Browsing by Author "Ogato, Tadesse"

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    Dynamics of Phytoplankton and Physicochemical Features of the Ethiopian Soda Lakes Chitu and Shala, and Evaluation of the Potential of their Waters for the Production of Arthrospira (Spirulina) Fusiformis (Cyanophyceae) in Laboratory Cultures
    (Addis Ababa University, 2015-06) Ogato, Tadesse; Kifle, Demeke (PhD)
    Soda lakes provide great opportunity for the study and understanding of aquatic ecosystems, owing to their unique biotic assemblage of great ecological and economic values and high sensitivity to environmental changes. In addition, Arthrospira (Spirulina), an alga of high ecological values to soda lakes ecosystems and produced commercially using high-cost chemical medium as a nutritional supplement to humans, grows naturally in some of the soda lakes. In this study, physicochemical features and phytoplankton community of the soda lakes Chitu and Shala, morphology of Arthrospira fusiformis in its natural habitat and its production using waters of the soda lakes were investigated. Measured parameters of underwater light climate of both lakes depicted light-limited conditions, which were associated with the accumulation of Arthrospira in Lake Chitu and non-biological turbidity in Lake Shala. The water column of Lake Chitu exhibited superficial thermal stratification with weak vertical mixing and considerable variability of dissolved oxygen (DO). Lake Shala showed superficial thermal stratification, but with turbulently mixing and well-oxygenated water column. Inorganic nitrogen sources in the euphotic zone of both lakes were often very low or undetectable while phosphorus sources were considerably high, possibly due to their phosphatic mineral-rich rock basins. NH3 and soluble reactive phosphate (SRP) showed dramatic increase with depth in Lake Chitu due to internal loading. SiO2 was remarkably low in both lakes, which was probably associated with biological uptake, some chemical processes within the lakes and organic matter accumulation. The phytoplankton community of Lake Chitu, which was constituted by a few (15) species belonging to the algal classes Bacillariophyceae (60%), Cyanophyceae (26.7%) and Cryptophyceae (2.3%), was dominated by A. fusiformis throughout the annual cycle, accounting for up to 98% of total phytoplankton abundance. The observed high mean phytoplankton biomass (chlorophyll-a, 150 μg chl-a L-1 and dry weight, 0.26 g L-1) in Lake Chitu showed significant seasonality (P < 0.05) with higher levels occurring during the rainy season. The algal biomass and abundance were correlated strongly and positively with rainfall and negatively with alkalinity-salinity, probably suggesting that hydrological control of the salinity of the lake is the major driving force for algal dynamics in Lake Chitu. Phytoplankton community of Lake Shala, which was of relatively higher diversity (23 spp. belonging to Bacillariophyceae, Cryptophyceae, iv Cyanophyceae and Dinophyceae), was persistently dominated by Bacillariophyceae and Cryptophyceae, respectively, accounting for about 57% and 22% of the total number of species, and 28% and 69% of total phytoplankton abundance. Cryptomonas spp. were the most abundant, contributing about 59 to 95% of total phytoplankton abundance of Lake Shala, while Thalassiosira sp. was the second most abundant (1–35%). Although seasonality was not large, the observed variations in the abundance of the dominant species in this lake were associated with water transparency, SiO2, rainfall and its effects on salinity and pH. Mean algal biomass, which was generally low (17 μg chl-a L-1), also showed small seasonal variations in Lake Shala, with strong but inverse relationship with water transparency. In general, the notable variations in phytoplankton and physicochemical parameters in Lake Chitu, probably suggest its sensitiveness to environmental perturbations and that any environmental changes including increased salinization may greatly affect the key alga (A. fusiformis) and the flamingos, with eventual impairment of its ecosystem services. On the other hand, the observed small variations in the physicochemical and phytoplankton variables in Lake Shala seem to suggest the lake‟s resilience to the current level of environmental perturbations, owing to its presumably high stress absorbing capacity associated with its large size and great depth. Three distinct morphotypes of A. fusiformis, tightly coiled (H-type), spiral or loosely coiled (S-type) and intermediately coiled (C-type) were observed in Lake Chitu, with the H-type (50%) and S-type (40%) being the first and second most dominant morphotypes. The dominance of H-type and tightening of its helix pitch were strongly associated with NO3- and HCO3- deficiencies, and high levels of photosynthetically active radiation (PAR) and temperature of the lake, while the abundance of S-type was more strongly but negatively correlated with the high salinity of the lake. The variability in morphology seems to suggest that A. fusiformis undergoes morphological changes in response to environmental stresses in its natural habitat, resulting in the occurrence of various morphotypes of the same species. Comparable growth rate (μ) and biomass (B) of A. fusiformis were observed in both Lake Chitu water-based media (CBM) and Lake Shala water-based media (SBM), with slightly higher values in the latter. Both CBM and SBM supplemented with the standard Spirulina medium (SM) (25% and 50%) supported considerably higher μ and B, which were probably associated with the reduction in pH and salinity, and provision of the limiting nutrient nitrogen in the supplemented media. The higher μ and B in SBM, probably due to some of their aggregate chemical parameters that were closer to those in the SM, and abundant SRP, seems to suggest that Lake Shala water is more conducive to Arthrospira. We contend that 25% and 50%-supplemented Lake Shala water can be preferably used to produce Arthrospira, thereby reducing the cost of nutrients by 75% and 50%, respectivel
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    Dynamics of Phytoplankton in Relation to Physico-Chemical Factors in Lake Bishoftu, Ethiopia
    (Addis Ababa University, 2007-07) Ogato, Tadesse; Kifle, Demeke (PhD)
    The 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 Bishoftu