THE LIMNOLOGY OF THE SUB CATCHMENT HH Nkotagu and CB Athuman Geology Department, University of , P.O. Box 35052, Dar es Salaam, . ABSTRACT A limnological study covering the Lake Tanganyika sub catchment was conducted during the dry season at 20 accessible sites on 8 rivers, 2 lakes and a dam. Standard methods were used to determine the levels of abiotic parameters from water samples. Physical parameters including EC, Eh, turbidity, temperature, pH and secchi transparency were measured in situ while - 3- 2+ chlorophyll a was determined in the laboratory. Nutrients such as NO3 , SiO2, PO4 and Fe - were determined along with HCO3 . Significant changes in the levels of the abiotic parameters and nutrients have been observed at various sampling sites throughout the study area. The mean - 3- 2+ -1 variation of NO3 , SiO2, PO4 and Fe concentrations with depth ranged from 0.4 to 2.6 mg l , 2.7 to 35.3 mg l-1, 0.01 to 0.16 mg l-1 and from <0.010 to 0.020 mg l-1 respectively for the entire sub catchment. Data conclude that processes including dissolution, diffusion, adsorption, absorption, nitrification, denitrification, mixing and reduction along with the anthropogenic activities and increased photosynthetic activity of algae contribute to the variation of the abiotic parameters. It is recommended that quantification of river flows, sediment load and nutrient budget at various sampling points be determined seasonally for proper evaluation of the limnological functioning of the ecosystem.

INTRODUCTION little is known on the limnology of this The limnology of tropical rivers and lakes sub-catchment is known. along with dams has recently received a paramount and growing attention due to Present information available on the pressures from land, atmospheric, and limnology of the Lake Tanganyika riverine inputs. The Lake Tanganyika sub catchment is only the temporal abiotic catchment includes the Malagarasi determinations conducted by Langenberg et Moyowosi wetland ecosystem in the al. (2003). This study focused at the Western Tanzania, a polymictic freshwater confluence point of the Malagarasi River ecosystem designated as first Tanzanian among others with the Lake Tanganyika at Ramsar site and the world’s 1024th Ramsar the Delta and concluded that, the external site since 2000 (Nkotagu and Ndaro 2004). loading forms the main pathway for It is estimated that during the last ten years nutrients entering productive layers of the the depth of the wetland has systematically Lake Tanganyika during stratified and been reduced to a maximum of about 2 m. oligotrophic periods. The wetland is located in western Tanzania The present study aimed at identifying about 200 km NE of Lake Tanganyika (Fig. causes of changes in abiotic parameters from 1) The wetland is characterized with various surface water bodies including biodiversity including varieties of flora and rivers, lakes and a dam in the Lake fauna such as fish of which 50 species are Tanganyika sub catchment as observed at known with some being endemic, various accessible points. crocodiles, hippopotamus and other microorganisms (Nkotagu and Ndaro 2004). MATERIALS AND METHODS Study area These rivers and swamps support the life of The study was conducted for six weeks many people and other organisms within the during the dry season commencing Lake Tanganyika sub catchment and hence September to October 2004. The study form a unique ecosystem. However, very covered the rivers Igombe, Lugufu, Luiche, Nkotagu and Athuman – The limnology of Lake Tanganyika sub cathment

Makere, Malagarasi, Moyowosi, Ruchugi, 9=Malagarasi River at the Uvinza-Nguruka and Ugalla together with lakes Sagara and Bridge, 10=Lake Sagara at Amerika, Nyamagoma as well as the Igombe Dam 11=Ugalla River at Katumba, 12=Moyovosi within the eastern side of the Lake River mouth at the Lake Nyamagoma, Tanganyika catchment at the following sites 13=Igombe River at Mtega in Kaliua, 1 to 20 as shown on figure1. 1=Malagarasi 14=Igombe River at Uyowa in Urambo, River at the Delta, 2=Malagarasi River at 15=Igombe Dam in , 16=Moyovosi the Ilagala Ferry, 3=Luiche River at the River at the Kagera- Bridge, Kigoma- Bridge, 4=Luiche River at 17=Malagarasi River at the Kasulu-Kibondo the Delta in Ujiji, 5=Lugufu River at Bridge, 18=Makere River at the Bridge,19= Jackobsen's Farm, 6=Ruchugi River at the Malagarasi River at the -Tanzania Kigoma-Uvinza Bridge, 7=Malagarasi River Border and 20= Malagarasi River at the at the Nyanza Salt Mine, 8=Malagarasi Igamba Water fall. River at the Uvinza-Mpanda Bridge,

