Browsing by Author "Abate, Rediat"

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    Seasonal Studies on Phytoplankton In Relation to Some Biological and Physico-Chemical Factors in Lake Hora-Kilole, Ethiopia
    (Addis Ababa University, 2008-07) Abate, Rediat; Kifle, Demeke (PhD)
    The temporal dynamics of plankton, benthic macroinvertebrates and physico-chemical variables were studied at a central station in Lake Hora-Kilole from August, 2007 to May, 2008. The lake remained small and shallow and consequently underwent complete mixing throughout the study period. Secchi depth ranged from 0.15 m to 0.78 m with high values coinciding with lower wind speed. Vertical extinction coefficient varied temporally (2.39-10.23 ln units m-1) with high values coinciding with relatively low algal biomass and high abiogenic turbidity associated with wind-induced mixing. SRP and TP varied from 0.33 to 3.5 and 2.13 to 6.15 μg L-1 respectively. The concentration of NO3 + NO2-N (in μg L-1) ranged from 17 to 303 while that of NH4++NH3–N varied from 1.7 to 49.09 μg L-1. Molybdate reactive silica (in mg L-1) ranged from 10.4 to 69.7. The phytoplankton community was composed primarily of green algae, euglenoids, diatoms, dinoflagellates and cyanobacteria with the overwhelming dominance of dinoflagellates and Cyanobacteria corresponding to the seasonal peaks of Chl a biomass of phytoplankton. The most dominant species of Dinoflagellates and Cyanobacteria were Peridinium gatunense and Anabaena cf. agardhii and cylindrospermopsis curvispora respectively. Zooplankton abundance peaked in February, 2008 coincident with the largest peaks of phytoplankton biomass and abundance and with Thermocyclops decipiens, Brachionus sp. and Daphnia barbata as the most important taxa. Among the macroinvertebrates, chironomids and Lambriculidae-oligochaete were dominant during the dry period. Total phytoplankton biomass showed seasonal variations (≈36 to 148 μg L-1) with large peaks in October, 2007(≈148 μg L-1) and February, 2008 (≈147 μg L-1) and the seasonal minimum (≈ 36 μg L-1) in January, 2008. Among the three size-groups of phytoplankton, the netplankton (> 20 μm fraction) was the most important contributor to the total phytoplankton biomass with its biomass and percentage contributions to total phytoplankton biomass ranging from 13.21 mg m-3 and 32 % to 132.06 mg m-3 and 95 %, respectively. The percentage contributions of nanoplankton and picoplankton varied from about 0 to 64% and from 4 to 28 %, respectively. Depth profiles of gross photosynthesis were of typical pattern for phytoplankton without surface photoinhibition during the major rainy period. Light-saturated rate of gross photosynthesis (Amax) varied from 370 to 3843 mg O2 (≈111.5– 1199 mg C) m-3 h-1 with the maximum value corresponding to the seasonal maximum of phytoplankton biomass. Biomass-specific rate of gross photosynthesis at light-saturation varied from about 6 to 33 with positive and moderate correlation (r=0.632, r2=0.40 at p=0.0496) with SRP. Hourly integral rate of gross photosynthesis, (ΣA), which was positively and strongly correlated with Amax (r= 0.885, r2=0.7895 at P = 0.0006) and Zeu (r=0.7849, r2=0.616 at p=0.0072), ranged from 0.21 to 6 g O2 (≈ 0.065 - 1.87 g C) m-2 h-1. The marked temporal variations in phytoplankton parameters are discussed in relation to physicochemical and biological variables.and the general knowledge of some biological characteristics of the species (Garcia et al., 1989). Limnology in the tropics has only recently developed past the stage of exploration, but the need of limnological knowledge is, as pressing at tropical latitudes as in the temperate (Melack, 1996; Talling and Lemoalle, 1998, Lewis, 2000). Despite the perturbation by humans, vast number of lakes of varied size and shape are found in most tropical regions. They are often inhabited by densely populated biota and have been the subject of some of the most scientifically informative studies of tropical aquatic ecosystems (Talling, 2001). In Africa, the first assessment of phytoplankton seasonality stemmed from collections made in 1899 by the Fullenborn expedition to Lake Nyassa, and later to Lake Malawi (Talling, 1986). Extensive efforts were made after 1950, especially in East and Central Africa (Talling, 1986). Despite their tropical location, African lakes exhibit considerable seasonality related to alternations of warm, wet, cooler and dry seasons. Although numerous studies have been made on the species composition and photosynthetic production of phytoplankton in various East African lakes (Talling and Lemoalle, 1998), relatively little has been done on this aspect in Ethiopian lakes. Ethiopia is rich in both natural water bodies such as rivers and lakes and man-made lakes (reservoirs) compared to other east African countries. Ethiopia also possesses many great crater lakes (Prosser et al., 1968), among these are the Bishoftu crater lakes, which form an extensive series of volcanic explosion craters in the vicinity of the city of Bishoftu (Debrezeit). The Bishoftu crater lakes are grouped among the most scientifically known lakes of Ethiopia until recently (Zinabu Gebre-Mariam, 1994). Volcanic crater lakes include lakes located in crates formed after eruption and they have small surface area and steep crater wall (Hutchinson, 1957). We have some