Kifle, Demeke (PhD)Ogato, Tadesse2018-07-112023-11-082018-07-112023-11-082007-07http://etd.aau.edu.et/handle/123456789/7976The seasonal dynamics of phytoplankton in relation to some physico-chemical and biological parameters were studied at a near-shore and central stations in Lake Bishoftu from August, 2006 to May, 2007. Water transparency (ZSD) ranged from 0.50 to 1,05 and from 0.46 to 1.0 m at the Central and Near Shore stations respectively, with high and low values corresponding to dry and rainy periods respectively. Mean vertical extinction coefficient varied temporally with higher values coincident with high algal biomass and abiogenic turbidity following rainy periods. The lake exhibited both superficial and deep-seated thermal stratifications, with ill-defined thermoclines on some occasions and never underwent complete mixing during the study period. Algal nutrients except nitrate-nitrogen showed temporal variations whose changes were inversely correlated with Chl a.The phytoplankton community of the lake, which consisted of five algal classess was dominated by cyanobacteria with Microcystis spp. as the most abundant and persistent taxa and with their contributions to the total abundance of the phytoplankton assemblage of the lake ranging from 58 to 99%. The zooplankton community was dominated by Calanoids and Rotifers with Lovenula africana and Brachionus spp.as the most numerically important taxa. Phytoplankton biomass measured as Chl a showed seasonal variations with maximum values during the rainy and post rainy months at both stations. Depth profile of photosynthesis showed a region of photoinhibition and condensed photosynthetic zone, a feature of some east African lakes. Light-saturated rates of gross photosynthesis (Amax) at the central station varied from a minimum of 410 to a maximum of 1630 mg O2 m-3 h- and with fairly strong and positive correlations with Chl a (R2=0.38, p=0.08), Soluble Reactive Phosphate, SRP (R2=0.46, p=0.07) and Light utilization efficiency, Φ (R2=0.61, p=0.02). Biomass specific rate at light-saturation exhibited unusually high values (29-91) and was positively and significantly correlated with SRP (R2=0.59, p=0.02), Areal rates varied from 0.89 to 3.88 g O2 m-2 h- 1 in association with biomass and Amax. The possible causes for the observed saptio-temporal variations in physico-chemical and biological features of the lake are discussed and suitable conservation strategies recommended.phytoplankton was based on short-term observations, which were not systematic. Wood and Talling (1988) have compiled the sporadic information on the dominant or most common species of phytoplankton reported in various limnological papers. The systematic and detailed taxonomic studies conducted on phytoplankton to date are those of Tsegaye Mihrete-Ab (1988), Elizabeth Kebede and Amha Belay (1994), Elizabeth Kebede (1996) and Elizabeth Kebede and Willén (1998). Also several studies (Talling et al., 1973; Amha Belay and Wood, 1984; Girma Tilahun, 1988; Brook Lemma, 1994; Taylor and Zinabu Gebre-Mariam 1989; Demeke Kifle and Amha Belay, 1990 and Eyasu Shumbulo, 2004; Girma Tilahun, 2006) have investigated the dynamics of phytoplankton in relation to some limnological aspects in Ethiopian lakes. Although the Bishoftu crater lakes have been subjects of many limnological investigations, some dating as far back to the early 1930's (Prosser et al., 1968), information on the temporal dynamics of the community structure and photosynthetic production of phytoplankton in relation to physico-chemical factors in Lake Bishoftu is non-existent. The early short-term comparative studies made on the Bishoftu lakes focused mainly on the assessment of general trends in physico-chemical limnological features. Even the few published reports on the phytoplankton biomass (Zinabu Gebre-Mariam, 1994; Wood and Talling,1988) and photosynthetic production (Talling et al., 1973) in the Bishoftu crater lakes were based on short-term observations although it was indicated that phytoplankton of tropical waters exhibit temporal changes in their species composition, biomass and primary production (Talling,1986). In Lake Bishoftu, some-short term studies have been reported on bathymetry, stratification, some chemical and morphometric aspects by expeditionary limnologists such as Baxter et al. (1965), Baxter and Wood (1965), and Wood and Talling (1988). These short-term studies were based on occasional sampling for the purpose of comparison among crater and tropical Rift Valley lakeComparative study with another soda lake, Lake Aranguade, on zooplankton structure and dynamics has been conducted by Afeworki Ghebrai (1992). Therefore, in comparison with other Debre-Zeit and Rift Valley lakes, someone can confidently talk of that Lake Bishoftu is not studied in phytoplankton aspect over extended periods. Zinabu Gebre-Mariam (1994), Brook Lemma (2002) and Zinabu Gebre-Mariam et al. (2002) have noted that the Ethiopian Rift Valley and crater lakes have been undergoing changes in their limnological features during the last two decades or so because of increased human interventions. The present study lake is no exception. Lake Bishoftu is surrounded by a fast growing town, Debre Zeit, and its shores are used for washing clothes, watering livestock and recreation and the like. Shoreline modifications made on almost all sides of the lake for various purposes (e.g. for the construction of resort hotels and residences in the catchments) introduce enormous amounts of particulate wastes, which form suspensions in the water column thereby reducing light penetration at least in the near-shore regions of the lake. The lake has also been serving as a major dumping site for domestic wastes (solid and liquid wastes) originating from households, hotels and small industrial operations, including a privately-owned tannery. These activities could lead to changes in the physico-chemical limnological features of the lake resulting in the disruption of the lake ecosystem, with consequent changes in the species composition and photosynthetic production of phytoplankton, which determine the pelagic food web structure and fish production in the lake. It is also possible that degradation of this aquatic ecosystem may go on unnoticed for a long time and eventually result in unexpected disastrous effects. The assessment of qualitative and/or quantitative changes taking place in this lake, therefore, necessitates the generation of basic information on the physical, chemical and biological aspects of this water body over an extended period of time.Thus, the purpose of this research project was to look into the temporal variation in the species composition, seasonal abundance and photosynthetic production of phytoplankton in relation to some physico-chemical variables in Lake BishoftuenBiologyThesis