl a. The Chl a represents 1.5 % of the dry weight of organic matter (ash-free weight) of algae. The
Chl a content was multiplied by a factor of 67 (APHA, 1998) to get the dry weight of algae. Photosynthetic
rates were determined by inserting transparent and opaque bottles at 0, 10, 20 and 30 m of water for 2
hours. DO concentration in the bottles was determined at the beginning and the end of exposure period
(APHA, 1998). Samples of phytoplankton were collected using a 10 µ open net and 53 µ plankton closing
net. Vertical plankton hauls were made at 100 m to the surface and preserved using 4 % formaline.
Identification was done by using X 40 Olympus microscope. Enumeration (total cell counts) was done by
making 10 random counts using a Sedge wick rafter counter.
Results
1. Physical and chemical parameters
- Temperature: During the sampling period, mean variation in temperature ranged between 21.1 and
25.8°C. The mean thermal stratification depth recorded during this period ranged between 70 and 90m.
This could be attributed to meteorological factors and subsequent shift in hydrodynamics of the Lake. For
example, on 22nd and 23rd July strong winds lashed the lake triggering a protracted two days mixing.
Consequently, the thermocline declined 90 m (Fig. 1). Those waves will be best remembered for the high
Figure 1: Physical parameters versus depth in Kigoma area
Temperature (C) VS Depth (m)
16/07/01
24/07/01
19/07/01
23.0 24.0 25.0 26.0 27.0 28.0
23.0 24.0 25.0 26.0 27.0 28.0
0
20
40
60
80
100
0
10
20
30
40
50
60
70
80
90
100
8h50
10h40
30/07/01
26/07/01
23.0 24.0 25.0 26.0 27.0 28.0
23.0 24.0 25.0 26.0 27.0 28.0
02/08/01
23.0 24.0 25.0 26.0 27.0 28.0
23.0 24.0 25.0 26.0 27.0 28.0
0
0
0
0
20
20
20
20
40
40
40
40
60
60
60
60
80
80
80
80
100
100
100
100
12h47
8h50
14h40
9h00
9h30
12h22
12h30
9h00
12h30
Turbidity (NTU) VS Depth (m)
16/07/01
0.0
0.5
1.0
1.5
24/07/01
19/07/01
2.0
2.5
0
0.0
0.5
1.0
1.5
2.0
40
60
80
100
10h40
0.5
1.0
1.5
2.0
2.5
0
10
20
30
40
50
60
70
80
90
100
0
10
20
30
40
50
60
70
80
90
100
20
0.0
2.5
8h50
0.0
0.5
1.0
1.5
2.0
8h50
0.0
2.5
0.5
1.0
1.5
2.0
2.5
0
10
20
30
40
50
60
70
80
90
100
0
10
20
30
40
50
60
70
80
90
100
12h47
02/08/01
30/07/01
26/07/01
14h40
9h00
0.0
0.5
1.0
1.5
2.0
2.5
0
10
20
30
40
50
60
70
80
90
100
9h30
12h22
12h30
9h00
12h30
Secchi Disc Transparency (m)
16/07/01
24/07/01
19/07/01
02/08/01
30/07/01
26/07/01
0.0
0.0
0.0
0.0
0.0
0.0
2.0
2.0
2.0
2.0
2.0
2.0
4.0
4.0
4.0
4.0
4.0
8 h3 0
6.0
6.0
6.0
8h30
8h50
12h47
14h30
8.0
6.0
9h00
8.0
8.0
10.0
10.0
10.0
10.0
12.0
12.0
12.0
12.0
14.0
14.0
14.0
14.0
12h22
8.0
4.0
6.0
9h30
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
55.0
60.0
65.0
70.0
75.0
80.0
85.0
90.0
95.0
100.0
105.0
24/07/01
19/07/01
8h30
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
55.0
60.0
65.0
70.0
75.0
80.0
85.0
90.0
95.0
100.0
105.0
8h50
12h47
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
55.0
60.0
65.0
70.0
75.0
80.0
85.0
90.0
95.0
100.0
105.0
10.0
10.0
12.0
12.0
14.0
14.0
30/07/01
26/07/01
8h30
14h30
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
55.0
60.0
65.0
70.0
75.0
80.0
85.0
90.0
95.0
