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26 Assessment of Zooplankton Community Structure of the Bahir Ethiopian Journal of Environmental Studies and Management Vol.1 No.2 June. 2008 Assessment of Zooplankton Community Structure of the Bahir Dar Gulf of Lake Tana, Ethiopia *Imoobe, T. O. T. and **Akoma O. C. Abstract The zooplankton composition of the Bahir Dar gulf of Lake Tana was studied in June and July, 2007. A total of forty four zooplankton species made up of 16 species of rotifers, 16 species of cladocerans and 12 species of copepods and their developing stages were recorded in the following order of dominance; Rotifera > Cladocera > Cylopoida > Calanoida. The number of species was not too different among the three sampling Stations, however, average zooplankton abundance, was significantly higher (P<0.05) at the littoral stations 3 and 2 than the open water Station 1 due to the small-bodied nauplii stages, small rotifers and cladocerans particularly Bosmina longirostris. In general, species richness, evenness, and diversity increased as average abundance increased in the littoral zones of the lake. The dominant zooplankton were Bosmina longirostris, Daphnia lumholtzi, Thermodiaptomus galebi, Thermocyclops ethiopiensis, Diaphanosoma sarsi, Keratella sp., Brachionus sp., Filinia sp. and Trichocerca longiseta. The rotifers particularly, the brachionids consistently occurred at all stations and was the most dominant zooplankton group in terms of abundance and diversity. This study showed that the lake is not polluted, and has a high potential for thriving fishery. Key words: Zooplankton, Bahir Dar Gulf, Lake Tana, Ethiopia. Introduction In this paper we examined the The dominant zooplankton in zooplankton community structure and the freshwater ecosystems are rotifers and micro- physico-chemical conditions in the Bahir Dar crustaceans made up of cladocerans and gulf of Lake Tana, because the abundance of copepods. Generally, zooplankton occupies a major zooplankton divisions (e.g., Rotifera, central position in the trophic link between Copepoda) are important to show changes in primary producers and higher trophic levels; trophic state of a water body (Gannon & they are also good bio-indicators of the Stemberger, 1978; Pace, 1986). physical and chemical conditions of aquatic environments which cause changes in the Materials and Methods qualitative and quantitative composition of Study sites zooplankton and influence their densities The location of Lake Tana in Ethiopia, (Gliwicz, 1974; Radwan 1973, 1976; its catchment area and its shape is shown in Hillbricht-Ilkowska, 1977; Karabin, 1985; Fig 1. Lake Tana is located in the north- Matveeva, 1991). western range of the Ethiopian highlands in In Lake Tana, zooplankton has been the Amhara region of the Federal Democratic reported as an essential component of the Republic of Ethiopia. It straddles the provinces biodiversity. A few existing investigations of of Gojjam and Gondar and it is Ethiopia’s the zooplankton in Lake Tana have been those largest lake and the source of the Blue Nile. At by Brunelli & Cannicci (1940), Rzóska approximately 1,800 m above sea level, it is a (1976), and Wudneh (1998). Recently Dejen et high altitude lake. According to Mohr (1961) al. (2002) reported the importance of micro- the Tana-rift in which Lake Tana lies is a crustaceans in the diet of two ‘small barbs’, shallow trough which is not directly connected Barbus tanapelagius and B. humilis which are to the main Eastern Rift Valley but is certainly the basis for the commercial fish production in related to it. From geological evidences the the Lake, while Dejen et al . (2004) lake probably formed through volcanic investigated the temporal and spatial blocking of the Blue Nile in early Pleistocene distribution of micro-crustacean zooplankton times. Subsequently the lake basin filled up in relation to turbidity and other environmental and now covers an area of approximately 3150 factors in the lake, and Wondie & Mengistou km 2. The lake is bordered by low plains in the (2006) investigated the duration of north, east and south-west that are often development, biomass and rate of production flooded in the rainy season forming extensive of the dominant copepods (calanoida and wetlands and some steep rocky shores in the cyclopoida). west and north-west (Nagelkerke, 1997). *Department of Animal and Environmental Biology,University of Benin, Benin-City, Nigeria. **Department of Biology, Bahir Dar University,Bahir Dar, Ethiopia. [email protected] 26 Assessment of Zooplankton ……. Imoobe and Akoma EJESM Vol.1 no.2 2008 At the southernmost tip of the lake is the city macrophytes - Ceratophyllum of Bahir Dar and the gulf extends for demersum L. (Ceratophyllaceae), approximately 10 km from the shore line. Typha latifolia L.