Figure 1: The Lake Tanganyika sub catchment showing the sampling sites

Sampling was conducted in a horizontal transect across Sampling sites were initially decided basing the river at three positions including the on water depths as measured at various centre, right and left banks. positions on the rivers, lakes and the dam using a SCUBA device. Water samples were Aliquots of 400 ml and 800 ml of each collected by a 2-l water sampler at selected water sample were filtered using pore size depth intervals up to maximum depths in glass fibre 47 mm size filter papers. The half and one-litre plastic bottles filled to the filtrate aliquots were transported on ice to brim. However, at some rivers, sampling the laboratory for Silica (SiO2), Nitrate 20 Tanz. J. Sci. Vol 33 2007

- 3- 2+ (NO3 ), Phosphate (PO4 ) and Iron (Fe ) Multi probe meter 340i model. In addition, determinations. The remaining unfiltered turbidity and water transparency were also water samples were similarly transported to measured using a HACH turbidimeter the laboratory for alkalinity determination. 2100P model and a 20 cm diameter Secchi disk, respectively. Filters were placed into a test tube on to which 10 ml of 90 % ethanol were added. Laboratory work The test tubes were then wrapped with The filtered water samples were stored at 4 aluminium foil, marked and stored °C before analysis. Fluorescence readings for overnight in a cooler at 4 °C ready for chlorophyll a were taken using a chlorophyll a readings the following day. Fluorometer before and after acidifying the sample residues with 0.1 N HCl. The In situ physical parameters including absorbance readings were then converted into electrical conductivity (EC), dissolved concentrations as chlorophyll a using the oxygen (DO), pH and temperature were formula according to McIntyre (2004 measured at each sampling site using a unpublished data):

-1 Chl a (µg l ) = 0.003 x (Fbefore acid - Fafter acid) x Ethanol Extraction Volume (ml)/Volume of Filtered Water (L) where: F is the Fluorescence of the sample, which is equivalent to the absorbance on a Spectrophotometer and Chl a is Chlorophyll a in µg l-1.

Unfiltered water samples were tested for the catchment such as agriculture and alkalinity using a titrimetric method with pastoralism. Chlorophyll a decreased with - 0.1 N HCl and results expressed as HCO3 depth suggesting that turbidity is primarily (mg l-1) as explained by APHA et al. (1998). a function of increased suspended sediments - 3- Nutrients including SiO2, NO3 , PO4 and and bioturbation of clayed riverbed. The low Fe2+ were determined from the filtered water secchi transparency measured at the Kasulu- samples using a HACH Spectrophotometer Kibondo Bridge along the Malagarasi River DR/2010 model according to HACH (2002). as compared to the rest of the sampling sites supports the observed turbidity profile. RESULTS AND DISCUSSION Turbidity The mean variation of chlorophyll a with The mean turbidity values ranged from 2.1 turbidity shows a strong positive correlation to 23.5 NTU throughout the sampling sites (Fig. 2). This indicates that the observed and increased with depth at the sites trend of turbidity at some sites is a attributing to increased suspended sediments consequence of increased phytoplankton in water due to anthropogenic activities in abundance complementing the sediments surface and decreased with depth. This may The reciprocal relationship between be attributed to increased photosynthetic chlorophyll a and turbidity was observed at activities of algae at the water surface. the Malagarasi River Delta implying that However, the low oxygen levels recorded at turbidity at this site is mainly a function of the bottom at these sites according to Wetzel suspended sediments. (2001) may be attributed to oxidation of Dissolved Oxygen organic matter especially at the sediment- The mean concentration of dissolved oxygen water interface. According to Horne and varied from 0.02 to 9.16 mg l-1 as measured Goldman (1994) the organic matter may at the sampling sites. Higher levels of have originated from non point sources, dissolved oxygen were recorded at the water such as dead leaves and animal excreta.

21 Nkotagu and Athuman – The limnology of Lake Tanganyika sub cathment

Malagarasi River at the Uvinza-Mpanda Bridge 1.6

1.5 )

-1 y = 0.1744x + 0.4327 2

g l R = 0.7383

! 1.4 ( a 1.3

1.2

Chlorophyll 1.1

1 4 4.5 5 5.5 6 6.5 Turbidity (NTU)

Figure 2: Mean variation of chlorophyll a (µg l-1) with turbidity (NTU) at the Uvinza- Mpanda Bridge

The mean variation of dissolved oxygen the presence of increased photosynthetic concentration increases with chlorophyll a at algae activity as summarized in the most sampling sites and are both strongly equation: correlated as shown on the Malagarasi River at Delta (Fig. 3). This observation supports Light Energy + 6 CO2 + 6 H2O _ C6H12O6 + 6 O2

Malagarasi River at the Delta

0.022

) 0.02 -1

g l y = 0.0209x - 0.042 ! 0.018 2

( R = 0.6846 a 0.016

0.014

Chlorophyll 0.012

0.01 2.5 2.6 2.7 2.8 2.9 3 Dissolved Oxygen (mg l -1)