100.0
105.0
9h00
12h22
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0 9h30
50.0 12h30
55.0
60.0
65.0
70.0
75.0
80.0
85.0
90.0
95.0
100.0
105.0
9h00
9.4
8.0
Euphotic zone at different times of the day
16/07/01
6.0
12h30
8.0
Figure 2: Basic water parameters vs depth in Kigoma Area
Dissolved Oxygen (mg/l) vs depth (m)
0.0
2.0
24/07/01
19/07/01
16/07/01
4.0
6.0
0.0
8.0
2.0
4.0
6.0
0.0
8.0
2.0
26/07/01
4.0
6.0
8.0
0.0
2.0
02/08/01
30/07/01
4.0
6.0
8.0
0.0
2.0
4.0
6.0
0.0
8.0
0
0
0
0
0
0
20
20
20
20
20
20
40
40
40
40
40
40
60
60
60
60
80
80
80
100
100
8h50
8h50
14h40
9h00
12h22
6.0
8.0
80
100
100
4.0
60
80
80
100
10h40
60
2.0
100
9h30
9h00
12h30
12h30
Conductivity (uS/cm) vs depth (m)
16/07/01
600
625
650
19/07/01
675
700
600
0
0
20
20
40
40
60
60
80
80
100
100
625
24/07/01
650
675
700
600
625
650
26/07/01
675
700
0
10
20
30
40
50
60
70
80
90
100
10h40
8h50
12h47
8h50
600
625
650
30/07/01
675
700
600
625
650
02/08/01
675
700
600
0
0
0
20
20
20
40
40
40
60
60
60
80
80
80
100
100
100
14h40
9h00
12h22
9h30
625
650
12h30
675
9h00
700
12h30
Alkalinity (mg/l CaCO3) vs depth (m)
200
225
250
24/07/01
19/07/01
16/07/01
275
300
200
0
0
20
20
40
40
60
60
80
80
100
100
225
250
275
300
200
225
250
26/07/01
275
300
0
10
20
30
40
50
60
70
80
90
100
10h40
8h50
8h50
12h47
200
225
250
30/07/01
275
300
200
225
250
02/08/01
275
300
200
0
0
0
20
20
20
40
40
40
60
60
60
80
80
80
100
100
100
14h40
9h00
12h22
9h30
225
250
12h30
275
9h00
300
12h30
Total Phosphorous (mg/l) vs depth (m)
16/07/01
19/07/01
0.0 0.2 0.4 0.6 0.8 1.0
0
20
20
40
40
60
60
80
80
100
100
8h50
10h40
0
10
20
30
40
50
60
70
80
90
100
0
10
20
30
40
50
60
70
80
90
100
12h47
8h50
14h40
02/08/01
30/07/01
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.2 0.4 0.6 0.8 1.0
0
26/07/01
24/07/01
9h00
0.0
0.0 0.2 0.4 0.6 0.8 1.0
5.0
10.0
0
10
20
30
40
50
60
70
80
90
100
0
10
20
30
40
50
60
70
80
90
100
12h22
9h30
9h00
12h30
12h30
Total inorganic Nitrogen (mg/l) vs depth (m)
16/07/01
0.0
0.1
0.2
19/07/01
0.0
0.3
0
0
20
20
40
40
60
60
80
80
100
100
0.1
24/07/01
0.2
0.3
0.0
0.1
0.2
8h50
0.0
0.3
0
10
20
30
40
50
60
70
80
90
100
10h40
30/07/01
26/07/01
0.1
0.2
0.3
0
10
20
30
40
50
60
70
80
90
100
8h50
12h47
0.0
0.1
0.2
02/08/01
14h40
9h00
0.0
0.3
0.1
0.2
0.3
0
10
20
30
40
50
60
70
80
90
100
0
10
20
30
40
50
60
70
80
90
100
12h22
9h30
12h30
9h00
12h30
Silica (mg/l) vs depth (m)
19/07/01
16/07/01
0.0
1.0
2.0
0.0
3.0
0
0
20
20
40
40
60
60
80
80
100
100
10h40
1.0
2.0
24/07/01
3.0
0.0
1.0
2.0
3.0
0
10
20
30
40
50
60
70
80
90
100
8h50
12h47
30/07/01
26/07/01
0.0
1.0
2.0
14h40
Figure 3: Vertical nutrients profile vs depth in Kigoma area
1.0
2.0
9h00
12h22
0.0
3.0
0
10
20
30
40
50
60
70
80
90
100
0
10
20
30
40
50
60
70
80
90
100
8h50
0.0
3.0
02/08/01
1.0
2.0
3.0
0
10
20
30
40
50
60
70
80
90
100
9h30
12h30
9h00
12h30
fish catches on 24th night of Lates stapersii. However, a sad reminder of the waves will be the nine lives
lost when “Fatuma’ boat en route to the D.R. Congo capsized. The mean surface temperature ranged
between 25.3 to 26.0o C and were close from 24.4o C at 100 m. This is typical of Lake Tanganyika (Plisnier
et al., 1999).
