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Limnological Studies of the Pelagic Zone of Lake Tanganyika at Kigoma, Tanzania

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Limnological Studies of the Pelagic Zone of Lake Tanganyika at Kigoma, Tanzania LIMNOLOGICAL STUDIES OF THE PELAGIC ZONE OF LAKE TANGANYIKA AT KIGOMA, TANZANIA. FMM Chale Faculty of Science, Technology and Environmental Studies, The Open University of Tanzania, P.O. Box 23409, Dar es Salaam,Tanzania. [email protected] ABSTRACT Some limnological parameters and chlorophyll a were determined for twelve months at a pelagic station near Kigoma. Temperatures ranged between 25.7 ºC and 27.3 ºC at the surface, and 24.1 oC and 24.7 oC at100 m. Thermal stratification persisted throughout the year with the thermocline oscillating between 15 m in October and 60 m in May. Dissolved oxygen concentrations were high at the surface, ranging between 6.59 mg l-1 and 8.15 mg l-1, while at 100 m the concentrations ranged from less than 0.20 mg l-1 to 1.94 mg l-1. Nitrate-nitrogen concentrations in surface water ranged between 26 !g N l-1 in May and 74 !g N l-1in December. Nitrate -nitrogen concentrations generally increased with depth to 80 m, after which they decreased. Soluble reactive phosphorus concentrations in the photosynthetic zone were always very low, ranging between 0.30 !g P l-1 and 4.0 !g P l-1. The concentrations increased with depth. Total phosphorus concentrations in the surface water were also low, ranging between 2.0 !g P l-1 in March and 12.0 !g P l-1 in August. The concentrations increased with depth. Chlorophyll a concentrations were generally low throughout the year, except for October and April-May when high concentrations were found. In October the mean concentration was 4.52 !g Chl a l-1, while January had the lowest concentrations with a mean of 0.36 !g Chl a l-1. Chlorophyll a concentrations decreased with depth. INTRODUCTION May and September, causing upwelling in Lake Tanganyika is the deepest lake in the southern end of the lake (Coulter 163, Africa and contains the second largest Hecky op. cit., Coulter & Spigel op. cit.). volume of anoxic water after the Black Sea However, rainfall is reported to contribute (Hecky 1991). The lake lies between 30 31' more than 50 % of total dissolved fixed and 80 50' S and 290 05' and 310 15' E nitrogen in the lake (Edmond et al. 1993; (Plisner et al. 1999). It is about 650 km Hecky op. cit.). Hecky and Kling (1981) long, with an average width of 50 km, lying also reported that nitrogen fixation by at an elevation of 773 m asl. (Edmond et al. Anabaena spp. is likely to be a very 1993). important process in the lake. In the mixolimnion, the water does not mix Chale (2004) reported minimum secchi disc completely but there is a stable stratification readings and very high chlorophyll a levels in the pelagic zone (Coulter 1963). In the in the offshore water around Kigoma in northern basin around Kigoma, vertical October. The low secchi disc visibility and mixing is confined above the thermocline at high chlorophyll a had been shown to be a 50 to 80 m (Coulter & Spigel 1991). It has result of Anabaena spp. blooms which take been shown that in Lake Tanganyika, most place during that period (Hecky and Fee of the nutrients are found in the deep 1981, Plisnier et al. 1999). monimolimnion (Hecky et al. 1991) and their upward transport occurs mainly during Phytoplankton biomass expressed as the dry season when maximum circulation chlorophyll a has been reported to be takes place (Hecky 1991). Strong south generally low (Hecky and Kling 1981), with easterly winds blow over the lake between the exception of Octber-November when Chale – Limnological studies of the pelagic zone of Lake Tanganyika … very high concentrations have been found Sample treatments for nitrate-nitrogen, (Chale 2004). These high levels have been soluble reactive phosphorus and chlorophyll attributed to the heterocystous cyanobacteria a were carried out as described in Chale Anabeana spp. which bloom during that (2004). period (Hecky and Fee 1981, Salonen et al. 1999, Hecky 1991). All sample analyses were carried out in triplicate. Kurki et al. (1999) reported cyclopoid copepods to dominate the zooplankton in RESULTS the pelagic zone throughout the year with Surface water temperatures were always peaks in October-November and at the end high, ranging between 25.7 oC in August of the wet season in April and May. The and 27.3 oC in February (Fig. 1). Bottom zooplankton could have a negative effect on water temperatures did not differ much the phytoplankton due to grazing. throughout the study period, with values between 24.1 oC in October and 24.7 oC in In the