An inquiry into suspended matter in the Luiche and Malagarasi Rivers:
Provenance and pathways to Lake Tanganyika, East Africa
Student: Fidelis Makililo
Mentor: Dr Catherine O’Reilly
Introduction
Lake Tanganyika is the world’s second largest lake and second deepest lake, containing 18% of the world’s
total fresh water. It belongs to the western branch of the East Africa Rift System. It is also one of the oldest
lakes, with an estimated age of 9-12Ma (Cohen, 1993). The Lake Tanganyika drainage basin includes the
Luiche and Malagarasi wetlands. It covers most of the northwest central and southwest part of Tanzanian
highlands. Malagarasi and Luiche rivers have a major share of the in flowing waters to the lake. Similarly
they obviously contribute most of the suspended matter load to the lake. There is little information about
actual measured suspended matter, turbidity and chlorophyll-a concentration in the incoming rivers. In this
study different limnological parameters were sampled, analyzed and related to environment parameters.
These parameters were turbidity, chlorophyll-a and organic matter concentrations.
Study areas, Materials and Methodology
The study areas were the Malagarasi wetland and its associates (Moyowosi and Lake Sagara), and the
Luiche river and its tributaries (Fig.1). Samples from Luiche river and its tributaries (Dove, Mngonya,
Balenzi, and Kaseke) were taken from surface water. Samples from Lake Sagara and the Moyowosi
wetland were taken at depths of 0m, 1m, and 2m in order to investigate chlorophyll-a and turbidity vertical
change. Water sampling at the Luiche River and its tributaries was done on 15th July 2002 at different
bridges located along the Kigoma-Kasulu road. Samples were taken from the Dove, Mngonya, Kaseke,
Luiche pumping station, and Balenzi rivers with GPS locations at S 04.84068° E 029.65589°, S 04.83907°
E 029.66663°, S 04.853°, 04 E 029.73323°, S 04.86809° E 029.74029° and S 04.85121° E 029.70903°
respectively. The water sampling at the Malagarasi wetland (Moyowosi and Sagara) was done on 23rd July
2002. The sampling points were accessed by man-powered canoes with GPS location at S 4o58.323" E
31o06.000" for Moyowosi and S 5o12.598" E 31o03.000" and S 5o11.630" E 31o03.904" at Lake Sagara.
Water samples from the Malagarasi River were collected at different bridges located along the UvinzaNguruka road on 24th July 2002. We sampled at the Malagarasi River at the Railway bridge, Ruchugi River
and the Malagarasi upstream and downstream from Uvinza at Uvinza ward, with GPS locations at S
5o58.323" E 30o49.433"S 5o05.784" E 30o23.147"S 5o06.473" E 30o23.99" and S 5o06.923" E 30o22.816°
respectively. Surface water samples from each sampling point were taken by using a 2L standard Van
Dorne water sampler. Three liters of unfiltered water sample per each location were taken for turbidity,
chlorophyll-a and organic matter determination.
Turbidity was measured using a turbidity meter (Hach) and results were expressed as nephelometric turbid
units (NTU). Chlorophyll-a was measured by filtering 120-1000 ml of sample water through a Whatman
GF/C filter. The filter paper was kept in tube and 10ml of 90% acetone was added. It was then left in a light
proof container in the refrigerator for overnight extraction. The filter paper was removed, the liquid
centrifuged and the absorbance reading taken using a Spectronic Spectrophotometer at 665, 750, 480nm
wavelengths respectively.
The concentration of Chlorophyll-a(ug/l)were determined using the equation:
[chlorophyll-a] ug/l=(O.D.665-O.D.750)*{(13.9*v)/(V*l)}
where:
O.D.665=optical density (absorbance) at 665nm,a distinctive peak for chlorophyll-a
O.D.750=optical density at 750nm,a correction for any background turbidity
v =the volume of the extract in ml
V =the volume of water filtered in litres
l=the path-length of the cuvettes used in cm
13.9=an absorption coefficient (a constant) for chlorophyll-a
For organic matter, 120-1000ml water samples were filtered to obtain filtrate on Whatman filter paper.