scientific information on the Bishoftu crater lakes dating as far back as the early 1930`s and 1940`s (Omer-Cooper, 1930; Vatova, 1940; Loffredo and Maldura, 1941; Cannicci and Almacia, 1947, cited in Prosser et al., 1968). In the early 60`s, different aspects of these lakes were studied (Baxter et al.,1965; Baxter and wood, 1968; Prosser et al., 1968; Talling et al., 1973; Wood et al., 1976, 1976, 1984; Wood and Talling, 1988). It is these investigations which tempted Zinabu Gebre-Mariam (1994) to describe the Bishoftu crater lakes as being among the most studied lakes of Ethiopia. There is, however, scanty scientific information on several aspects of these lakes, particularly on some biological aspects. Lakes throughout the temperate and tropical latitude have been drastically altered as a result of the burden they carry when there is an increase in population density, economic growth, and change in land cover (Lewis, 2000). It is also known that climatic change especially rainfall, have a prominent effect on the limnology of a lake, resulting in changes in different parameters of a lake. Human interventions are the main cause of lake deterioration and it has various consequences. Human interferences are frequently reflected as changes in the trophic status of the lake, volume of the water, and consequent ecological changes. This is a phenomenon observed throughout the world, with some lakes changing from oligotrophic to eutrophic through mesotrophic or the reverse could happen. Increase or decease in the biomass of plankton and level of nutrients and alteration in the underwater climate are some of the symptoms of eutrophicaton and/or oligotrophication. Thus, the trophic status of a lake can be determined from different physico-chemical and biological parameters. Euphotic depth, concentration of a limiting nutrient, phytoplankton biomass, primary productivity, and phytoplankton abundance are some of the basic and direct indicators of the trophic status of Lake. One can also look at the abundance and biomass of consumers (Zooplankton and fishes) to predict tropic status. These, the same parameters can be used to determine long term changes of lakes, and for comparison of two or more lakes (Brook Lemma, 2002). The assessment of these physico-chemical and biological parameters is, therefore, crucial to evaluate the ecological health of aquatic ecosystems, optimize their exploitation, manage and conserve their resources. Most of the Ethiopian lakes including the Bishoftu crater lakes, which are in the vicinity of a fast-growing town surrounded by agricultural lands, are subjected to shoreline modification, waste disposal, and other practices associated with population growth (Zinabu Gebre-Mariam, 1998; Zinabu Gebre-Maraim, et al., 2002). Furthermore, many water bodies (predominantly rivers and lakes) in Ethiopia have been physically degraded or altered seriously by human manipulation (e.g. L. Hora-Kilole) and habitats have consequently been lost (e.g. L Haramaya) (Brook Lemma, 2002). Human-induced changes of lakes in Ethiopia are exemplified by the eutrophication of Lake Hayk (Elizabeth Kebede et al., 1992), shrinkage of Lake Alemaya (Brook Lemma, 1994; 2002) and drastic changes in the water chemistry and species composition and productivity of phytoplankton in Lake Hora-Kilole (Brook Lemma, 1994; 2002). Lake Hayk has changed greatly in its phytoplankton biomass and transparency (Elizabeth Kebede et al., 1992). This was possible because of the introduction of planktivorous fish which freed the phytoplankton from grazers’ control. The lake was stocked with tilapia species from a crater lake, probably Lake Hora in 1978 by the Fisheries Department, Ministry of Agriculture, to provide food, and harvest by a newly established fish gillnet. The commercial fishery of the lake increased to 200 tones per year (84Kg per hectare) (Elizabeth Kebede et al., 1992). Lake Alemaya has been used for irrigation, animal watering and household consumption and consequently dramatic changes occurred in its volume has occurred (Brook Lemma, 1994; 2002). Lake Hora-Kilole was once grouped among the unique saline lakes of Africa, such as Arenguade, Chitu, Abijata and Shalla in Ethiopia and Nakuru in Keneya (Prosser et al., 1968; Talling et al., 1973; Vareschi, 1982; Wood et al., 1984; Elizabeth Kebede et al., 1986; Wood and Talling 1988; Green 1986; 1993; Tudorancea et al., 1999). This lake was also known for its superabundance of Spirulina spp. assemblage and avifauna. In 1989, the Ministry of Agriculture (MOA), Addis Ababa, in an attempt to use the lake as a reservoir to gravitationally irrigate the southern and eastern low-lying plains, diverted River Mojo into Hora-Kilole resulting in the complete transformation of the lake ecology (Brook Lemma, 1994; 2002). After the diversion of the river, few investigations have tried to show ecological changes in the Lake (Brook lemma, 1994; 2002, Zinabu Gebre-Mariam, 1994; Zinabu Gebre-Mariam et al., 2002). Diversion of R. Mojo into Hora-Kilole has caused substantial changes in the morphometric, biological and physicochemical characteristics of the lake. So the aim of this study is to complement and update the physico-chemical and biological data generated for the lake in previous studies, which may help us show the seasonal and long-term changes that have occurred in the lake.