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Lake Hayq, a highland lake in Ethiopia, was stocked with Tilapia fish (Oreochromis niloticus) in late 1970s, offering an opportunity to study the effect of fish predation in a natural lake. Since 1930s, some limnological surveys have been done sporadically documenting a change in zooplankton composition including the disappearance of cladocerans, hypothesizing the stocked planktivorous fish could be a cause. Nevertheless, no detailed research was conducted to identify potential effects of fish stocking predominantly due to its remote location. The article presents data about zooplankton composition, abundance and biomass done between October 2007 and January 2009 on short-time intervals including the underlying limnological variables. The zooplankton community was depauperate comprising two copepods, three cladocerans, and six rotifers taxa, as typical for tropical lakes. Total mean standing biomass of all crustacean zooplankton was 237 mg dry mass m−3, which gave Lake Hayq an intermediate position when compared with other tropical lakes. Of copepods, Thermocyclops ethiopiensis was almost an exclusive species, and its temporal variation was influenced by food supply and water temperature. We refute the hypothesis that Tilapia was the cause for the seasonal disappearance of cladocerans, and attribute it to the adverse effect of episodic mixing. Nevertheless, the planktivorous fish probably plays a key role in structuring the cladocerans in particular the large-sized Daphnia magna. In January 2008, we observed a massive planktivorous fish mortality that triggered high algal biomass, which was later grazed by large-sized D. magna demonstrating the trophic cascade hypothesis in a natural ecosystem. 相似文献
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E. Aynekulu M. DenichD. Tsegaye R. AertsB. Neuwirth H.J. Boehmer 《Journal of Arid Environments》2011,75(5):499-503
Forests are highly susceptible to dieback under ongoing climate warming. In degraded forests, dead standing trees, or snags, have become such prominent features that they should be taken into account when setting management interventions. This study investigated (1) the extent and spatial pattern of standing dead stems of Juniperus procera and Olea europaea subsp. cuspidata along an elevational gradient, and (2) the effect of dieback on forest stand structure. We quantified abundance, size, and spatial pattern of tree dieback in 57 plots (50 m × 50 m) established at 100 m intervals along five transects. The snag density and basal area (mean ± SE) of the two species combined were 147 ± 23 stems ha−1 and 5.35 ± 0.81 m2 ha−1, respectively. The percentages of snags were extremely high for both J. procera (57 ± 7%) and O. europaea subsp. cuspidata (60 ± 5%), but showed a decreasing trend with increasing elevation suggesting that restoration is even more urgent at the lower elevations. Snags of the two species accounted for 31 and 45% of total stand density and basal area, respectively. Living stems exhibited truncated inverse-J-shaped diameter and height class distributions, indicating serious regeneration problems of these foundation species in the study area. In addition to direct interventions to assist recruitment of climax tree species, sites with high dieback would probably benefit from snag reduction to prevent fire incidents in the remaining dry Afromontane forests in northern Ethiopia. 相似文献
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Solomon Tadesse 《Gondwana Research》2001,4(1):97
Trace element distribution patterns are reported for whole rocks (granites, aplites, greisens, pegmatites, alaskites) and minerals from the Kenticha pegmatite field. The data shed light on the evolution, regional and local zonal pattern of the granitepegmatites and associated mineralization in the Kenticha belt. The complex mineralization of commercial concentrations of Ta, Nb, Hf, Zr, REE, U and Th is related to Be, Li, Cs, Rbbearing zones of pegmatites and is structurally controlled. Whole rock chemical signatures of the suite of felsic rocks of the Kenticha belt are predominantly similar to those generated by subduction in modern magmatic arcs and indicate a mantle derivation. Columbotantalite concentrates extracted from the pegmatitic ores represent the basic raw materials from which a number of possible byproducts can be recovered. 相似文献
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Analyzing the runoff response to soil and water conservation measures in a tropical humid Ethiopian highland 总被引：1，自引：0，他引：1
Dagnenet Sultan Atsushi Tsunekawa Nigussie Haregeweyn Enyew Adgo Mitsuru Tsubo Derege Tsegaye Meshesha 《自然地理学》2013,34(5):423-447