Samples were prepared for analysis by drying in an oven for a minimum of 18 hours at 70 °C to remove
water. Then the dry filtrate, which was a mixture of organic matter and solids, was weighed. Sample weight
(W4) was determined using an analytical balance. Samples were transferred into a Thermolyne 1400
muffle furnace and burned at 550° for two hours to remove organic matter. After cooling in a dessicator,
samples were re-weighed (W7). The organic matter percentages (%) were determined as follows:
%OM=100*(W4-W7)/(W4)
Results
The results are summarized and presented in Table 1 and Figures 2 – 4. The highest turbidity from Luiche
River and its tributaries was 8.93 NTU, which was measured near Luiche Railway Bridge. Turbidity
ranged between 1.7 NTU and 8.93 NTU, and decreased as you go upstream from the Luiche River Railway
Bridge. The highest turbidity for Malagarasi River, Moyowosi wetland and Lake Sagara was 34.9 NTU,
which was measured at 2m depth at Lake Sagara. Turbidity ranged between 1.16 NTU to 34.9 NTU. The
highest concentration of chlorophyll-a was observed at Lake Sagara on surface water and was 0.1056µg/L,
and the lowest 0.0007ug/l was measured at the Moyowosi wetland. The percentage organic matter
obtained from the Luiche River and its tributaries ranged from 42.88% OM to 96.77% OM, while those for
the Malagarasi River, Moyowosi wetland and Lake Sagara ranged from 21.70% OM to 77.03% OM .
Discussion
Organic Matter – The percent organic matter obtained from the Luiche River and its tributaries (Table 1)
showed that high percentages of organic matter were obtained from Kaseke stream. This high value may
be a result of a decrease in clastic dilution and the presence of vegetation of either side of this stream.
There was no direct link between organic matter percentage and turbidity levels from different sites.
Turbidity – Turbidity was high 34.9 NTU in Lake Sagara compared to the other value obtained from other
sites. Lake Sagara is large, shallow and open lake. Therefore, this lake is exposed to strong wind
movement which lead to frequent mixing. Second, the lake has a sufficiently high concentration of
particulate organic matter. The turbidity at the water surface measured 24.3 NTU, at 1m depth it was
measured 2.73 NTU (Fig. 4), and at 2m depth it was measured 34.9 NTU. Turbidity at the water surface
was high due to presence of chlorophyll-a, and the decrease of turbidity at 1m depth was due to lower
chlorophyll-a concentration. Turbidity increased at 2m depth; this was due to re-suspended bottom
sediments. In comparison, at Moyowosi the situation was very different because there are small islands of
aquatic plants. These islands act as wind barriers that protect the water from strong waves. For the turbidity
measurements taken from the Luiche river and its tributaries, the results show that turbidity was increasing
downstream. This increase in suspended matter may be due to input from the many small streams which
drain from different areas and are affected by scattered lowland agriculture and livestock grazing. In
general there was less difference in turbidity levels at the four Malagarasi River sites (Malagarasi R
downstream , Ruchugi river, Malagarasi Uvinza upstream and Malagarasi bridge). Large differences in
turbidity occurred between Moyowosi and Lake Sagara.
Chlorophyll-a – There were high concentrations of chlorophyll-a in Lake Sagara compared to other sites.
This may be due to the fact that it is an open lake and therefore it gets enough sunlight The chlorophyll-a
was decreasing deeper in the water column, this was most likely a result from a decrease in sunlight,
nutrients, and increase in turbidity from suspended sediments. There were slight differences in chlorophyll
concentration at the different sites. Turbidity and chlorophyll concentration from these sites were not
directly related. The mixing caused by surface wind movement, amount of suspended sediments, and water
speed may be a controlling factor for chlorophyll-a and turbidity levels.
