Ecosystem structure, trophic link and functioning of a shallow rift valley lake: the case of Lake Ziway (Ethiopia

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Addis Ababa University


A trophic model was constructed for the Lake Ziway ecosystem using Ecopath with Ecosim application software. This model was used to evaluate and analyze the food web structure and other properties of this ecosystem. The mean Chlorophyll a biomass of 70.46 μg L−1was found which is higher than those of earlier reports. Wet weight value of phytoplankton was estimated to be 15.99 tons km-2. A total of 4 cladocerans and 2 copepods were identified. The biomass of zooplankton is dominated by Thermocyclops (127.73 mg WW m-3). A total of fourteen families of macro-zoobenthos were identified which were dominated by chironomids (15924 individuals m-2) that were attached with the macrophyte Typha anguistifola. The above ground biomass of macrophytes vegetation of Lake Ziway was estimated as 3011.68 tons WW km-2. The total mortality value for Carp and Catfish was 0.74 and 1.34 year-1 respectively. The total annual catch from Lake Ziway was estimated at 1127 tons based on the catch and effort data (2008– 2010). The estimated annual catch in tons per gear type was 683.11, 62.97 and 295.98 for beach seine, gill net and long line, respectively. The results of Ecopath with Ecosim modeling showed the presence of six aggregated trophic levels (TLs). The fractional trophic level assigned TLs of Lake Ziway ecosystem between 1.0 and 3.29. The biomass flow was highly restricted to I and II trophic levels which contributed 99.76 %. The mean transfer efficiency was only 4.4%. The fishery catch consumed 2.5% of the primary production in the lake. Low ecotrophic efficiency (EE) of 0.47 was calculated for the phytoplankton indicating that most of the production remains within the system unutilized. A high value of EE was noted for carp (0.99), indicating that it is subjected to high fishing pressure despite its low biomass. In Lake Ziway, EE values were high for all fish groups, indicating that the fish groups were highly constrained by a combination of fishing and predation mortality. Management scenario using Ecosim analysis indicated that increasing catfish mortality by 1.50 times resulted in increment of tilapia to 1.6 times the current biomass in four years period and leveling of its biomass to 1.16 times the current value at the end of the tenth year. Doubling in mortality of catfish resulted in increment of tilapia biomass to 1.12 times the current biomass. Decreasing the effort of beach seine did not benefit the tilapia group as their biomass declined to 0.98 and 0.66 which is related to biomass increment of catfish. The final scenario of increasing the effort long line by 1.50 and 2.00 times benefited tilapia and significantly decreased the catfish community from the ecosystem. The total analysis indicated that lake Ziway ecosystem has an open niche which can be filled with obligate herbivores to increase the energy transfer efficiency of the lake. Key words: Ecopath with Ecosim, Biomass, Transfer efficiency, Trophic modeling, Species introduction



Ecopath with Ecosim, Biomass, Transfer efficiency, Trophic modeling, Species introduction