Figure 3: Mean variation of chlorophyll a (µg l-1) with dissolved oxygen (mg l-1) at Malagarasi River Delta

22 Tanz. J. Sci. Vol 33 2007

The orthorgrade oxygen profiles were distribution of heat from the surface to the observed on the Malagarasi River e.g. riverbed. Igamba Water Fall. This is probably due to the geomorphology of the river governed by Electrical Conductivity the presence of high waterfall gradient of Electrical conductivity values ranged from about 20 m. The pH values ranged from 32.0 to 544.0 µS cm-1. The highest 6.34 to 9.0 at the sampling sites and measured salinity may be attributed to the increased with increasing depth inferring to observed brine leakages into the river as increased photosynthetic activity with depth artisanal salt works are noted to be operating thus supporting the observed oxygen profile. about a kilometre upstream of the bridge at Uvinza. In addition, the pore-surface water The observed pH values are supported by mixing effects could also be attributing to - - values of HCO3 (18.3 to 140.3 mg l ) that high salinity values. increased with depth. This is probably due to increasing CO2 consumption with depth. However, salinity increased with depth at Temperature some sites attributing to increased dissolved The mean temperature variation with depth ions at higher depths promoted by ranged from 20.23 to 27.28 °C with the favourable redox conditions. Furthermore, minimum temperature observed on the the salinity of water decreased with depth at Lugufu River at Jackobsen’s Farm and the the sampling sites as a consequence of maximum observed on Lake Sagara at increased autotrophs that take up ions for Amerika. These observations may be use in photosynthesis at low light levels as consequent to presence of dense vegetation indicated by low secchi transparency at these cover and openness that provides and denies sites. Similarly according to Wetzel and shade to the water surface respectively. Likens (1990), the adsorption of ions on the sediments may also significantly contribute The temperature-decreasing trend with depth to the decreased salinity with depth. was recorded at the sampling sites e.g. Igamba Water Fall. Horne and Goldman Silica (1994) and Cohen (2003) explain this The mean variation of silica concentration phenomenon as a consequence of decreasing with depth ranges from 2.7 to 35.3 mg l-1 light intensity with depth from the water for the entire sub-catchment at the sampling surface. sites and decreases with depth at most sites. The decrease of silica with depth at these However, an increasing trend of the mean sites may be attributed to increased primary temperature with depth from water surface productivity as diatoms use silica for was observed on the Malagarasi River at the building up their cell walls. Kasulu-Kibondo and at the Nyanza Salt Mine. This is consequent to terrestrial heat The observed decrease of silica is supported resulting from absorption of significant solar by the strong positive correlation between radiation by sediments and slowly releasing silica and chlorophyll a as shown at the it to the overlying water column. Igamba Water Fall (Fig. 4). This implies that the observed silica concentration may At very shallow depth sites such as Makere predominantly be biogenic at these sites. River at the Kasulu-Kibondo Bridge, temperature was more-or-less the same However, at some sites, the mean throughout the water column. This is due to concentration of silica shows a weak the fact that solar radiation penetrates down negative correlation with chlorophyll a to the riverbed causing a uniform implying that at these sites silica

23 Nkotagu and Athuman – The limnology of Lake Tanganyika sub cathment

concentration may essentially be inorganic as observed at the Malagarasi River Delta.

Malagarasi River at the Igamba Water Fall

2.2

) 2 -1 y = 0.1856x - 1.2391 g l

! 1.8 2

( R = 0.5992 a 1.6

1.4

Chlorophyll 1.2

1 13.5 14 14.5 15 15.5 16 16.5 17

-1 SiO2 (mg l )

-1 -1 Figure 4: Mean variation of chlorophyll a (µg l ) with SiO2 (mg l ) for Malagarasi River at Igamba Water Fall

Nitrate At sites including the Malagarasi River The mean variation of nitrate concentration Delta, Uvinza-Nguruka Bridge and Kasulu- with depth ranged from 0.4 to 2.6 mg l-1. Kibondo Bridge along with the Igombe Higher concentrations determined suggest Dam in Tabora nitrate concentration the wide use of fertilizers on the farms along decreased with depth as demonstrated on the lake, dam and riverbanks. In addition figure 5. This may, according to Appelo and increased pastoralism may attribute to the Postma (1994), be attributed to observed levels of nitrate concentration. decomposition of organic matter or increased ferrous iron as shown in the following chemical reaction:

2+ - + 10 Fe + 2 NO3 + 14 H2O & 10 FeOOH + N2 + 18 H resulting in the promotion of denitrification process.