(Typhaceae) In the present study, samples were Nymphaea sp. (Nymphaeaceae), collected from three points in the Bahir Dar Potamogeton heterophyllus Schreb. gulf area; two shoreline stations and one (Najadaceae) and Lemna sp. pelagic station as follows: • Station 3 is at the shoreline by the • Station 1 is on the open water about Bahir Dar Resort area. The station is 10 km from the southernmost point of adjacent to one of the lake wetlands the lake at the ‘Mango Park’ in Bahir used extensively for livestock grazing Dar City. and subsistent vegetable farming. • Station 2 is at the shoreline by Tana Typha latifolia and Nymphaea sp. Hotel/Shum-Abo Resort. The were observed as the dominant shoreline vegetation comprise vegetation. predominantly of the following 27 Ethiopian Journal of Environmental Studies and Management Vol.1 No.2 June. 2008 Environmental conditions was with the aid of relevant literatures Generally, water temperature ranged (Korinek, 1999; Flössner, 2000; Smirnov, between 22.2 and 25 oC throughout the study 1996; Defaye, 1988; Van de Velde, 1984), in with the highest mean temperature value the zooplankton Laboratory of the Department (24.13±0.59 oC) recorded in station 2. Depth of Animal and Environmental Biology, ranged between 0.96 and 4 m across the University of Benin, Benin City Nigeria. sampling stations, the pelagic zone of station 1 Results was significantly (P<0.001) deeper than other Zooplankton Composition, Abundance, sections of the lake in the study area. Secchi Diversity and Spatial Distribution transparency ranged from a minimum of 0.4 m Table 2 shows the zooplankton species (station 3) to a maximum of 0.9 m (recorded in encountered in Bahir Dar Gulf of Lake Tana station 2). Generally, the lake water was and their relative abundance at the sampled relatively turbid at all the stations. stations. Forty four zooplankton species made The water is fresh with conductivity up of 16 species of rotifers, 16 species of values ranging from 152 to 232µScm -1 with a cladocerans and 12 species of copepods and progressive increase from station 1 to station their developing stages were recorded. The 3. Lowest ionic content of the lake water was number of species was not too different among recorded in station 1 (156.3±1.93 µScm -1), the three sampling Stations, generally ranging while the highest record was for station 3 between 33 in the open water (Station 1) and (185.5±15.2). A weak alkaline pH range of 7.3 40 in the Littoral Station 2. Average – 8.5 was observed across the stations. zooplankton abundance, was significantly Dissolved oxygen concentration values higher (P<0.05) at stations 3 and 2 than Station (1.667mgl -1 - 7.7mgl -1) recorded in the Bahir 1. The highest average zooplankton abundance Dar gulf of Lake Tana were significantly (in station 3) was about 1.5x the average lower (P<0.05) in station 3 than other stations. abundance in the most sparsely populated The essential primary productivity nutrient, Station 1 (open water). In general, species nitrate was comparatively higher (0.92 - richness, evenness, and diversity increased as 4.18mgl -1) than phosphate (0.1 to 0.61mgl -1). average abundance increased in the littoral A summary of some physical and chemical zones of the lake (Table 3). conditions of the study stations is presented in The cyclopoid copepods which Table 1. constituted 75% of the copepod species found Sampling and analysis in the lake were of relatively high densities at Fortnightly sampling of the Bahir Dar the three stations particularly the littoral gulf of Lake Tana was carried out in the early stations 2 and 3. Out of the nine cyclopoid wet season months of June and July 2007 to copepod species, Thermocyclops ethiopiensis investigate physico-chemical conditions, and and Mesocyclops aequatorialis similis were plankton assemblages. Samples were the most abundant. Calanoids, nauplii and collected from the three stations using a 3.5 copepodites were also abundant at all three liter capacity Van Dorn water sampler. stations. The calanoid copepod Triplicate samples were collected with the Thermodiaptomus galebi lacustris was the water sampler at different depth intervals and dominant calanoid copepod. Most of the homogenized before being sub sampled for cladocerans were in the families bosminidae, physico-chemical analyses. Temperature, sididae and daphnidae. Chydorids, and depth and Secchi disc transparency were taken moinids were found in relatively fewer in-situ during the survey. Conductivity, numbers at all three stations while dissolved oxygen and pH were also recorded macrothricids were found only at the littoral in-situ using a WTW water sampler probe. Stations 2 and 3. Bosmina longirostris , Nitrate and phosphate were determined Daphnia hyalina, Daphnia lumholtzi and colometrically using Palintest analytical kit. Diaphanosoma sarsi were the most abundant Vertical plankton hauls were made at each of cladoceran species. The rotifers particularly, the sample stations using a 55µm net and the brachionids consistently occurred at all immediately fixed in 4% formaldehyde stations and was the most dominant solution. Observation and identification of zooplankton group in terms of abundance and zooplankton to species level was done with an diversity. The zooplankton groups occurred in Olympus model microscope and classification 28 Assessment of Zooplankton …….
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