AbstractDifferent soil and water conservation (SWC) practices have been implemented in many drought-prone parts of Ethiopia since the 1980s. We assessed the effect of SWC practices on runoff response and experimentally derived and tested the validity of the runoff curve number (CN) model parameter for the tropical humid highland climate of the Kasiry watershed in northwestern Ethiopia. We recorded daily rainfall and runoff depth from 18 runoff plots (30 m long × 6 m wide) representing the five main land-use types with various SWC practices and two slope classes (gentle and steep). CN values were derived using the lognormal geometric mean CN procedure. Runoff was significantly less from plots with SWC measures, with average reductions of 44 and 65% observed in cultivated and non-agricultural lands, respectively. Runoff on plots representing non-agricultural land was relatively accurately predicted with the derived CN method, but predictions were less accurate for plots treated with a SWC practice. We conclude that predicting the effect of SWC practices on runoff requires parameterization with separate sets of CN value for each SWC practice. 相似文献
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Solomon Tadesse 《Gondwana Research》1999,2(3):439
Primary and placer gold deposits are mined from the Pan-African Adola volcano-sedimentary sequence, in southern Ethiopia. Two major mineralized belts can be recognized: the Megado (‘Gold Belt’) and the Kenticha Belts. The Kenticha Belt is also known for its rare metal mineralization. Extensive exploration of the area resulted in two most important primary gold deposits of Lega Dembi and Sakaro. The primary gold deposits are classified into four classes based on their geological setting:
- - auriferous veins, lodes, stockworks and silicified zones disseminated in schistose rocks
- - gold associated with quartzite
- - gold mineralization confined to conglomerates and meta-arkoses
- - auriferous quartz veins in high grade gneiss rocks
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Ayla Tankut Marjorie Wilson Tadesse Yihunie 《Journal of Volcanology and Geothermal Research》1998,85(1-4)
Detailed field mapping in the Güvem area in the Galatia province of NW Central Anatolia, Turkey, combined with K–Ar dating, has established the existence of two discrete Miocene volcanic phases, separated by a major unconformity. The magmas were erupted in a post-collisional tectonic setting and it is possible that the younger phase could be geodynamically linked to the onset of transtensional tectonics along the North Anatolian Fault zone. The Early Miocene phase (18–20 Ma; Burdigalian) is the most voluminous, comprising of over 1500 m of potassium-rich intermediate-acid magmas. In contrast, the Late Miocene volcanic phase (ca. 10 Ma; Tortonian) comprises a single 70-m-thick flow unit of alkali basalt. The major and trace element and Sr–Nd isotope compositions of the volcanics suggest that the Late Miocene basalts and the parental mafic magmas to the Early Miocene series were derived from different mantle sources. Despite showing some similarities to high-K calc-alkaline magma series from active continental margins, the Early Miocene volcanics are clearly alkaline with higher abundances of high field strength elements (Zr, Nb, Ti, Y). Crustal contamination appears to have enhanced the effects of crystal fractionation in the petrogensis of this series and some of the most silica-rich magmas may be crustal melts. The mantle source of the most primitive mafic magmas is considered to have been an asthenospheric mantle wedge modified by crustally-derived fluids rising from a Late Cretaceous–Early Tertiary Tethyan subduction zone dipping northwards beneath the Galatia province. The Late Miocene basalts, whilst still alkaline, have a Sr–Nd isotope composition indicating partial melting of a more depleted mantle source component, which most likely represents the average composition of the asthenosphere beneath the region. 相似文献
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Tilaye Tadesse Alexei A. Pevtsov T. Wiegelmann P. J. MacNeice S. Gosain 《Solar physics》2014,289(11):4031-4045