- Dissolved oxygen (D.O): surface dissolved oxygen level was 6.7 to 7.8 mg/L and decreased to near anoxia
at 100 m (0.3-1.7 mg/L). D.O levels are consistent in temperature with a oxycline depth of 60 m. The
oxycline depth varied between 50 and 70 m and was a function of the above factors.
- Transparency and light extinction: transparency recorded, using a Secchi Disc, fluctuated between 8.5 and
13.4 m. The highest water transparency was observed on 24th July a day after strong waves (Fig.1). The
corresponding euphotic zone deepened to 80 m suggesting very high water clarity (Fig.1) and this could
significantly influence the distribution of biotic community in the lake.
- Alkalinity and conductivity: the alkalinity values (Table 1) decreased with depth (range surface: 233.0293.0 mg/L CaCO3; range bottom: 229.0-282.0 mg/L CaCO3). This vertical profile is inversely compared
to electrical conductivity (Table 1) which ranged at surface between 611.0 and 661.0 µS/cm (range bottom:
633.0-682.0 µS/cm). The increasing of conductivity and the decreasing of alkalinity with depth is
considered typical of Lake Tanganyika (Kimirei and Nahimana, 2000; Plisnier, 1999).
- Nutrients: results are presented in table 1. The main nutrient measured were: reactive phosphorous (range
surface: 0.17-0.5 mg/L; range bottom: 0.27-0.70 mg/L), total inorganic nitrogen (range surface: 0.03-0.08
mg/L; range bottom: 0.04-0.21 mg/L) and silica (range surface: 0.5-1.5 mg/L; range bottom: 1.0-3.0 mg/L).
Those values are comparable to values obtained the last year (Kimirei and Nahimana, 2000).
2. Chlorophyll a and algal biomass
- Chlorophyll a values measured ranged between 0.41 and 2.10 mg.m-3. This is comparably to past values
(Table 2) obtained in this lake (Salonen et al., 1999; Haupert et al., 1998, Johannes et al., 1999 and
Baldwin, 2000). The correlation with physical parameters is weak, but there is a distinct positive
correlation between Chl a and fundamental nutrients (fig. 4). The vertical profile of Chl a is consistent with
findings in other lakes. The mean maximum Chl a value was at 10 m. This depth corresponds to the highest
mean value of nutrients (fig. 4) and is considered most favorable for phytoplankton development because
nutrients and light among other growth factors are within thresholds.
Algal biomass was consistently low and declined rapidly below 30-m depths. This was consistent with Chl
a and respiration rate measurements, and agrees with previous studies. However, on 24th July, the winds
induced mixing. There was also important light penetration.
- Net photosynthetic rates: The mean daily primary production measured was estimated to be 1mg O2
/m3/day. Because phytoplankton biomass observed was low (27.3 to 140.4 mg/m3) the rate of primary
production observed suggest high algal growth rates ranging from 0.1–4.6 mg/L O2m-3.d. –1.. Corresponding
vertical light extinction coefficients were low during most visits. The euphotic depth ranged between 30.3
m and 10l.5 m, the lastest being a very unusual observation. The phytoplankton growth rates observed has
been described as one of the highest for tropical fresh water lakes ( Sarvala et al., 1999).