following, profiles of monthly April and May. Thermal stratification was limnological data and chlorophyll a in the observed throughout the year, though the offshore water of L. Tanganyika near depths at which the thermocline occurred Kigoma are presented. The study was differed from month to month. conducted between July, 1998 and June, 1999. Dissolved oxygen concentrations in the surface water ranged between 6.59 mg l-1 MATERIALS AND METHODS and 8.15 mg l-1 in August and April, The study was carried out at an offshore respectively (Fig. 2). There was always station about 5 km from shore where the oxygen at 100 m, though the concentrations depth was more than 500 m (Chitamwebwa differed during the different months. In 1999). Sampling was conducted in the November, April and May, the oxygen mixolimnion to the depth of 100 m. In concentrations were 1.94 mg l-1, 1.07 mg l-1 order to minimize changes in the water and 1.60 mg l-1, respectively at 100 m. Low characteristics, sampling was always carried oxygen was found in October from the depth out between 9:30 am an 10:30 am. of 20 m. At 20 m the concentration was only 3.03 mg l-1. Water temperature and dissolved oxygen were determined at 5 m intervals using a Nitrate-nitrogen concentrations in surface Yellow Springs Incorporated, Model 50B water ranged between 26 !g l-1 in May and Dissolved Oxygen Meter with a cord length 66 !g l-1 in December (Fig. 3). From the of 80 m. Temperature and dissolved oxygen surface to 40 m, NO3-N concentrations values between 85 m and 100 m depths were generally increased slightly. But after 40 m obtained on samples brought on deck the steep increases to 80 m were observed. For vessel. example, the mean NO3-N concentration was 61 !g N l-1, while at 60 m it was 118 !g N Samples nutrients and chlorophyll a were l-1 and at 80 m it was 218 !g N l-1. After 80 collected at 20 m intervals using a 7.4 litre m, the concentrations generally decreased. capacity (Limnos, Finland) water sampler. 24 Tanz. J. Sci. Vol 32(2) 2006 Figure 1: Monthly Temperature (oC) Profiles at Various Depths. Soluble reactive phosphorus (SRP) Chlorophyll a concentrations are presented concentrations in the euphotic zone were in Fig. 5. In July, February and March, peak usually very low (Fig. 4). The lowest values levels were obtained at 40 m. October had were obtained in August, December, January the highest levels throughout the water and March, when the concentrations were column with a mean concentration of 4.52 close to the limit of detection (0.25 !g P l-1 !g Chl a l-1 (range 3.2 and 5.11 !g Chl a l- with a 4 cm cell). The mean SRP in the 1). euphotic zone was 1.75 !g P l-1. 25 Chale – Limnological studies of the pelagic zone of Lake Tanganyika … Figure 2: Monthly Dissolved Oxygen Concentrations (mg l-1) with Depth. 26 Tanz. J. Sci. Vol 32(2) 2006 Figure 3: Nitrate- nitrogen (!g l-1) with Depth at Sampling Location. 27 Chale – Limnological studies of the pelagic zone of Lake Tanganyika … Figure 4: Monthly Soluble Reactive Phosphorus Concentrations (!g ll-1) at Sampling Location. 28 Tanz. J. Sci. Vol 32(2) 2006 Figure 5: Chlorophyll a Levels (!g l-1) at Sampling Location. DISCUSSION between November and June. This covered Thermal stratification was observed to the wet season and part of the dry season. In persist throughout the year, with maximum the present study, the deepening of the depth during the dry season in May and July thermocline was observed during both and also in December. The permanency of periods as reported by Plisnier et al. (1999). the thermocline in the northern basin has The highest surface temperature (27.3 oC) also been reported (Coulter 1963, Coulter was recorded in February, while the lowest and Spigel 1991, Chitamwebwa 1999, was recorded in August. A similar situation Plisnier et al. 1999). Coulter and Spigel was reported by Coulter and Spigel(1991) (op. cit.) stated that stratification was at its for Kigoma. At 100 m, the temperature maximum during the rainy season. ranged between 24.1 oC and 24.7 oC, a range However, Plisnier et al. (op. cit.) observed which had been reported before deepening of the thermocline at Kigoma (Chitamwebwa 1999, Edmond et al. 1993). 29 Chale – Limnological studies of the pelagic zone of Lake Tanganyika … In the present study, the thermocline was photosynthetic zone may be due to uptake observed to oscillate between 15 m and 60 by phytoplankton (Hecky and Kling 1981; m. Hecky et al. 1991). Oxygen was present throughout the study In L. Tanganyika, most of the nutrients are period from surface to 100 m, though the in the monimolimnion and their supply to bottom water had very low concentrations.
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