However, at most sites the mean bioturbation process and bottom currents at concentration of nitrate was observed to these sites which promote nitrate diffusion increase with depth. This indicates the process from the sediment pore waters into sediment-water interface disturbance through the overlying water column.

24 Tanz. J. Sci. Vol 33 2007

- -1 NO3 (mg l )

2.8 1.6 2.6 1.4 2.4 2.2 1.2 2 1

1.8 Bridge Tabora 1.6 0.8 Igombe Dam in

1.4 Malagarasi River at 0.6 the Kasulu-Kibondo 1.2 1 0.4 0 1 2 3 4 Depth (m) IGDM MAB-KS-KB

- -1 Figure 5: Mean variation of NO3 (mg l ) with depth (m) at the Igombe Dam in Tabora and Malagarasi River at the Kasulu-Kibondo Bridge

Phosphate Mtega in Kaliua and in Lake Sagara at The mean phosphate concentration varied Amerika. High values of phosphate may be with depth and ranges from 0.01 to 0.16 mg inferred to increased anthropogenic activities l-1 at the sampling sites with the highest within the sub catchment. values at the Igombe River as determined at

Malagarasi River at the Uvinza-Nguruka Bridge 0.08 0.025

0.07 0.02 ) ) -1 0.06 -1 0.015

(mg l 0.05 (mg l 3- 4 0.01 2+ 0.04 Fe PO

0.03 0.005

0.02 0 0 0.5 1 1.5 Depth (m) PO4 Fe2+

3- -1 2+ -1 Figure 6: Mean variation of PO4 (mg l ) with Fe (mg l ) at the Uvinza-Nguruka Bridge

25 Nkotagu and Athuman – The limnology of Lake Tanganyika sub cathment

ACKNOWLEDGEMENTS At most sites the mean concentration of The Royal Geographical Society (UK) and phosphate decreased with increasing depth the US National Geographical Society from the water surface. This is attributed to (NGS) funded this work. We are grateful to adsorption of phosphate on to sediments the University of Dar es Salaam and the enriched with ferric oxides and or ferrous Ministry of Natural Resources and Tourism hydroxide depending on the field stability for the permission to conduct this work. The conditions prevailing there. Kigoma and Tabora regions authorities are thanked for their support. The Tanzania However, the mean concentrations of ferrous Fisheries Research Institute (TAFIRI), the 2+ 3- (Fe ) iron and phosphate (PO4 ) were noted Nyanza Project and the support staff are to increase with depth at some sites greatly acknowledged for their assistance and including the Uvinza-Nguruka Bridge (Fig. cooperation. 6). This is possibly due to decomplexation of iron phosphate complex through REFERENCES reduction process. APHA, AWWA and WEF 1998 Standard Methods for Examination of Water and According to Wetzel (2001), these Wastewater. 20th ed. American Public observations may be attributed to reducing Health Association. Washington, DC. conditions. Redox potential and pH Appelo CAJ and Postma D 1994 correlated negatively thus supporting the Geochemistry, groundwater and field stability of ferrous (Fe2+) iron. pollution. 2nd ed. A.A. Balkema. Rotterdam. The Netherlands. 536pp. CONCLUSIONS Cohen AS 2003 Paleolimnology: The The data from the present study show History and Evolution of Lake Systems. significant changes in concentrations of Oxford University Press. Inc. New York. limnological parameters among the 20 500pp. sampled sites within the Lake Tanganyika HACH Company 2002 Water Analysis sub catchment. The depths and widths of Handbook. 4th ed. Loveland. Colorado. rivers, lakes and the dam vary from one USA. 1260pp. point to another due to anthropogenic Horne AJ and Goldman CR 1994 activities which result in high siltation and Limnology. 2nd ed. McGraw-Hill. Inc. erosion. USA. 576pp. Langenberg VT, Nyamushashu S, From these data we conclude also that, Roijackers R and Koelmans AA 2003 several processes such as dissolution, External Nutrient Sources for Lake diffusion, nitrification, denitrification, Tanganyika. Internat. Assoc. Great reduction, absorption and adsorption along Lakes Res., J.Great Lakes Res. 29 (2): with mixing control the levels of abiotic 169-180. parameters in the entire sub catchment. Nkotagu HH and Ndaro SGM (eds.) 2004 Future research work is recommended to The Malagarasi Wetland Ecosystem. Dar focus on the quantification of flow, es Salaam University Press. 141pp. sediment load and nutrient budget at various Wetzel RG and Likens GE 1990 points along the rivers, lakes and the dam in Limnological Analyses. 2nd ed. Springer- both dry and rain seasons in order to fully Verlag. New York. 391pp. understand the limnological functioning of Wetzel RG 2001 Limnology: Lake and the Lake Tanganyika sub catchment. River Ecosystems. 3rd ed. Academic Press. 1006pp.

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