3. Phytoplankton assemblages
During the survey, ten main families of various phytoplankton taxa were encountered. Composition
predominantly comprised algae belonging to Chlorophyceae (28.2%) and Bacillariophyceae (22% of the
total species taxa). Taxa (families) that attained of over 10% of phytoplanktonic assemblage were arbitrary
considered sub-dominant (Fig. 5). Abundance displayed a high degree of patchiness. Highest densities were
at between 0 to 30 m depth except on 24th July when shifts occurred on both the species predominance and
occurrence depth. Nitzschia spp, Surirella spp and Pediastrum spp. largely scant on the 16th and 19th
gained dominance almost to the total exclusion of earlier dominant species (Actinastrum spp., and
1
0.2
0.5
0.1
0
0
30
40
0.4
0.2
0
60
0
10
20
Depth (m)
Chla
PO4---
Chla
20
30
40
Chl a (mg/m3)
643
Cond.(uS/cm)
644
0.4
642
60
Chla
1.25
0.8
1.2
0.6
1.15
0.4
1.1
0.2
1.05
0
1
10
20
Cond.
Chl a (m g/m 3)
1.4
1.2
30
40
Depth (m)
Chla
SiO2
4
1
0.8
3
0.6
0.4
2
1
0
10
20
30
40
60
Figure 4: Chlorophyll a in
relation with soluble reactive
phosphorous, total inorganic
nitrogen, conductivity, silica,
and maximum net
photosynthesis.
Depth (m)
Chla
Net.P. Max
30
Figure 5: Relative abundance of
pelagic phytoplankton at Kigoma area.
25
16th July through 2nd August, 2001
20
15
10
5
Golenkiniaceae
Treubariaceae
Coelastraceae
S cenedesmaceae
Cyanophyceae
Hydrodictyaceae
Oocysticeae
Bacillariophyceae
0
Chlorophyceae
60
5
r =0.81
0.2
0
Taxa
1.3
1
0
0
1.35
r = 0.42
1.2
Depth (m)
Relative abundance (%)
Chl a (mg/m3)
645
0.9
10
60
Tot.I.N.
1.4
646
r = 0.06
0
40
Depth (m)
1.4
-0.1
30
SiO2 (mg/L)
20
0.8
0.6
Max. Net. Phot.
(m g O2.m -3.h-1)
10
1
r = 0.42
Dinophyceae
0
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Tot.I.N. (mg/L)
0.3
Chl a (mg/m3)
0.4
r = 0.70
PO4---(mg/L)
Chl a (mg/m3)
1.5
Gloeotrichia spp). Biomass analysis revealed a significant switch in algal abundance to between 70 and
100m depths on 24th July and was consistent to increased transparency, turbidity and nutrient levels. Upon
complete analysis of the samples a comprehensive species list will be compiled.
Conclusions
This preliminary results show the strong influences of meteorological parameters on the limnology and
consequently the biotic community of the lake. This observation justify the need to initiate a meteorological
and limnological monitoring program in the lake. Primary production studies are fundamental to lake
ecology and yet past research has been largely sporadic and gaps in data are important at Lake Tanganyika.
This necessitates that regular monitoring of the littoral and pelagic primary production is in place to fill gap
and tease out this complex and highly variable interactions.
Acknowledgements
We would like to sincerely thank our mentor, Dr. P.D. Plisnier, who provided limitless support, guidance
and encouragement throughout the project period. His experience, knowledge and organization is clearly
outstanding and many lessons have been learnt. Appreciation is also extended to the Limno- Teaching
Assistants, Willy Mbemba and Ismail Kimirei. They ensured that analytical detail and protocol is adhered
to, their continued support made the rigorous field work and laboratory analysis appear easy. The
TITANIC crew will be remembered for keeping time and bearing with us all through. Above all, we wish
to thank the Nyanza Project for giving us the opportunity to be part of the Nyanza “old boys” and for
recruiting highly competent scientists and resourceful lecturers: Dr. Michel, Dr. Lezzar, and Dr. Cohen.
Lake Tanganyika needs them.
References
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thermocline depth in Lake Tanganyika and its effect on nutrient levels in Kigoma Basin. Nyanza Project,
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