THE LIMNOLOGY of the LAKE TANGANYIKA SUB CATCHMENT HH Nkotagu and CB Athuman Geology Department, University of Dar Es Salaam, P.O
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THE LIMNOLOGY OF THE LAKE TANGANYIKA SUB CATCHMENT HH Nkotagu and CB Athuman Geology Department, University of Dar es Salaam, P.O. Box 35052, Dar es Salaam, TANZANIA. 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 Tabora, 16=Moyovosi the Ilagala Ferry, 3=Luiche River at the River at the Kagera-Kigoma Bridge, Kigoma-Kasulu 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 Burundi-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.