Solar eruptive phenomena, like flares and coronal mass ejections (CMEs), are governed by magnetic fields. To describe the structure of these phenomena one needs information on the magnetic flux density and the electric current density vector components in three dimensions throughout the atmosphere. However, current spectro-polarimetric measurements typically limit the determination of the vector magnetic field to only the photosphere. Therefore, there is considerable interest in accurate modeling of the solar coronal magnetic field using photospheric vector magnetograms as boundary data. In this work, we model the coronal magnetic field for global solar atmosphere using nonlinear force-free field (NLFFF) extrapolation codes implemented to a synoptic maps of photospheric vector magnetic field synthesized from the Vector Spectromagnetograph (VSM) on Synoptic Optical Long-term Investigations of the Sun (SOLIS) as boundary condition. Using the resulting three-dimensional magnetic field, we calculate the three-dimensional electric current density and magnetic energy throughout the solar atmosphere for Carrington rotation 2124 using our global extrapolation code. We found that spatially, the low-lying, current-carrying core field demonstrates a strong concentration of free energy in the active-region core, from the photosphere to the lower corona (about 70 Mm). The free energy density appears largely co-spatial with the electric current distribution. 相似文献
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T. Wiegelmann J. K. Thalmann B. Inhester T. Tadesse X. Sun J. T. Hoeksema 《Solar physics》2012,281(1):37-51
The Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) provides photospheric vector magnetograms with a high spatial and temporal resolution. Our intention is to model the coronal magnetic field above active regions with the help of a nonlinear force-free extrapolation code. Our code is based on an optimization principle and has been tested extensively with semianalytic and numeric equilibria and applied to vector magnetograms from Hinode and ground-based observations. Recently we implemented a new version which takes into account measurement errors in photospheric vector magnetograms. Photospheric field measurements are often affected by measurement errors and finite nonmagnetic forces inconsistent for use as a boundary for a force-free field in the corona. To deal with these uncertainties, we developed two improvements: i) preprocessing of the surface measurements to make them compatible with a force-free field, and ii) new code which keeps a balance between the force-free constraint and deviation from the photospheric field measurements. Both methods contain free parameters, which must be optimized for use with data from SDO/HMI. In this work we describe the corresponding analysis method and evaluate the force-free equilibria by how well force-freeness and solenoidal conditions are fulfilled, by the angle between magnetic field and electric current, and by comparing projections of magnetic field lines with coronal images from the Atmospheric Imaging Assembly (SDO/AIA). We also compute the available free magnetic energy and discuss the potential influence of control parameters. 相似文献
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Tadesse Yihunie 《International Journal of Earth Sciences》2002,91(5):922-933
Polydeformed and metamorphosed Neoproterozoic rocks of the East African Orogen in the Negele area constituted three lithostructurally distinct and thrust-bounded terranes. These are, from west to east, the Kenticha, Alghe and Bulbul terranes. The Kenticha and Bulbul terranes are metavolcano-sedimentary and ultramafic sequences, representing parts of the Arabian-Nubian Shield (ANS), which are welded to the central Alghe gneissic terrane of the Mozambique Belt affinity along N-S-trending sheared thrust contacts. Structural data suggest that the Negele basement had evolved through three phases of deformation. During D1 (folding) deformation, north-south upright and inclined folds with north-trending axes were developed. East and west-verging thrusts, right-lateral shearing along the north-oriented Kenticha and Bulbul thrust contacts and related structural elements were developed during D2 (thrusting) deformation. The pervasive D1 event is interpreted to have occurred at 620-610 Ma and the D2 event ended prior to 554 Ma. Right-lateral strike-slips along thrust contacts are interpreted to have been initiated during late D2. During D3, left-lateral strike-slip along the Wadera Shear Zone and respective strike-slip movements along conjugate set of shear zones were developed in the Alghe terrane, and are interpreted to have occurred later than 557 Ma. The structural data suggest that eastward thrusting of the Kenticha and westward tectonic transport of the Bulbul sequences over the Alghe gneissic terrane of the Mozambique Belt, during D2, were accompanied by right-lateral strike-slip displacements along thrust contacts. Right-lateral strike-slip movements along the Kenticha thrust contact, further suggest northward movement of the Kenticha sequence during the Pan-African orogeny in the Neoproterozoic. Left-lateral strike-slip along the orogen-parallel NNE-SSW Wadera Shear Zone and strike-slip movements along a conjugate set of shear zones completed final terrane amalgamation between the Arabian-Nubian Shield and the Mozambique Belt in Neoproterozoic southern Ethiopia. 相似文献