Lake Sagara was quite different from the other sites. Turbidity increased down stream in Luiche river and
its tributaries .The important role of turbidity in the aquatic ecosystem is that turbidity dictates the
transparency of the water, limiting the light transmission within it, and in this aspect it relates to
productivity and energy flow within the community. Chlorophyll–a content of algae would bear a relation
to the total organic material produced by algae, and thus the chlorophyll concentration in water would serve
as an index of productivity. Chlorophyll production and content in algae varies considerably with light,
nutrients, temperature and other factors. However, because the data collection in this study was limited,
more field and lab work should be carried out to determine the effect of turbidity and chlorophyll-a on the
aquatic ecosystem. Future work should include the following: sampling should occur at the same time
every day to avoid daily fluctuation, sample replication, and sampling over a longer period of time in order
to record the changes in chlorophyll-a and changes in weather, and make a better comparison.
Acknowledgements
I would like to thank the following: My mentor Dr Catherine O’Reilly provided endless support and
encouragement throughout the entire project. She and Willy Mnemba guided me through analysis of all
samples collected. Prof. Andrew S Cohen for his co-ordination of the Nyanza project. Dr Kiram Lezzar
and Dr Ellinor Michel for their nice lectures. Dr H Nkotagu for his Malagarasi field guidance. Willy
Mbemba, Richard Hartwell (RJ), Kamina Chororoka and Alwiya Shariff for there great help with
technical details. Dada Consolata Ngonyani and my friend Emanuel William for their help in field and lab
work. Above all I wish to thank the US National Science Foundation Grant #ATM 9619458 (the Nyanza
project), and the WWF for financial support of this project.
References
Cohen, A., M. Soreghan, and C. Scholz. 1993. Estimating the age of formation of lakes: An example from Lake Tanganyika, East
African Rift system. Geology 21:511-514.
Reid, G.K.. Ecology of Inland waters and estuaries 105-106:329-330.
Figure 1: Map of watersheds with sampling
locations (black circles).
Turbidity Chlorophyll % organic
(NTU)
(ug/L)
matter
Sample site
Luiche
Watershed
Dove
2.25
0.0042
43%
Mngonya
2.94
0.0017
66%
Kaseke
1.7
0.0010
97%
Luiche
2
0.0020
45%
Balenzi
3
0.0053
79%
8.93
0.0000
Sagara 0m
24.3
0.105640
43%
Sagara 1m
2.73
0.094520
58%
Sagara 2m
34.9
0.054210
77%
Sagara n. shore
13.4
0.013900
74%
Malagarasi Bridge
Upstream from Uvinza
surface
6.52
0.001807
22%
5.16
0.000973
35%
Moyowosi-20D
Malagarasi
R.downstream
1.37
0.001112
51%
5.79
0.001390
58%
Moyowosi-21
1.25
0.000618
40%
Ruchugi R.
7.42
0.001251
75%
Moyowosi-20
1.16
0.001251
76%
Luiche rw bridge
Malagarasi Watershed
0.000695
Moyowosi-20B
Table 1: Results for the Luiche and Malagarasi watersheds.
0.12
0.006
0.10
0.005
0.08
0.004
0.06
0.003
0.04
0.002
0.02
0.001
0.00
0.000
0
10
20
30
40
turbidity (NTU)
Figure 2: Chlorophyll and turbidity
Luiche chlorophyll (ug/L)
Malagarasi chlroophyll a (ug/L)
Malagarasi
Luiche
Malagarasi
Luiche
0.8
0.6
0.4
0.2
0.0
0
10
20
30
40
turbidity (NTU)
Figure 3: Organic matter and turbidity
% OM
0.4
0.6
0.8
1.0
Turbidity (NTU)
0
20
40
0
depth (m)
Organic matter
1.0
Turbditiy
OM
chlorophyll
1
2
0.05
0.10
Chlorophyll (ug/L)
Figure 4: Depth profiles for Lake Sagara